Allow using polymorphic_downcast and polymorphic_cast in constexpr (#30)

This is part of the effort to make the Boost libraries "modular" for build and consumption. See https://lists.boost.org/Archives/boost/2024/01/255704.php and https://github.com/grafikrobot/boost-b2-modular/blob/b2-modular/README.adoc for more information.

This PR depends on the following other PRs being merged to both develop and master branches of the respective repos:

- https://github.com/boostorg/boost/pull/854

This PR will be changed to ready for review, i.e. not draft, when the above are merged. Do not merge this one until that time.

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
           python tools/boostdep/depinst/depinst.py --include benchmark --include example --include examples --include tools --git_args "--depth 10 --jobs 3" $LIBRARY
           ./bootstrap.sh
           ./b2 -d0 headers
-          ./b2 -j4 variant=debug tools/inspect/build
+          ./b2 -j4 variant=debug tools/inspect
 
       - name: Run tests
         run: |

README.md

@@ -5,8 +5,8 @@ Boost.Conversion is one of the [Boost C++ Libraries](https://github.com/boostorg
 
 @               | Build         | Tests coverage | More info
 ----------------|-------------- | -------------- |-----------
-Develop branch: | [![CI](https://github.com/boostorg/conversion/actions/workflows/ci.yml/badge.svg?branch=develop)](https://github.com/boostorg/conversion/actions/workflows/ci.yml) [![Build status](https://ci.appveyor.com/api/projects/status/1cky1hrunfa46bdx/branch/develop?svg=true)](https://ci.appveyor.com/project/apolukhin/conversion/branch/develop) | [![Coverage Status](https://coveralls.io/repos/github/boostorg/conversion/badge.svg?branch=develop)](https://coveralls.io/github/boostorg/conversion?branch=develop) | [details...](https://www.boost.org/development/tests/develop/developer/conversion.html)
-Master branch:  | [![CI](https://github.com/boostorg/conversion/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/boostorg/conversion/actions/workflows/ci.yml) [![Build status](https://ci.appveyor.com/api/projects/status/1cky1hrunfa46bdx/branch/master?svg=true)](https://ci.appveyor.com/project/apolukhin/conversion/branch/master) | [![Coverage Status](https://coveralls.io/repos/github/boostorg/conversion/badge.svg?branch=master)](https://coveralls.io/github/boostorg/conversion?branch=master) | [details...](https://www.boost.org/development/tests/master/developer/conversion.html)
+Develop branch: | [![CI](https://github.com/boostorg/conversion/actions/workflows/ci.yml/badge.svg?branch=develop)](https://github.com/boostorg/conversion/actions/workflows/ci.yml) [![Build status](https://ci.appveyor.com/api/projects/status/1cky1hrunfa46bdx/branch/develop?svg=true)](https://ci.appveyor.com/project/apolukhin/conversion/branch/develop) | [![Coverage Status](https://coveralls.io/repos/github/boostorg/conversion/badge.svg?branch=develop)](https://coveralls.io/github/boostorg/conversion?branch=develop) | [details...](https://regression.boost.io/develop/developer/conversion.html)
+Master branch:  | [![CI](https://github.com/boostorg/conversion/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/boostorg/conversion/actions/workflows/ci.yml) [![Build status](https://ci.appveyor.com/api/projects/status/1cky1hrunfa46bdx/branch/master?svg=true)](https://ci.appveyor.com/project/apolukhin/conversion/branch/master) | [![Coverage Status](https://coveralls.io/repos/github/boostorg/conversion/badge.svg?branch=master)](https://coveralls.io/github/boostorg/conversion?branch=master) | [details...](https://regression.boost.io/master/developer/conversion.html)
 
