GHA fix 3

.github/workflows/ci.yml

@@ -142,8 +142,9 @@ jobs:
           - toolset: clang
             compiler: clang++-16
             cxxstd: "03,11,14,17,20,2b"
-            os: ubuntu-24.04
+            os: ubuntu-latest
             install: clang-16
+            container: ubuntu:24.04
           - toolset: clang
             compiler: clang++-17
             cxxstd: "03,11,14,17,20,2b"
@@ -224,6 +225,10 @@ jobs:
             cxxstd: 14,latest
             addrmd: 32,64
             os: windows-2019
+          - toolset: msvc-14.1
+            cxxstd: "11,14,17,20,latest"
+            addrmd: 32,64
+            os: windows-2019
           - toolset: msvc-14.2
             cxxstd: "14,17,20,latest"
             addrmd: 32,64

doc/00_optional.qbk

@@ -1,8 +1,8 @@
 [library Boost.Optional
-    [quickbook 1.4]
+    [quickbook 1.7]
     [authors [Cacciola Carballal, Fernando Luis]]
     [copyright 2003-2007 Fernando Luis Cacciola Carballal]
-    [copyright 2014-2026 Andrzej Krzemieński]
+    [copyright 2014-2026 Andrzej Krzemieński]
     [category miscellaneous]
     [id optional]
     [dirname optional]

doc/16_optional_references.qbk

@@ -27,7 +27,7 @@ rather than the reference itself.
 On compilers that do not conform to Standard C++ rules of reference binding,
 some operations on optional references are disabled in order to prevent subtle
 bugs. For more details see
-[link boost_optional.dependencies_and_portability.optional_reference_binding Dependencies and Portability section].
+[link optional_reference_binding Dependencies and Portability section].
 ]
 
 [heading Rvalue references]
@@ -39,6 +39,8 @@ Rvalue references and lvalue references to const have the ability in C++ to exte
 
 [endsect]
 
+
+[#optional_ref_rebinding_semantics]
 [section Rebinding semantics for assignment of optional references]
 
 If you assign to an ['uninitialized ] `optional<T&>` the effect is to bind (for

doc/17_in_place_factories.qbk

@@ -1,5 +1,5 @@
 
-[section In-Place Factories]
+[section:in_place_factories In-Place Factories][#boost_optional_factories]
 
 One of the typical problems with wrappers and containers is that their
 interfaces usually provide an operation to initialize or assign the
@@ -138,4 +138,4 @@ The factories are implemented in the headers: __IN_PLACE_FACTORY_HPP__ and __TYP
 
 [caution The support for in-place factories is deprecated. Use constructor taking `in_place_init` tag and function `.emplace()` instead.]
 
-[endsect]
+[endsect:in_place_factories]

doc/1A_type_requirements.qbk

@@ -8,7 +8,7 @@ The very minimum requirement of `optional<T>` is that `T` is a complete type and
     assert(!o);        //
     o.value();         // always throws
 
-But this is practically useless. In order for `optional<T>` to be able to do anything useful and offer all the spectrum of ways of accessing the contained value, `T` needs to have at least one accessible constructor. In that case you need to initialize the optional object with function `emplace()`, or if your compiler does not support it, resort to [link boost_optional.design.in_place_factories In-Place Factories]:
+But this is practically useless. In order for `optional<T>` to be able to do anything useful and offer all the spectrum of ways of accessing the contained value, `T` needs to have at least one accessible constructor. In that case you need to initialize the optional object with function `emplace()`, or if your compiler does not support it, resort to [link boost_optional_factories In-Place Factories]:
 
     optional<T> o;
     o.emplace("T", "ctor", "params");

doc/21_ref_none.qbk

@@ -2,7 +2,7 @@
     Boost.Optional
 
     Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
-    Copyright (c) 2015 Andrzej Krzemienski
+    Copyright (c) 2015 Andrzej Krzemieński
 
     Distributed under the Boost Software License, Version 1.0.
     (See accompanying file LICENSE_1_0.txt or copy at
@@ -15,14 +15,25 @@
 ```
 namespace boost {
 
-class none_t {/* see below */};
+class none_t
+{
+  friend constexpr bool operator==(none_t, none_t) = default;
+  /* see below */
+};
 
 inline constexpr none_t none (/* see below */);
 
 } // namespace boost
 ```
 
-Class `none_t` is meant to serve as a tag for selecting appropriate overloads of from `optional`'s interface. It is an empty, trivially copyable class with disabled default constructor.
+Class `none_t` is meant to serve as a tag for selecting appropriate overloads of from `optional`'s interface.
+It is an empty class.
+
+It is [@https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable.html trivially copyable].
+
+It is ['not] [@https://en.cppreference.com/w/cpp/named_req/DefaultConstructible default-constructible].
+
+It models concept [@https://en.cppreference.com/w/cpp/concepts/equality_comparable `std::equality_comparable`].
 
 Constant `none` is used to indicate an optional object that does not contain a value in initialization, assignment and relational operations of `optional`.
 

doc/28_ref_optional_semantics.qbk

@@ -281,7 +281,7 @@ factory.
 * [*Postconditions: ] `*this` is [_initialized] and its value is ['directly given]
 from the factory `f` (i.e., the value [_is not copied]).
 * [*Throws:] Whatever the `T` constructor called by the factory throws.
-* [*Notes:] See [link boost_optional.design.in_place_factories In-Place Factories]
+* [*Notes:] See [link boost_optional_factories In-Place Factories]
 * [*Exception Safety:] Exceptions can only be thrown during the call to
 the `T` constructor used by the factory; in that case, this constructor has
 no effect.
@@ -524,7 +524,7 @@ factory.
 * [*Postconditions: ] `*this` is [_initialized] and its value is ['directly given]
 from the factory `f` (i.e., the value [_is not copied]).
 * [*Throws:] Whatever the `T` constructor called by the factory throws.
-* [*Notes:] See [link boost_optional.design.in_place_factories In-Place Factories]
+* [*Notes:] See [link boost_optional_factories In-Place Factories]
 * [*Exception Safety:] Exceptions can only be thrown during the call to
 the `T` constructor used by the factory; in that case, the `optional` object
 will be reset to be ['uninitialized].
@@ -956,7 +956,7 @@ __SPACE__
 
 * [*Postconditions:] `bool(*this) == bool(rhs)`.
 
-* [*Notes:] This behaviour is called ['rebinding semantics]. See [link boost_optional.design.optional_references.rebinding_semantics_for_assignment_of_optional_references here] for details.
+* [*Notes:] This behaviour is called ['rebinding semantics]. See [link optional_ref_rebinding_semantics here] for details.
 
 * [*Example:]
 ``

doc/29_ref_optional_convenience.qbk

@@ -11,7 +11,7 @@
 
 [section Header <boost/optional.hpp>]
 
-This is an alias for header [link boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional `<boost/optional/optional.hpp>`].
+This is an alias for header [link ref_header_optional_optional_hpp `<boost/optional/optional.hpp>`].
 
 
 [endsect]

doc/90_dependencies.qbk

@@ -13,13 +13,55 @@
 
 
 [section Minimum System Requirements]
-This library requires C++11 as minimum. However some features are disabled.
+This library requires C++11 as minimum. However, in C++11 some features are disabled.
 
-For C++14 and higher this library provides a `constexpr` interface for non-mutable
-member functions and some mutable member functions.
+[section:constexpr Support for `constexpr`]
+
+[section C++11]
+For compilers fully supporting C++11 (including unconstrained unions and ref-qualifiers),
+for trivially-destructible `T`s, `optional<T>` is a ['literal type] and its constructors
+with `this->has_value() == false` as postcondition:
+
+* `optional()`,
+* `optional(none_t)`,
+
+are ['core constant expressions]. Even for other `T`s, these constructors are guaranteed to
+perform ['constant initialization]: they are never subject to "initialization order fiasco".
+
+Other constructors with `this->has_value() == true`
+as postcondition are core constant expressions if the expression required to initialize
+the contained value is a core constant expression. This includes constructors:
+
+* `template <typename... Args> optional(in_place_init, Args&&...)`,
+* `template <typename U> optional(U&&)`,
+* `optional(const T&)`,
+* `optional(T&&)`.
+
+Other constructors, including the copy and move constructos, are ['not] core constant expressions.
+
+Member functions `.has_value`, `operator bool` and (non)equality comparisons with `none` are core-constant expressions for trivially-destructible `T`s.
+
+Also all `const`-qualified non-static member functions and comparison operators are core constant expressions, if the corresponding operations on `T`
+are core constant expressions.
+[endsect]
+
+[section C++14]
+For C++14 and higher this library, for trivially-destructible `T`s provides the `constexpr` interface for all mutable and non-mutable
+member functions, as long as:
+
+* the corresponding operations on `T` are core constant expressions and
+* the member never attempts to change the `optional`'s state from not containing a value to containing a value.
 
 [note For types that overload unary `operator&` (address) some member functions in `optional`, like `operator->`,
 cannot be implemented as `constexpr` on some compilers.]
+[endsect]
+
+[section C++17]
+In C++17 all non-deprecated constructors are core constant expressions as long as
+`T` is a literal type and its constructor used is a core constant expression.
+[endsect]
+
+[endsect:constexpr]
 
 [endsect]
 

doc/91_comparison_with_std.qbk

@@ -11,8 +11,7 @@
 
 [section:std_comp Comparison with `std::optional`]
 
-[table
-  []
+[table Comparison with `std::optional`
   [ [[*`boost::optional`]] [[*`std::optional`]] [] ]
   [ [`optional<int> o = none;`] [`optional<int> o = nullopt;`] [Different name for no-value tag.] ]
   [ [`optional<X> o {in_place_init, a, b};`] [`optional<int> o {in_place, a, b};`] [Different name for in-place initialization tag.] ]
@@ -42,6 +41,12 @@
   [ [`make_optional(cond, v);`] [] [No `make_optional` with condition in `std`.] ]
   [ [] [`make_optional<T>(a, b);`] [No `make_optional` with specified `T` in `boost`.] ]
   [ [`std::cout << optional<int>{};`] [] [No printing to IOStreams in `std`.]]
+
+  [ [`std::vector<optional<int>> rng;`
+
+  `std::ranges::find(rng, boost::none)`] [`std::vector<optional<int>> rng;`
+
+  `std::ranges::find(rng, boost::optional<int>{})`] [optional<T> is never [@https://en.cppreference.com/w/cpp/concepts/equality_comparable `std::equality_comparable_with`] `nullopt_t` in `std`. ] ]
 ]
 
 

doc/92_relnotes.qbk

@@ -11,9 +11,35 @@
 
 [section:relnotes Release Notes]
 
+[heading Boost Release 1.91]
 
+* For compilers with full C++11 support (including "unrestricted  unions" and ref-qualifiers)
+  changed the implementation from aligned storage to union storage. This enables the gradual
+  `constexpr` support:
 
+    * In C++11, some constructors and `const`-qualified accessors become usable
+      in compile-time contexts (are ['core constant expressions] for types
+      satisfying certain constraints.
+    * In C++14 even some mutating operations become core constant expressions (those
+      that do not require changing the state of `optional` from not having a value to
+      having a value), for co-operating types.
+    * In C++17 all constructors (including copy and move) become core constant expressions
+      for co-operating types.
 
+  This addresses [@https://github.com/boostorg/optional/issues/143 issue #143].
+
+* *Breaking change.* In the said implementation, abandoned the mechanism for customizing
+  `swap`. Hardly anyone knows about this mechanism and it was never documented.
+
+* In the said implementation, applied a couple of small changes to get closer to the interface of `std::optional`:
+  * Enabled syntax `o = {}` for putting optional objects to no-value state.
+  * Enabled syntax `o.value_or({})`, which uses a default-constructed `T`.
+  * Construct `o = u`, where `o` is of type `optional<T>` and `u` is of type `U` convertible to `T`,
+    does not create a temporary `T`.
+
+* `none_t` is now `std::equality_comparable`, which means that `none_t` and `optional<T>`
+  model concept `std::equality_comparable_with` (for `std::equality_comparable` `T`s),
+  which means that you can `std::ranges::find(rng, boost::none)` for a range of optional objects.
 
 [heading Boost Release 1.87]
 

include/boost/none_t.hpp

@@ -25,7 +25,7 @@
 
 #elif defined(BOOST_NO_CXX11_REF_QUALIFIERS) || defined(BOOST_NO_CXX11_NOEXCEPT) || defined(BOOST_NO_CXX11_DEFAULTED_MOVES)
 
-BOOST_PRAGMA_MESSAGE("C++03 support is deprecated in Boost.Optional 1.83 and will be removed in Boost.Optional 1.88.")
+BOOST_PRAGMA_MESSAGE("C++03 support is deprecated in Boost.Optional 1.83 and will be removed in Boost.Optional 1.92.")
 
 #endif
 
@@ -46,6 +46,11 @@ struct none_t
 {
   struct init_tag{};
   explicit BOOST_CONSTEXPR none_t(init_tag){} // to disable default constructor
+
+#ifndef BOOST_OPTIONAL_DISABLE_EQUALITY_FOR_NONE
+  friend BOOST_CONSTEXPR bool operator==(none_t, none_t) { return true; }
+  friend BOOST_CONSTEXPR bool operator!=(none_t, none_t) { return false; }
+#endif
 };
 
 #endif // old implementation workarounds

include/boost/optional/detail/optional_common_defs.hpp

@@ -22,14 +22,22 @@
 #include <boost/config.hpp>
 #include <boost/core/addressof.hpp>
 #include <type_traits>
+#include <boost/optional/detail/optional_factory_support.hpp>
 
-#ifndef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 #include <boost/type_traits/decay.hpp>
 #include <boost/type_traits/is_base_of.hpp>
 #endif
 
+// This is needed for C++11, where constexpr functions must contain a single expression.
+// We want to assert and then return.
+#if defined NDEBUG
+# define BOOST_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
+#else
+# define BOOST_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([]{BOOST_ASSERT(!(#CHECK));}(), (EXPR)))
+#endif
 
-#ifndef BOOST_NO_CXX14_CONSTEXPR
+#ifdef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 # define BOOST_OPTIONAL_DECAY(T) typename ::std::decay<T>::type
 # define BOOST_OPTIONAL_IS_TAGGED(TAG, U) ::std::is_base_of<TAG, BOOST_OPTIONAL_DECAY(U)>
 #else

include/boost/optional/detail/optional_reference_spec.hpp

@@ -13,13 +13,13 @@
 #define BOOST_OPTIONAL_DETAIL_OPTIONAL_REFERENCE_SPEC_AJK_03OCT2015_HPP
 
 #ifdef BOOST_OPTIONAL_CONFIG_NO_PROPER_ASSIGN_FROM_CONST_INT
-#ifndef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 #include <boost/type_traits/is_integral.hpp>
 #include <boost/type_traits/is_const.hpp>
 #endif
 #endif
 
-#ifdef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifdef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 # define BOOST_OPTIONAL_TT_PREFIX ::std
 #else
 # define BOOST_OPTIONAL_TT_PREFIX boost
@@ -135,27 +135,27 @@ public:
     typedef T* pointer_type;
     typedef T* pointer_const_type;
 
-    BOOST_CXX14_CONSTEXPR optional() BOOST_NOEXCEPT : ptr_() {}
-    BOOST_CXX14_CONSTEXPR optional(none_t) BOOST_NOEXCEPT : ptr_() {}
+    BOOST_CONSTEXPR optional() BOOST_NOEXCEPT : ptr_() {}
+    BOOST_CONSTEXPR optional(none_t) BOOST_NOEXCEPT : ptr_() {}
 
     template <class U>
-    BOOST_CXX14_CONSTEXPR explicit optional(const optional<U&>& rhs) BOOST_NOEXCEPT : ptr_(rhs.get_ptr()) {}
-    BOOST_CXX14_CONSTEXPR optional(const optional& rhs) BOOST_NOEXCEPT : ptr_(rhs.get_ptr()) {}
+    BOOST_CONSTEXPR explicit optional(const optional<U&>& rhs) BOOST_NOEXCEPT : ptr_(rhs.get_ptr()) {}
+    BOOST_CONSTEXPR optional(const optional& rhs) BOOST_NOEXCEPT : ptr_(rhs.get_ptr()) {}
 