 [Latest developer documentation](https://www.boost.org/doc/libs/develop/doc/html/conversion.html)
 

build.jam

@@ -0,0 +1,26 @@
+# Copyright René Ferdinand Rivera Morell 2023-2024
+# Distributed under 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)
+
+require-b2 5.2 ;
+
+constant boost_dependencies :
+    /boost/assert//boost_assert
+    /boost/config//boost_config
+    /boost/smart_ptr//boost_smart_ptr
+    /boost/throw_exception//boost_throw_exception ;
+
+project /boost/conversion
+    : common-requirements
+        <include>include
+    ;
+
+explicit
+    [ alias boost_conversion : : : : <library>$(boost_dependencies) ]
+    [ alias all : boost_conversion test ]
+    ;
+
+call-if : boost-library conversion
+    ;
+

doc/conversion.qbk

@@ -1,6 +1,6 @@
 [/
     Copyright 2016 Mikhail Maximov.
-    Copyright Antony Polukhin, 2020-2023.
+    Copyright Antony Polukhin, 2020-2025.
 
     Distributed under the Boost Software License, Version 1.0. (See accompanying
     file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)
@@ -12,7 +12,7 @@
     [id conversion]
     [version 1.7]
     [authors [Stroustrup, Bjarne], [Abrahams, Dave], [Rasin, Boris], [Polukhin, Antony]]
-    [copyright 2001 Beman Dawes, 2014-2023 Antony Polukhin]
+    [copyright 2001 Beman Dawes, 2014-2025 Antony Polukhin]
     [license
         Distributed under the Boost Software License, Version 1.0.
         (See accompanying file LICENSE_1_0.txt or copy at
@@ -30,8 +30,7 @@ The Conversion Library improves program safety and clarity by performing
 otherwise messy conversions. It includes cast-style function templates designed 
 to complement the C++ Standard's built-in casts.
 
-To reduce coupling, particularly to standard library IOStreams, 
-the Boost Conversion Library is supplied by several headers:
+To reduce coupling, the Boost Conversion Library is supplied by several headers:
 
 # The [@boost:boost/polymorphic_cast.hpp boost/polymorphic_cast.hpp] header
   provides [link polymorphic_cast `polymorphic_cast<>`] and
@@ -42,8 +41,6 @@ the Boost Conversion Library is supplied by several headers:
   [link polymorphic_pointer_cast `polymorphic_pointer_downcast<>`]
 # The [@boost:boost/implicit_cast.hpp boost/implicit_cast.hpp] header provides `implicit_cast<>` 
   to perform implicit casts only (no down-cast, no void*->T*, no U->T if T has only explicit constructors for U).
-# The [@boost:boost/lexical_cast.hpp boost/lexical_cast.hpp] header 
-  provides [@boost:doc/libs/release/libs/lexical_cast/ `lexical_cast<>`] general literal text conversions, such as an `int` represented as a `string`, or vice-versa. 
 
 [endsect]
 
@@ -153,10 +150,10 @@ This includes C++ built-in pointers, `std::shared_ptr`,
     class Banana : public Fruit {};
 
     // Use one of these:
-    typedef Fruit* FruitPtr;
-    typedef std::shared_ptr<Fruit> FruitPtr;
-    typedef boost::shared_ptr<Fruit> FruitPtr;
-    typedef boost::intrusive_ptr<Fruit> FruitPtr;
+    using FruitPtr = Fruit*;
+    using FruitPtr = std::shared_ptr<Fruit>;
+    using FruitPtr = boost::shared_ptr<Fruit>;
+    using FruitPtr = boost::intrusive_ptr<Fruit>;
 
     void f(FruitPtr fruit) {
       // ... logic which leads us to believe it is a banana
@@ -176,17 +173,17 @@ This includes C++ built-in pointers, `std::shared_ptr`,
     // Throws: std::bad_cast if ( dynamic_cast<Derived>(x) == 0 )
     // Returns: dynamic_cast<Derived>(x)
     template <class Derived, class Base>
-    inline Derived polymorphic_cast(Base* x);
+    constexpr Derived polymorphic_cast(Base* x);
 