     // the following two implement a 'conditionally explicit' constructor: condition is a hack for buggy compilers with screwed conversion construction from const int
     template <class U,
               BOOST_OPTIONAL_REQUIRES(detail::is_same_decayed<T, U>),
               BOOST_OPTIONAL_REQUIRES(detail::is_const_integral_bad_for_conversion<U>)>
-      BOOST_CXX14_CONSTEXPR explicit
-      optional(U& rhs) BOOST_NOEXCEPT
-      : ptr_(boost::addressof(rhs)) {}
+    BOOST_CONSTEXPR explicit
+    optional(U& rhs) BOOST_NOEXCEPT
+    : ptr_(boost::addressof(rhs)) {}
 
     template <class U,
               BOOST_OPTIONAL_REQUIRES(detail::is_same_decayed<T, U>),
               BOOST_OPTIONAL_REQUIRES(!detail::is_const_integral_bad_for_conversion<U>)>
-      BOOST_CXX14_CONSTEXPR
-      optional(U& rhs) BOOST_NOEXCEPT
-      : ptr_(boost::addressof(rhs)) {}
+    BOOST_CONSTEXPR
+    optional(U& rhs) BOOST_NOEXCEPT
+    : ptr_(boost::addressof(rhs)) {}
 
     BOOST_CXX14_CONSTEXPR optional& operator=(const optional& rhs) BOOST_NOEXCEPT { ptr_ = rhs.get_ptr(); return *this; }
     template <class U>
@@ -164,50 +164,47 @@ public:
 
 
     BOOST_CXX14_CONSTEXPR void swap(optional& rhs) BOOST_NOEXCEPT { ::std::swap(ptr_, rhs.ptr_); }
-    BOOST_CXX14_CONSTEXPR T& get() const { BOOST_ASSERT(ptr_); return   *ptr_; }
+    constexpr T& get() const { return BOOST_OPTIONAL_ASSERTED_EXPRESSION(ptr_, *ptr_); }
 
-    BOOST_CXX14_CONSTEXPR T* get_ptr() const BOOST_NOEXCEPT { return ptr_; }
-    BOOST_CXX14_CONSTEXPR T* operator->() const { BOOST_ASSERT(ptr_); return ptr_; }
-    BOOST_CXX14_CONSTEXPR T& operator*() const { BOOST_ASSERT(ptr_); return *ptr_; }
+    constexpr T* get_ptr() const BOOST_NOEXCEPT { return ptr_; }
+    constexpr T* operator->() const { return BOOST_OPTIONAL_ASSERTED_EXPRESSION(ptr_, ptr_); }
+    constexpr T& operator*() const { return BOOST_OPTIONAL_ASSERTED_EXPRESSION(ptr_, *ptr_); }
 
-    BOOST_CXX14_CONSTEXPR T& value() const
+    constexpr T& value() const
     {
-      if (this->is_initialized())
-        return this->get();
-      else
-        boost::throw_exception(boost::bad_optional_access());
+      return this->is_initialized() ?
+             this->get() :
+             (boost::throw_exception(boost::bad_optional_access()), this->get());
     }
 
-    BOOST_CXX14_CONSTEXPR explicit operator bool() const BOOST_NOEXCEPT { return ptr_ != 0; }
+    constexpr explicit operator bool() const BOOST_NOEXCEPT { return ptr_ != 0; }
 
     BOOST_CXX14_CONSTEXPR void reset() BOOST_NOEXCEPT { ptr_ = 0; }
 
-    BOOST_CXX14_CONSTEXPR bool is_initialized() const BOOST_NOEXCEPT { return ptr_ != 0; }
-    BOOST_CXX14_CONSTEXPR bool has_value() const BOOST_NOEXCEPT { return ptr_ != 0; }
+    constexpr bool is_initialized() const BOOST_NOEXCEPT { return ptr_ != 0; }
+    constexpr bool has_value() const BOOST_NOEXCEPT { return ptr_ != 0; }
 
     template <typename F>
-    BOOST_CXX14_CONSTEXPR optional<typename optional_detail::result_of<F, reference_const_type>::type>
+    constexpr optional<typename optional_detail::result_of<F, reference_const_type>::type>
     map(F f) const
     {
-      if (this->has_value())
-        return f(this->get());
-      else
-        return none;
+      return this->has_value() ?
+             f(get()) :
+             optional<typename optional_detail::result_of<F, reference_const_type>::type>();
     }
 
     template <typename F>
-    BOOST_CXX14_CONSTEXPR optional<typename optional_detail::result_value_type<F, reference_const_type>::type>
+    constexpr optional<typename optional_detail::result_value_type<F, reference_const_type>::type>
     flat_map(F f) const
       {
-        if (this->has_value())
-          return f(get());
-        else
-          return none;
+        return this->has_value() ?
+               f(get()) :
+               optional<typename optional_detail::result_value_type<F, reference_const_type>::type>();
       }
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 
-    BOOST_CXX14_CONSTEXPR optional(T&& /* rhs */) BOOST_NOEXCEPT { detail::prevent_binding_rvalue<T&&>(); }
+    optional(T&& /* rhs */) BOOST_NOEXCEPT { detail::prevent_binding_rvalue<T&&>(); }
 
     template <class R, BOOST_OPTIONAL_REQUIRES(detail::no_unboxing_cond<T, R>)>
         BOOST_CXX14_CONSTEXPR optional(R&& r) BOOST_NOEXCEPT

include/boost/optional/detail/optional_relops.hpp

@@ -25,32 +25,32 @@ namespace boost {
 //
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator == ( optional<T> const& x, optional<T> const& y )
 { return bool(x) && bool(y) ? *x == *y : bool(x) == bool(y); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator < ( optional<T> const& x, optional<T> const& y )
 { return !y ? false : (!x ? true : (*x) < (*y)); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator != ( optional<T> const& x, optional<T> const& y )
 { return !( x == y ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator > ( optional<T> const& x, optional<T> const& y )
 { return y < x ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator <= ( optional<T> const& x, optional<T> const& y )
 { return !( y < x ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator >= ( optional<T> const& x, optional<T> const& y )
 { return !( x < y ) ; }
 
@@ -59,32 +59,32 @@ bool operator >= ( optional<T> const& x, optional<T> const& y )
 // optional<T> vs T cases
 //
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator == ( optional<T> const& x, T const& y )
 { return x && (*x == y); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator < ( optional<T> const& x, T const& y )
 { return (!x) || (*x < y); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator != ( optional<T> const& x, T const& y )
 { return !( x == y ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator > ( optional<T> const& x, T const& y )
 { return y < x ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator <= ( optional<T> const& x, T const& y )
 { return !( y < x ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator >= ( optional<T> const& x, T const& y )
 { return !( x < y ) ; }
 
@@ -93,32 +93,32 @@ bool operator >= ( optional<T> const& x, T const& y )
 //
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator == ( T const& x, optional<T> const& y )
 { return y && (x == *y); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator < ( T const& x, optional<T> const& y )
 { return y && (x < *y); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator != ( T const& x, optional<T> const& y )
 { return !( x == y ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator > ( T const& x, optional<T> const& y )
 { return y < x ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator <= ( T const& x, optional<T> const& y )
 { return !( y < x ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator >= ( T const& x, optional<T> const& y )
 { return !( x < y ) ; }
 
@@ -128,32 +128,32 @@ bool operator >= ( T const& x, optional<T> const& y )
 //
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator == ( optional<T> const& x, none_t ) BOOST_NOEXCEPT
 { return !x; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator < ( optional<T> const&, none_t ) BOOST_NOEXCEPT
 { return false; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator != ( optional<T> const& x, none_t ) BOOST_NOEXCEPT
 { return bool(x); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator > ( optional<T> const& x, none_t y ) BOOST_NOEXCEPT
 { return y < x ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator <= ( optional<T> const& x, none_t y ) BOOST_NOEXCEPT
 { return !( y < x ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator >= ( optional<T> const& x, none_t y ) BOOST_NOEXCEPT
 { return !( x < y ) ; }
 