     // Effects: assert( dynamic_cast<Derived>(x) == x );
     // Returns: static_cast<Derived>(x)
     template <class Derived, class Base>
-    inline Derived polymorphic_downcast(Base* x);
+    constexpr Derived polymorphic_downcast(Base* x);
 
     // Effects: assert( dynamic_cast<Derived>(&x) == &x );
     // Returns: static_cast<Derived>(x)
     template <class Derived, class Base>
-    inline Derived polymorphic_downcast(Base& x);
+    constexpr Derived polymorphic_downcast(Base& x);
 
     // Throws: std::bad_cast if ( dynamic_pointer_cast<Derived>(x) == 0 )
     // Returns: dynamic_pointer_cast<Derived>(x)

include/boost/implicit_cast.hpp

@@ -17,7 +17,7 @@ namespace detail {
 
 template<class T> struct icast_identity
 {
-    typedef T type;
+    using type = T;
 };
 
 } // namespace detail

include/boost/polymorphic_cast.hpp

@@ -14,7 +14,7 @@
 //  08 Nov 14  Add polymorphic_pointer_downcast (Boris Rasin)
 //  09 Jun 14  "cast.hpp" was renamed to "polymorphic_cast.hpp" and
 //             inclusion of numeric_cast was removed (Antony Polukhin)
-//  23 Jun 05  numeric_cast removed and redirected to the new verion (Fernando Cacciola)
+//  23 Jun 05  numeric_cast removed and redirected to the new version (Fernando Cacciola)
 //  02 Apr 01  Removed BOOST_NO_LIMITS workarounds and included
 //             <boost/limits.hpp> instead (the workaround did not
 //             actually compile when BOOST_NO_LIMITS was defined in
@@ -31,7 +31,7 @@
 //  19 Oct 00  Fix numeric_cast for floating-point types (Dave Abrahams)
 //  15 Jul 00  Suppress numeric_cast warnings for GCC, Borland and MSVC
 //             (Dave Abrahams)
-//  30 Jun 00  More MSVC6 wordarounds.  See comments below.  (Dave Abrahams)
+//  30 Jun 00  More MSVC6 workarounds.  See comments below.  (Dave Abrahams)
 //  28 Jun 00  Removed implicit_cast<>.  See comment below. (Beman Dawes)
 //  27 Jun 00  More MSVC6 workarounds
 //  15 Jun 00  Add workarounds for MSVC6
@@ -62,6 +62,12 @@
 # include <typeinfo>
 # include <type_traits>
 
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
+#define BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST constexpr
+#else
+#define BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST inline
+#endif
+
 namespace boost
 {
 //  See the documentation for descriptions of how to choose between
@@ -74,7 +80,7 @@ namespace boost
     //  section 15.8 exercise 1, page 425.
 
     template <class Target, class Source>
-    inline Target polymorphic_cast(Source* x)
+    BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST Target polymorphic_cast(Source* x)
     {
         Target tmp = dynamic_cast<Target>(x);
         if ( tmp == 0 ) boost::throw_exception( std::bad_cast() );
@@ -93,7 +99,7 @@ namespace boost
     //  Contributed by Dave Abrahams
 
     template <class Target, class Source>
-    inline Target polymorphic_downcast(Source* x)
+    BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST Target polymorphic_downcast(Source* x)
     {
         BOOST_ASSERT( dynamic_cast<Target>(x) == x );  // detect logic error
         return static_cast<Target>(x);
@@ -109,11 +115,11 @@ namespace boost
     //  Contributed by Julien Delacroix
 
     template <class Target, class Source>
-    inline typename std::enable_if<
+    BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST typename std::enable_if<
         std::is_reference<Target>::value, Target
     >::type polymorphic_downcast(Source& x)
     {
-        typedef typename std::remove_reference<Target>::type* target_pointer_type;
+        using target_pointer_type = typename std::remove_reference<Target>::type*;
         return *boost::polymorphic_downcast<target_pointer_type>(
             std::addressof(x)
         );
@@ -121,4 +127,6 @@ namespace boost
 