@@ -162,32 +162,32 @@ bool operator >= ( optional<T> const& x, none_t y ) BOOST_NOEXCEPT
 //
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator == ( none_t , optional<T> const& y ) BOOST_NOEXCEPT
 { return !y; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator < ( none_t , optional<T> const& y ) BOOST_NOEXCEPT
 { return bool(y); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator != ( none_t, optional<T> const& y ) BOOST_NOEXCEPT
 { return bool(y); }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator > ( none_t x, optional<T> const& y ) BOOST_NOEXCEPT
 { return y < x ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator <= ( none_t x, optional<T> const& y ) BOOST_NOEXCEPT
 { return !( y < x ) ; }
 
 template<class T>
-inline BOOST_CXX14_CONSTEXPR
+inline BOOST_CONSTEXPR
 bool operator >= ( none_t x, optional<T> const& y ) BOOST_NOEXCEPT
 { return !( x < y ) ; }
 

include/boost/optional/detail/optional_select_implementation.hpp

@@ -0,0 +1,36 @@
+// Copyright (C) 2026 Andrzej Krzemieński.
+//
+// Use, modification, and distribution is subject to the Boost Software
+// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/libs/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  akrzemi1@gmail.com
+//
+//
+// This header provides definitions required by any specialization of
+// optional<>.
+
+#ifndef BOOST_OPTIONAL_DETAIL_OPTIONAL_SELECT_IMPLEMENTATION_01FEB2026_HPP
+#define BOOST_OPTIONAL_DETAIL_OPTIONAL_SELECT_IMPLEMENTATION_01FEB2026_HPP
+
+#include <boost/config.hpp>
+
+#if !defined(BOOST_NO_CXX11_CONSTEXPR) &&             \
+    !defined(BOOST_NO_CXX11_REF_QUALIFIERS) &&        \
+    !defined(BOOST_NO_CXX11_TRAILING_RESULT_TYPES) && \
+    !defined(BOOST_NO_CXX11_UNRESTRICTED_UNION) &&    \
+    !defined(BOOST_NO_CXX11_NOEXCEPT) &&              \
+    !defined(BOOST_NO_CXX11_DEFAULTED_MOVES)
+# define BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
+#endif
+
+
+// In C++20 we have `std::construct_at()` which is a constexpr equivalent of
+// placement-new. We can then make more functions constexpr.
+// TBD: This additional constexpr-ication is left for the future.
+# define BOOST_OPTIONAL_CXX20_CONSTEXPR
+
+#endif //BOOST_OPTIONAL_DETAIL_OPTIONAL_SELECT_IMPLEMENTATION_01FEB2026_HPP

include/boost/optional/detail/union_optional.hpp

@@ -13,13 +13,12 @@
 // This header provides definitions required by any specialization of
 // optional<>.
 
-#ifndef BOOST_OPTIONAL_DETAIL_CONSTEXPR_OPTIONAL_01FEB2026_HPP
-#define BOOST_OPTIONAL_DETAIL_CONSTEXPR_OPTIONAL_01FEB2026_HPP
+#ifndef BOOST_OPTIONAL_DETAIL_UNION_OPTIONAL_01FEB2026_HPP
+#define BOOST_OPTIONAL_DETAIL_UNION_OPTIONAL_01FEB2026_HPP
 
-#include <initializer_list>
+//#include <initializer_list>
 #include <boost/assert.hpp>
 #include <boost/core/invoke_swap.hpp>
-#include <boost/core/invoke_swap.hpp>
 #include <boost/throw_exception.hpp>
 #include <boost/optional/bad_optional_access.hpp>
 
@@ -30,12 +29,20 @@
 # define BOOST_OPTIONAL_REQUIRES(...) typename ::std::enable_if<__VA_ARGS__::value, bool>::type = false
 
 
-// In C++20 we have `std::construct_at()` which is a constexpr equivalent of
-// placement-new. We can then make more functions constexpr.
-// TBD: This additional constexpr-ication is left for the future.
-# define BOOST_OPTIONAL_CXX20_CONSTEXPR
+# ifdef __cpp_guaranteed_copy_elision
+#   if __cpp_guaranteed_copy_elision
+#     define BOOST_OPTIONAL_CONSTEXPR_COPY
+#   endif
+# endif
 
 
+template <typename T, typename U>
+struct fail_hard_on_nonconvertible
+{
+  static_assert(::std::is_convertible<U&&, T>::value, "The argument must be convertible to T");
+  using type = bool;
+};
+
 // Missing C++17 type traits
 namespace boost { namespace optional_detail {
 
@@ -47,7 +54,7 @@ struct conjunction<B1> : B1 {};
 
 template <class B1, class... Bn>
 struct conjunction<B1, Bn...>
-    : ::std::conditional_t<bool(B1::value), conjunction<Bn...>, B1> {};
+    : ::std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
 
 }}
 
@@ -108,13 +115,13 @@ struct constexpr_guarded_storage
     template <class... Args> explicit constexpr constexpr_guarded_storage(optional_ns::in_place_init_t, Args&&... args)
       : init_(true), storage_(forward_<Args>(args)...) {}
 
-    template <class U, class... Args, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, ::std::initializer_list<U>>)>
-    constexpr explicit constexpr_guarded_storage(optional_ns::in_place_init_t, ::std::initializer_list<U> il, Args&&... args)
-      : init_(true), storage_(il, forward_<Args>(args)...) {}
+    // template <class U, class... Args, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, ::std::initializer_list<U>>)>
+    // constexpr explicit constexpr_guarded_storage(optional_ns::in_place_init_t, ::std::initializer_list<U> il, Args&&... args)
+    //   : init_(true), storage_(il, forward_<Args>(args)...) {}
 
-    constexpr void reset () noexcept { init_ = false; }
+    BOOST_CXX14_CONSTEXPR void reset () noexcept { init_ = false; }
 
-    ~constexpr_guarded_storage() = default;
+    //~constexpr_guarded_storage() = default;
 };
 
 
@@ -130,12 +137,12 @@ struct fallback_guarded_storage
 
     explicit constexpr fallback_guarded_storage(T&& v) : init_(true), storage_(move_(v)) {}
 
-    template <class... Args> explicit fallback_guarded_storage(optional_ns::in_place_init_t, Args&&... args)
+    template <class... Args> explicit constexpr fallback_guarded_storage(optional_ns::in_place_init_t, Args&&... args)
         : init_(true), storage_(forward_<Args>(args)...) {}
 
-    template <class U, class... Args, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, ::std::initializer_list<U>>)>
-    explicit fallback_guarded_storage(optional_ns::in_place_init_t, ::std::initializer_list<U> il, Args&&... args)
-        : init_(true), storage_(il, forward_<Args>(args)...) {}
+    // template <class U, class... Args, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, ::std::initializer_list<U>>)>
+    // explicit fallback_guarded_storage(optional_ns::in_place_init_t, ::std::initializer_list<U> il, Args&&... args)
+    //     : init_(true), storage_(il, forward_<Args>(args)...) {}
 
     void reset() noexcept
     {
@@ -169,16 +176,16 @@ namespace boost {
   {
     using storage_t = optional_detail::guarded_storage<T>;
     storage_t storage;
-    static_assert( !::std::is_same<typename std::decay<T>::type, none_t>::value, "bad T" );
-    static_assert( !::std::is_same<typename std::decay<T>::type, in_place_init_t>::value, "bad T" );
+    static_assert( !::std::is_same<typename std::decay<T>::type, none_t>::value, "optional<none_t> is illegal" );
+    static_assert( !::std::is_same<typename std::decay<T>::type, in_place_init_t>::value, "optional<in_place_init_t> is illegal" );
+    static_assert( !::std::is_same<typename std::decay<T>::type, in_place_init_if_t>::value, "optional<in_place_init_if_t> is illegal" );
 
-
-    constexpr typename ::std::remove_const<T>::type* dataptr() { return ::boost::addressof(storage.storage_.value_); }
+    BOOST_CXX14_CONSTEXPR typename ::std::remove_const<T>::type* dataptr() { return ::boost::addressof(storage.storage_.value_); }
     constexpr const T* dataptr() const { return ::boost::addressof(storage.storage_.value_); }
 
     constexpr const T& contained_val() const& { return storage.storage_.value_; }
-    constexpr T&& contained_val() && { return optional_detail::move_(storage.storage_.value_); }
-    constexpr T& contained_val() & { return storage.storage_.value_; }
+    BOOST_CXX14_CONSTEXPR T&& contained_val() && { return optional_detail::move_(storage.storage_.value_); }
+    BOOST_CXX14_CONSTEXPR T& contained_val() & { return storage.storage_.value_; }
 
     template <typename... Args>
     BOOST_OPTIONAL_CXX20_CONSTEXPR void initialize(Args&&... args)
@@ -188,6 +195,31 @@ namespace boost {
       storage.init_ = true;
     }
 
+  #ifdef BOOST_OPTIONAL_CONSTEXPR_COPY
+    // The conditional initialization of storage needs to employ a factory
+    // function, so that we can utilize the guaranteed copy elision on the
+    // return type.
+    // an alternative -- using a conditional operator in the initializer --
+    // does not work in MSVC 14.2 and 14.3.
+    template <typename... U>
+    static constexpr storage_t conditional_storage_from_values(bool cond, U&&... v)
+    {
+      if (cond)
+        return storage_t(in_place_init, optional_detail::forward_<U>(v)...);
+      else
+        return storage_t();
+    }
+
+    template <typename OU>
+    static constexpr storage_t conditional_storage_from_optional(OU&& ou)
+    {
+      if (ou.has_value())
+        return storage_t(in_place_init, *optional_detail::forward_<OU>(ou));
+      else
+        return storage_t();
+    }
+  #endif
+
   public:
     using value_type = T;
     using unqualified_value_type = typename ::std::remove_const<T>::type;
@@ -209,36 +241,97 @@ namespace boost {
     constexpr optional(const T& v) : storage(v) {}
     constexpr optional(T&& v) : storage(optional_detail::move_(v)) {}
 
+#ifdef BOOST_OPTIONAL_CONSTEXPR_COPY
     constexpr optional(bool cond, const T& v)
-    : storage(cond ? storage_t(v) : storage_t())
+    : storage(conditional_storage_from_values(cond, v))
     {}
 
     constexpr optional(bool cond, T&& v)
-    : storage(cond ? storage_t(optional_detail::move_(v)) : storage_t())
+    : storage(conditional_storage_from_values(cond, optional_detail::move_(v)))
     {}
 
     constexpr optional(const optional& rhs)
-    : storage(rhs.is_initialized() ? storage_t(*rhs) : storage_t())
+    : storage(conditional_storage_from_optional(rhs))
     {}
 
     constexpr optional(optional&& rhs)
       noexcept(::std::is_nothrow_move_constructible<T>::value)
-    : storage(rhs.is_initialized() ? storage_t(*optional_detail::move_(rhs)) : storage_t())
+    : storage(conditional_storage_from_optional(optional_detail::move_(rhs)))
     {}
 
     template <typename U, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U const&>)>
     constexpr explicit optional(optional<U> const& rhs)
-    : storage(rhs.is_initialized() ? storage_t(*rhs) : storage_t())
+    : storage(conditional_storage_from_optional(rhs))
     {}
 
     template <typename U, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U&&>)>
     constexpr explicit optional(optional<U> && rhs)
-    : storage(rhs.is_initialized() ? storage_t(*optional_detail::move_(rhs)) : storage_t())
+    : storage(conditional_storage_from_optional(optional_detail::move_(rhs)))
     {}
 
+    template <typename... Args>
+    constexpr explicit optional( in_place_init_if_t, bool cond, Args&&... args )
+    : storage(conditional_storage_from_values(cond, optional_detail::forward_<Args>(args)...))
+    {}
+#else
+    optional(bool cond, const T& v)
+    : storage()
+    {
+      if (cond)
+        initialize(v);
+    }
+
+    optional(bool cond, T&& v)
+    : storage()
+    {
+      if (cond)
+        initialize(optional_detail::move_(v));
+    }
+
+    optional(const optional& rhs)
+    : storage()
+    {
+      if (rhs.is_initialized())
+        initialize(*rhs);
+    }
+
+    optional(optional&& rhs)
+      noexcept(::std::is_nothrow_move_constructible<T>::value)
+    : storage()
+    {
+      if (rhs.is_initialized())
+        initialize(*optional_detail::move_(rhs));
+    }
+
+    template <typename U, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U const&>)>
+    explicit optional(optional<U> const& rhs)
+    : storage()
+    {
+      if (rhs.is_initialized())
+        initialize(*rhs);
+    }
+
+    template <typename U, BOOST_OPTIONAL_REQUIRES(::std::is_constructible<T, U&&>)>
+    explicit optional(optional<U> && rhs)
+    : storage()
+    {
+      if (rhs.is_initialized())
+        initialize(*optional_detail::move_(rhs));
+    }
+
+    template <typename... Args>
+    explicit optional( in_place_init_if_t, bool cond, Args&&... args )
+    : storage()
+    {
+      if (cond)
+        initialize(optional_detail::forward_<Args>(args)...);
+    }
+#endif // BOOST_OPTIONAL_CONSTEXPR_COPY
+
     template <typename FT,
               BOOST_OPTIONAL_REQUIRES(optional_detail::is_typed_in_place_factory<FT>)>
-    constexpr explicit optional (FT&& factory)
+    /*non-constexpr (deprecated)*/
+    explicit optional (FT&& factory)
     : storage()
     {
       factory.apply(this->dataptr());
@@ -247,10 +340,11 @@ namespace boost {
 
     template <typename FT,
               BOOST_OPTIONAL_REQUIRES(optional_detail::is_in_place_factory<FT>)>
-    constexpr explicit optional (FT&& factory)
+    /*non-constexpr (deprecated)*/
+    explicit optional (FT&& factory)
     : storage()
     {
-      factory.template apply<T>(this->dataptr());
+      boost_optional_detail::construct<value_type>(factory, this->dataptr());
       storage.init_ = true;
     }
 
@@ -265,12 +359,7 @@ namespace boost {
     : storage(in_place_init, optional_detail::forward_<Args>(args)...)
     {}
 
-    template <typename... Args>
-    constexpr explicit optional( in_place_init_if_t, bool cond, Args&&... args )
-    : storage( cond ? storage_t(in_place_init, optional_detail::forward_<Args>(args)...) : storage_t() )
-    {}
-
-    constexpr void reset() noexcept
+    BOOST_CXX14_CONSTEXPR void reset() noexcept
     {
       storage.reset();
     }
@@ -283,13 +372,13 @@ namespace boost {
     template <typename... Args>
     BOOST_OPTIONAL_CXX20_CONSTEXPR void emplace(Args&&... args)
     {
-      reset(); static_assert(noexcept(reset()));
+      reset();
       // <-- now we are not containing a value
       initialize(optional_detail::forward_<Args>(args)...);
     }
 
 
-    constexpr optional& operator=(none_t) noexcept
+    BOOST_CXX14_CONSTEXPR optional& operator=(none_t) noexcept
     {
       reset();
       return *this;
@@ -386,15 +475,17 @@ namespace boost {
     }
 
     template <class F, BOOST_OPTIONAL_REQUIRES(optional_detail::is_in_place_factory<F>)>
+    /*non-constexpr (deprecated)*/
     optional& operator=(F&& factory)
     {
       reset();
-      factory.template apply<T>(this->dataptr());
+      boost_optional_detail::construct<value_type>(factory, this->dataptr());
       storage.init_ = true;
       return *this;
     }
 
     template <class F, BOOST_OPTIONAL_REQUIRES(optional_detail::is_typed_in_place_factory<F>)>
+    /*non-constexpr (deprecated)*/
     optional& operator=(F&& factory)
     {
       reset();
@@ -406,7 +497,6 @@ namespace boost {
     BOOST_OPTIONAL_CXX20_CONSTEXPR
     void swap(optional& rhs)
       noexcept(::std::is_nothrow_move_constructible<T>::value && noexcept(boost::core::invoke_swap(*rhs, *rhs)))
-      // I am cheating here. I need "swapabe" not "assignnable" trait. But this is best I can do in C++14
     {
       if (is_initialized())
       {
@@ -428,8 +518,8 @@ namespace boost {
     constexpr bool has_value() const noexcept { return this->is_initialized(); }
     constexpr explicit operator bool() const noexcept { return this->is_initialized(); }
 
-    constexpr reference_const_type get() const { BOOST_ASSERT(this->is_initialized()) ; return this->contained_val(); }
-    constexpr reference_type       get()       { BOOST_ASSERT(this->is_initialized()) ; return this->contained_val(); }
+    constexpr             reference_const_type get() const { return BOOST_OPTIONAL_ASSERTED_EXPRESSION(this->is_initialized(), this->contained_val()); }
+    BOOST_CXX14_CONSTEXPR reference_type       get()       { BOOST_ASSERT(this->is_initialized()) ; return this->contained_val(); }
 