 } // namespace boost
 
+#undef BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST
+
 #endif  // BOOST_POLYMORPHIC_CAST_HPP

include/boost/polymorphic_pointer_cast.hpp

@@ -1,6 +1,6 @@
 //  boost polymorphic_pointer_cast.hpp header file  ----------------------------------------------//
 //  (C) Copyright Boris Rasin, 2014-2021.
-//  (C) Copyright Antony Polukhin, 2014-2023.
+//  (C) Copyright Antony Polukhin, 2014-2025.
 //  Distributed under 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)

index.html

@@ -3,7 +3,7 @@
  Copyright (c) 2016 Mikhail Maximov
  vigorous.activity at gmail dot com
 
- Copyright (c) Antony Polukhin, 2021-2023
+ Copyright (c) Antony Polukhin, 2021-2024
 
  Distributed under the Boost Software License,
  Version 1.0. (See accompanying file LICENSE_1_0.txt
@@ -33,7 +33,7 @@ boost-no-inspect
     <a href="../../doc/html/conversion.html">../../doc/html/conversion.html</a>
   </p>
   <p>
-    &copy; Antony Polukhin, 2014-2023
+    &copy; Antony Polukhin, 2014-2024
   </p>
 </body>
 </html>

test/Jamfile.v2

@@ -1,18 +1,20 @@
 # Copyright (C) 2001-2003 Douglas Gregor
-# Copyright (C) Antony Polukhin, 2011-2023
+# Copyright (C) Antony Polukhin, 2011-2024
 #
 # Distributed under 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)
 #
 
+require-b2 5.0.1 ;
+import-search /boost/config/checks ;
+import config : requires ;
 import testing ;
 import feature ;
 
-import ../../config/checks/config : requires ;
-
 project
     : requirements
-        [ requires cxx11_rvalue_references ]
+        <library>/boost/conversion//boost_conversion
+        [ requires cxx11_decltype ]
         # default to all warnings on:
         <warnings>all
         # set warnings as errors for those compilers we know we get warning free:

test/appveyor.yml

@@ -2,7 +2,7 @@
 # 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)
 #
-# Copyright Antony Polukhin, 2016-2023.
+# Copyright Antony Polukhin, 2016-2025.
 
 #
 # See https://svn.boost.org/trac/boost/wiki/TravisCoverals for description of this file

test/cast_test.cpp

@@ -38,6 +38,48 @@ namespace
     };
 }
 
+constexpr bool compile_time_polymorphic_cast_check() {
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
+    Derived derived;
+    Base* base = &derived;
+    return polymorphic_cast<Derived*>(base) != nullptr;
+#endif
+    return true;
+}
+
+static_assert(
+    compile_time_polymorphic_cast_check(),
+    "polymorphic_cast does not work at compile time"
+);
+
+constexpr bool compile_time_polymorphic_downcast_check() {
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
+    Derived derived;
+    Base* base = &derived;
+    return polymorphic_downcast<Derived*>(base) != nullptr;
+#endif
+    return true;
+}
+
+static_assert(
+    compile_time_polymorphic_downcast_check(),
+    "polymorphic_downcast does not work at compile time"
+);
+
+constexpr bool compile_time_polymorphic_downcast2_check() {
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
+    Derived derived;
+    Base& base = derived;
+    Derived& derived_again = polymorphic_downcast<Derived&>(base);
+    (void)derived_again;
+#endif
+    return true;
+}
+
+static_assert(
+    compile_time_polymorphic_downcast2_check(),
+    "polymorphic_downcast does not work at compile time"
+);
 
 int main( int argc, char * argv[] )
 {

test/implicit_cast.cpp

@@ -18,8 +18,8 @@ struct foo
     operator long() const { return 0; }
 };
 
-typedef type<long> long_type;
-typedef type<foo> foo_type;
+using long_type = type<long>;
+using foo_type = type<foo>;
 
 int main()
 {