     //BOOST_DEPRECATED("use `value_or(v)` instead")
     reference_const_type get_value_or (reference_const_type v) const { return this->is_initialized() ? this->contained_val() : v; }
@@ -440,22 +530,19 @@ namespace boost {
     pointer_const_type get_ptr() const { return is_initialized() ? dataptr() : nullptr; }
     pointer_type       get_ptr()       { return is_initialized() ? dataptr() : nullptr; }
 
-    constexpr reference_const_type operator*() const& { return this->get(); }
-    constexpr reference_type       operator*() &      { return this->get(); }
-    constexpr reference_type_of_temporary_wrapper operator*() && { return optional_detail::move_(this->get()); }
+    constexpr             reference_const_type operator*() const& { return this->get(); }
+    BOOST_CXX14_CONSTEXPR reference_type       operator*() &      { return this->get(); }
+    BOOST_CXX14_CONSTEXPR reference_type_of_temporary_wrapper operator*() && { return optional_detail::move_(this->get()); }
 
-    constexpr pointer_const_type operator->() const { BOOST_ASSERT(this->is_initialized()) ; return this->dataptr(); }
-    constexpr pointer_type       operator->()       { BOOST_ASSERT(this->is_initialized()) ; return this->dataptr(); }
+    constexpr             pointer_const_type operator->() const { return BOOST_OPTIONAL_ASSERTED_EXPRESSION(this->is_initialized(), this->dataptr()); }
+    BOOST_CXX14_CONSTEXPR pointer_type       operator->()       { BOOST_ASSERT(this->is_initialized()) ; return this->dataptr(); }
 
     constexpr reference_const_type value() const&
     {
-      if (this->is_initialized())
-        return this->get();
-      else
-        boost::throw_exception(boost::bad_optional_access());
+      return this->is_initialized() ? this->get() : (boost::throw_exception(boost::bad_optional_access()), this->get());
     }
 
-    constexpr reference_type value() &
+    BOOST_CXX14_CONSTEXPR reference_type value() &
     {
       if (this->is_initialized())
         return this->get();
@@ -463,7 +550,7 @@ namespace boost {
         boost::throw_exception(boost::bad_optional_access());
     }
 
-    constexpr reference_type_of_temporary_wrapper value() &&
+    BOOST_CXX14_CONSTEXPR reference_type_of_temporary_wrapper value() &&
     {
       if (this->is_initialized())
         return optional_detail::move_(this->get());
@@ -471,17 +558,15 @@ namespace boost {
         boost::throw_exception(boost::bad_optional_access());
     }
 
-    template <class U = T>
+    template <class U = typename ::std::remove_cv<T>::type,
+              typename fail_hard_on_nonconvertible<T, U>::type = true>
     constexpr value_type value_or(U&& v) const&
     {
-      if (this->is_initialized())
-        return get();
-      else
-        return optional_detail::forward_<U>(v);
+      return this->is_initialized() ? get() : T(optional_detail::forward_<U>(v));
     }
 
-    template <class U>
-    constexpr value_type value_or(U&& v) &&
+    template <class U = typename ::std::remove_cv<T>::type>
+    BOOST_CXX14_CONSTEXPR value_type value_or(U&& v) &&
     {
       if (this->is_initialized())
         return optional_detail::move_(get());
@@ -489,17 +574,15 @@ namespace boost {
         return optional_detail::forward_<U>(v);
     }
 
-    template <typename F>
+    template <typename F,
+              typename fail_hard_on_nonconvertible<T, decltype(optional_detail::declval_<F>()())>::type = true>
     constexpr value_type value_or_eval(F f) const&
     {
-      if (this->is_initialized())
-        return get();
-      else
-        return f();
+      return this->is_initialized() ? get() : value_type(f());
     }
 
     template <typename F>
-    constexpr value_type value_or_eval ( F f ) &&
+    BOOST_CXX14_CONSTEXPR value_type value_or_eval ( F f ) &&
     {
       if (this->is_initialized())
         return optional_detail::move_(get());
@@ -508,7 +591,8 @@ namespace boost {
     }
 
     template <typename F>
-    constexpr optional<typename optional_detail::result_of<F, reference_type>::type> map(F f) &
+    BOOST_CXX14_CONSTEXPR optional<typename optional_detail::result_of<F, reference_type>::type>
+    map(F f) &
     {
       if (this->has_value())
         return f(get());
@@ -517,16 +601,17 @@ namespace boost {
     }
 
     template <typename F>
-    constexpr optional<typename optional_detail::result_of<F, reference_const_type>::type> map(F f) const&
+    constexpr optional<typename optional_detail::result_of<F, reference_const_type>::type>
+    map(F f) const&
     {
-      if (this->has_value())
-        return f(get());
-      else
-        return none;
+      return this->has_value() ?
+             f(get()) :
+             optional<typename optional_detail::result_of<F, reference_const_type>::type>();
     }
 
     template <typename F>
-    constexpr optional<typename optional_detail::result_of<F, reference_type_of_temporary_wrapper>::type> map(F f) &&
+    BOOST_CXX14_CONSTEXPR optional<typename optional_detail::result_of<F, reference_type_of_temporary_wrapper>::type>
+    map(F f) &&
     {
       if (this->has_value())
         return f(optional_detail::move_(this->get()));
@@ -535,7 +620,7 @@ namespace boost {
     }
 
     template <typename F>
-    constexpr optional<typename optional_detail::result_value_type<F, reference_type>::type>
+    BOOST_CXX14_CONSTEXPR optional<typename optional_detail::result_value_type<F, reference_type>::type>
     flat_map(F f) &
     {
       if (this->has_value())
@@ -548,14 +633,13 @@ namespace boost {
     constexpr optional<typename optional_detail::result_value_type<F, reference_const_type>::type>
     flat_map(F f) const&
     {
-      if (this->has_value())
-        return f(get());
-      else
-        return none;
+      return this->has_value() ?
+             f(get()) :
+             optional<typename optional_detail::result_value_type<F, reference_const_type>::type>();
     }
 
     template <typename F>
-    constexpr optional<typename optional_detail::result_value_type<F, reference_type_of_temporary_wrapper>::type>
+    BOOST_CXX14_CONSTEXPR optional<typename optional_detail::result_value_type<F, reference_type_of_temporary_wrapper>::type>
     flat_map(F f) &&
     {
       if (this->has_value())
@@ -577,6 +661,4 @@ namespace boost {
 }
 
 
-// https://godbolt.org/z/ov1ahMsv6
-
-#endif // BOOST_OPTIONAL_DETAIL_CONSTEXPR_OPTIONAL_01FEB2026_HPP
+#endif // BOOST_OPTIONAL_DETAIL_UNION_OPTIONAL_01FEB2026_HPP

include/boost/optional/optional.hpp

@@ -23,15 +23,7 @@
 #define BOOST_OPTIONAL_OPTIONAL_FLC_19NOV2002_HPP
 
 
-#if !defined(BOOST_NO_CXX14_CONSTEXPR) &&             \
-    !defined(BOOST_NO_CXX11_REF_QUALIFIERS) &&        \
-    !defined(BOOST_NO_CXX11_TRAILING_RESULT_TYPES) && \
-    !defined(BOOST_NO_CXX11_UNRESTRICTED_UNION)
-# ifdef BOOST_OPTIONAL_CONFIG_ENABLE_CONSTEXPR_IMPLEMENTATION // for now, only per manual request
-#  define BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
-# endif
-#endif
-
+#include <boost/optional/detail/optional_select_implementation.hpp>
 #include <boost/optional/detail/optional_common_defs.hpp>
 
 
@@ -56,8 +48,8 @@ operator<<(std::basic_ostream<CharType, CharTrait>& os, optional_detail::optiona
 #include <boost/none.hpp>
 
 
-#if defined BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
-#include <boost/optional/detail/constexpr_optional.hpp>
+#if defined BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
+#include <boost/optional/detail/union_optional.hpp>
 #else
 
 
@@ -92,10 +84,8 @@ operator<<(std::basic_ostream<CharType, CharTrait>& os, optional_detail::optiona
 #include <boost/type_traits/is_volatile.hpp>
 #include <boost/type_traits/is_scalar.hpp>
 
-
 #include <boost/optional/optional_fwd.hpp>
 #include <boost/optional/detail/optional_config.hpp>
-#include <boost/optional/detail/optional_factory_support.hpp>
 #include <boost/optional/detail/optional_aligned_storage.hpp>
 
 

include/boost/optional/optional_fwd.hpp

@@ -17,25 +17,32 @@
 #define BOOST_OPTIONAL_OPTIONAL_FWD_FLC_19NOV2002_HPP
 
 #include <boost/config.hpp>
+#include <boost/core/invoke_swap.hpp>
+#include <boost/optional/detail/optional_select_implementation.hpp>
+#include <type_traits>
+
 
 namespace boost {
 
 template<class T> class optional ;
 
 // This forward is needed to refer to namespace scope swap from the member swap
-#ifdef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
-template<class T> BOOST_OPTIONAL_CXX20_CONSTEXPR void swap ( optional<T>& , optional<T>& ) ;
+#ifdef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
+  template<class T> BOOST_OPTIONAL_CXX20_CONSTEXPR void swap ( optional<T>& lhs, optional<T>& rhs )
+    noexcept(::std::is_nothrow_move_constructible<T>::value && noexcept(boost::core::invoke_swap(*lhs, *rhs)));
 #else
-template<class T> void swap ( optional<T>& , optional<T>& ) ;
-#endif // BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+  template<class T> void swap ( optional<T>& , optional<T>& ) ;
+#endif // BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
+
 
 template<class T> struct optional_swap_should_use_default_constructor ;
 
+
 #ifndef BOOST_OPTIONAL_CONFIG_DONT_SPECIALIZE_OPTIONAL_REFS
 
-template<class T> class optional<T&> ;
+  template<class T> class optional<T&> ;
 
-template<class T> BOOST_CXX14_CONSTEXPR void swap ( optional<T&>& , optional<T&>& ) BOOST_NOEXCEPT;
+  template<class T> BOOST_CXX14_CONSTEXPR void swap ( optional<T&>& , optional<T&>& ) BOOST_NOEXCEPT;
 
 #endif
 

test/Jamfile.v2

@@ -39,6 +39,7 @@ run optional_test_assign.cpp ;
 run optional_test_swap.cpp ;
 compile optional_test_constexpr.cpp ;
 compile optional_test_wuninitialized.cpp ;
+compile optional_test_fwd_header.cpp ;
 run optional_test_conversions_from_U.cpp ;
 run optional_test_convert_from_T.cpp ;
 run optional_test_convert_assign.cpp ;
@@ -48,6 +49,7 @@ run optional_test_flat_map.cpp ;
 run optional_test_hash.cpp ;
 run optional_test_map.cpp ;
 run optional_test_tie.cpp : : : <library>/boost/tuple//boost_tuple ;
+run optional_test_ranges_find.cpp ;
 run optional_test_ref_assign_portable_minimum.cpp ;
 run optional_test_ref_assign_mutable_int.cpp ;
 run optional_test_ref_assign_const_int.cpp ;

test/optional_test_adl_for_get.cpp

@@ -0,0 +1,32 @@
+// Copyright (C) 2021 Andrzej Krzemienski.
+//
+// Use, modification, and distribution is subject to the Boost Software
+// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/lib/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  akrzemi1@gmail.com
+
+template <typename U>
+struct wrapper {};
+
+template <typename /*Tag*/, typename U>
+int get(wrapper<U> const&) { return 0; }
+
+#include "boost/optional/optional.hpp"
+
+namespace boost
+{
+  struct any_type_in_boost_namespace {};
+}
+
+class tag; // user-defined tag
+
+int main()
+{
+  // the following tests if boost::get for optional<> does
+  // not interfere with the global get
+  return get<tag>(wrapper<boost::any_type_in_boost_namespace>());
+}

test/optional_test_constexpr.cpp

@@ -10,8 +10,9 @@
 //  akrzemi1@gmail.com
 
 #include "boost/optional.hpp"
+#include <string>
 
-#ifdef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifdef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 
 struct Record
 {
@@ -20,43 +21,55 @@ struct Record
   constexpr int operator()() const { return i; }
 };
 
+struct Guard
+{
+  int i;
+  constexpr explicit Guard(int i) : i(i) {}
+  Guard(Guard&&) = delete;
+};
+
+static_assert(boost::none == boost::none, "");
+static_assert(!(boost::none != boost::none), "");
+
 namespace test_int
 {
   constexpr boost::optional<int> oN;
-  static_assert(!oN);
-  static_assert(oN == boost::none);
-  static_assert(oN <= boost::none);
-  static_assert(!(oN != boost::none));
-  static_assert(oN == oN);
-  static_assert(!oN.has_value());
-  static_assert(oN.value_or({}) == 0);
-  static_assert(oN.value_or(0) == 0);
-  static_assert(oN.value_or_eval(Record(9)) == 9);
+  static_assert(!oN, "");
+  static_assert(oN == boost::none, "");
+  static_assert(oN <= boost::none, "");
+  static_assert(!(oN != boost::none), "");
+  static_assert(oN == oN, "");
+  static_assert(!oN.has_value(), "");
+  static_assert(oN.value_or({}) == 0, "");
+  static_assert(oN.value_or(0) == 0, "");
+  static_assert(oN.value_or_eval(Record(9)) == 9, "");
+
+  constexpr boost::optional<int> o1 (1);
+  constexpr boost::optional<int> o2 {2};
+
+  static_assert(o1, "");
+  static_assert(o1.has_value(), "");
+  static_assert(o1 != boost::none, "");
+  static_assert(o1 != oN, "");
+  static_assert(o1 > oN, "");
+  static_assert(o1 >= oN, "");
+  static_assert(*o1 == 1, "");
+  static_assert(o1.value() == 1, "");
+  static_assert(o1.value_or(0) == 1, "");
+  static_assert(o1.value_or({}) == 1, "");
+  static_assert(o1.value_or_eval(Record(9)) == 1, "");
+  static_assert(o1 == 1, "");
+  static_assert(o2, "");
+  static_assert(o2 != o1, "");
+  static_assert(o2 > o1, "");
+
+#ifdef BOOST_OPTIONAL_CONSTEXPR_COPY
   constexpr boost::optional<int> oNc = oN;
   constexpr boost::optional<int> oNd = boost::none;
   constexpr boost::optional<int> oNe = {};
-  static_assert(oNc == oN);
-  static_assert(oNd == oN);
-  static_assert(oNe == oN);
-
-  constexpr boost::optional<int> o1 = 1;
-  constexpr boost::optional<int> o2 {2};
-
-  static_assert(o1);
-  static_assert(o1.has_value());
-  static_assert(o1 != boost::none);
-  static_assert(o1 != oN);
-  static_assert(o1 > oN);
-  static_assert(o1 >= oN);
-  static_assert(*o1 == 1);
-  static_assert(o1.value() == 1);
-  static_assert(o1.value_or(0) == 1);
-  static_assert(o1.value_or({}) == 1);
-  static_assert(o1.value_or_eval(Record(9)) == 1);
-  static_assert(o1 == 1);
-  static_assert(o2);
-  static_assert(o2 != o1);
-  static_assert(o2 > o1);
+  static_assert(oNc == oN, "");
+  static_assert(oNd == oN, "");
+  static_assert(oNe == oN, "");
 
   constexpr bool test_reset() {
     boost::optional<int> o = 1;
@@ -66,31 +79,75 @@ namespace test_int
     return o == boost::none;
   }
 
-  static_assert(test_reset());
+  static_assert(test_reset(), "");
+#endif
 }
 
 namespace test_record
 {
-  constexpr boost::optional<Record> rN = boost::none;
-  constexpr boost::optional<Record> r1 = Record(1);
+  constexpr boost::optional<Record> rN (boost::none);
+  constexpr boost::optional<Record> r1 (Record(1));
   constexpr boost::optional<Record> r2 (boost::in_place_init, 2);
 
-  static_assert(!rN);
-  static_assert(rN == boost::none);
-  static_assert(r1);
-  static_assert(r1 != boost::none);
-  static_assert(r2);
-  static_assert(rN.value_or(Record(9)).i == 9);
-  static_assert(r1->i == 1);
-  static_assert(r2.value().i == 2);
+  static_assert(!rN, "");
+  static_assert(rN == boost::none, "");
+  static_assert(r1, "");
+  static_assert(r1 != boost::none, "");
+  static_assert(r2, "");
+  static_assert(rN.value_or(Record(9)).i == 9, "");
+  static_assert(r1->i == 1, "");
+  static_assert(r2.value().i == 2, "");
+}
+
+namespace test_guard
+{
+  constexpr boost::optional<Guard> g1 {boost::in_place_init, 1};
+  constexpr boost::optional<Guard> gNa {boost::none};
+  constexpr boost::optional<Guard> gNb;
+
+  static_assert(g1, "");
+  static_assert(!!g1, "");
+  static_assert(g1 != boost::none, "");
+  static_assert(!(g1 == boost::none), "");
+  static_assert(g1.has_value(), "");
+
+  static_assert(!gNa, "");
+  static_assert(!gNa.has_value(), "");
+  static_assert(gNa == boost::none, "");
+  static_assert(!(gNa != boost::none), "");
+
+  static_assert(!gNb, "");
+  static_assert(!gNb.has_value(), "");
+  static_assert(gNb == boost::none, "");
+  static_assert(!(gNb != boost::none), "");
 }
 
 namespace test_optional_ref
 {
   constexpr int gi = 9;
   constexpr boost::optional<const int&> iref = gi;
-  static_assert(iref);
-  static_assert(*iref == 9);
+  static_assert(iref, "");
+  static_assert(*iref == 9, "");
 }
 
-#endif // BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+namespace test_msvc_14_1
+{
+  struct Aggr
+  {
+    boost::optional<std::string> os;
+  };
+
+  void test()
+  {
+    Aggr a;
+    Aggr b = a;
+    b = a;
+    Aggr c = Aggr();
+    b = Aggr();
+    (void)a;
+    (void)b;
+    (void)c;
+  }
+}
+
+#endif // BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION

test/optional_test_constructible_from_other.cpp

@@ -55,5 +55,6 @@ int main()
 {
     // Invokes boost::optional copy constructor. Should not invoke wrapper constructor from U.
     boost::optional< wrapper< int > > res = foo();
+    (void)res;
     return 0;
 }

test/optional_test_fail_explicit_convert_in_value_or.cpp

@@ -29,4 +29,3 @@ void test_implicit_conversion_to_bool()
   boost::optional<T> opt;
   opt.value_or(U());
 }
-

test/optional_test_fwd_header.cpp

@@ -0,0 +1,23 @@
+// Copyright (C) 2026 Andrzej Krzemienski.
+//
+// Use, modification, and distribution is subject to the Boost Software
+// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/lib/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  akrzemi1@gmail.com
+
+#include "boost/optional/optional_fwd.hpp"
+#include "boost/optional/optional.hpp"
+
+void statically_test_basic_instantiations()
+{
+  boost::optional<int> oN, o1(1);
+  swap(oN, o1);
+
+  int i = 1;
+  boost::optional<int> rN, ri(i);
+  swap(rN, ri);
+}

test/optional_test_ranges_find.cpp

@@ -0,0 +1,70 @@
+// Copyright (C) 2026 Andrzej Krzemienski.
+//
+// Use, modification, and distribution is subject to the Boost Software
+// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/lib/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  akrzemi1@gmail.com
+
+
+#include "boost/optional/optional.hpp"
+#include "boost/core/lightweight_test.hpp"
+#include <type_traits>
+
+#ifndef BOOST_NO_CXX20_HDR_RANGES
+#include <ranges>
+#include <concepts>
+#include <iterator>
+#include <array>
+#include <string>
+
+static_assert(std::equality_comparable<boost::none_t>, "boost::none shall be equality comparable");
+
+
+template <typename T>
+void test_that_you_can_find_none_in_a_range_of_optional(T x, T y)
+{
+  static_assert(std::equality_comparable_with<boost::optional<T>, boost::none_t>, "boost::none shall satisfy the concept");
+  static_assert(std::equality_comparable_with<boost::none_t, boost::optional<T> >, "boost::none shall satisfy the concept");
+  static_assert(std::indirect_binary_predicate<std::ranges::equal_to, const boost::optional<T>*, const boost::none_t*>, "boost::none shall satisfy the concept");
+
+  //                                        [0]     [1]     [2]
+  std::array<boost::optional<T>, 3> arr = {{ x, boost::none, y }};
+  auto it = std::ranges::find(arr, boost::none);
+  BOOST_TEST_EQ(std::distance(arr.begin(), it), 1);
+}
+#endif // BOOST_NO_CXX20_HDR_RANGES
+
+
+# if (defined __GNUC__) && (!defined BOOST_INTEL_CXX_VERSION) && (!defined __clang__)
+#  if (__GNUC__ <= 4)
+#   define BOOST_OPTIONAL_TEST_INSUFFICIENT_TYPE_TRAIT_SUPPORT
+#  endif
+# endif
+
+#ifndef BOOST_OPTIONAL_TEST_INSUFFICIENT_TYPE_TRAIT_SUPPORT
+static_assert(std::is_trivially_copyable<boost::none_t>::value, "boost::none shall be trivially copyable");
+#endif
+
+
+void test_that_none_is_equal_to_none()
+{
+  BOOST_TEST(boost::none == boost::none);
+  BOOST_TEST(!(boost::none != boost::none));
+  auto None = boost::none;
+  BOOST_TEST(None == boost::none);
+  BOOST_TEST(!(None != boost::none));
+}
+
+int main()
+{
+#ifndef BOOST_NO_CXX20_HDR_RANGES
+  test_that_you_can_find_none_in_a_range_of_optional(1, 2);
+  test_that_you_can_find_none_in_a_range_of_optional<std::string>("one", "two");
+#endif
+  test_that_none_is_equal_to_none();
+  return boost::report_errors();
+}

test/optional_test_static_properties.cpp

@@ -20,7 +20,7 @@
 #include "boost/type_traits/is_base_of.hpp"
 #include "boost/optional/detail/experimental_traits.hpp"
 
-#ifndef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 #ifndef BOOST_OPTIONAL_DETAIL_NO_DEFAULTED_MOVE_FUNCTIONS
 
 struct PrivDefault
@@ -136,11 +136,11 @@ void test_trivial_copyability()
 }
 
 #endif
-#endif // BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#endif // BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 
 int main()
 {
-#ifndef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 #ifndef BOOST_OPTIONAL_DETAIL_NO_DEFAULTED_MOVE_FUNCTIONS
   test_type_traits();
   test_trivial_copyability();

test/optional_test_swap.cpp

@@ -210,7 +210,7 @@ namespace boost {
 // Compile time tweaking on whether or not swap should use the default constructor:
 //
 
-#ifndef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 template <> struct optional_swap_should_use_default_constructor<
   optional_swap_test::class_whose_default_ctor_should_be_used> : true_type {} ;
 
@@ -219,7 +219,7 @@ template <> struct optional_swap_should_use_default_constructor<
 
 template <class T> struct optional_swap_should_use_default_constructor<
   optional_swap_test::template_whose_default_ctor_should_be_used<T> > : true_type {} ;
-#endif // BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#endif // BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
 
 //
 // Specialization of boost::swap:
@@ -353,14 +353,14 @@ void test_swap_member_function( T const* )
 void test_swap_tweaking()
 {
   ( test_swap_function( ARG(optional_swap_test::class_without_default_ctor) ) );
-#ifndef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
   ( test_swap_function( ARG(optional_swap_test::class_whose_explicit_ctor_should_be_used) ) );
   ( test_swap_function( ARG(optional_swap_test::class_whose_default_ctor_should_be_used) ) );
   ( test_swap_function( ARG(optional_swap_test::class_whose_default_ctor_should_not_be_used) ) );
   ( test_swap_function( ARG(optional_swap_test::template_whose_default_ctor_should_be_used<char>) ) );
 #endif
   ( test_swap_member_function( ARG(optional_swap_test::class_without_default_ctor) ) );
-#ifndef BOOST_OPTIONAL_USES_CONSTEXPR_IMPLEMENTATION
+#ifndef BOOST_OPTIONAL_USES_UNION_IMPLEMENTATION
   ( test_swap_member_function( ARG(optional_swap_test::class_whose_explicit_ctor_should_be_used) ) );
   ( test_swap_member_function( ARG(optional_swap_test::class_whose_default_ctor_should_be_used) ) );
   ( test_swap_member_function( ARG(optional_swap_test::class_whose_default_ctor_should_not_be_used) ) );