feat: 💥 unit_can_be_prefixed removed - from now on all named units can be prefixed

Resolves #604

docs/users_guide/framework_basics/concepts.md

@@ -117,20 +117,8 @@ and is satisfied by:
 
 ### `PrefixableUnit<T>` { #PrefixableUnit }
 
-`PrefixableUnit` concept is satisfied by all units derived from a `named_unit` class template for
+`PrefixableUnit` concept is satisfied by all units derived from a `named_unit` class template.
-which a customization point `unit_can_be_prefixed<T{}>` was not explicitly set to `false`. Such
+Such units can be passed as an argument to a `prefixed_unit` class template.
-units can be passed as an argument to a `prefixed_unit` class template.
-
-??? abstract "Examples"
-
-    All units in the [SI](../../appendix/glossary.md#si) can be prefixed with SI-defined prefixes.
-
-    Some [off-system units](../../appendix/glossary.md#off-system-unit) like `non_si::day`
-    can't be prefixed. To enforce that, the following has to be provided:
-
-    ```cpp
-    template<> inline constexpr bool unit_can_be_prefixed<non_si::day> = false;
-    ```
 
 
 ### `UnitOf<T, V>` { #UnitOf }

docs/users_guide/framework_basics/systems_of_units.md

@@ -190,21 +190,3 @@ inline constexpr struct mag_pi final : magnitude<std::numbers::pi_v<long double>
 ```cpp
 inline constexpr struct degree final : named_unit<{u8"°", "deg"}, mag_pi / mag<180> * si::radian> {} degree;
 ```
-
-!!! tip
-
-    The ISO 8000 and [SI](../../appendix/glossary.md#si) standards explicitly forbid using prefixes
-    with some units (e.g., day, minute, hour, degree Celsius). This is why the library disallows
-    this as well by providing specializations of the `unit_can_be_prefixed` variable template for
-    such units. Thanks to it trying to create a prefixed version for them will result in
-    a compile-time error.
-
-    However, some projects are not aware of those limitations and use prefixed version of such units
-    (e.g., [linux kernel uses millidegrees Celsius](https://github.com/search?q=repo%3Atorvalds%2Flinux+millidegree&type=code)).
-    To enable compatibility with those projects we can workaround the limitation in **mp-units**
-    by providing a scaled version of the unit explicitly:
-
-    ```cpp
-    inline constexpr struct milli_degree_celsius final : named_unit<symbol_text{u8"m℃", "m`C"}, mag_ratio<1, 1000> * si::degree_Celsius> {} milli_degree_celsius;
-    inline constexpr auto mdeg_C = milli_degree_celsius;
-    ```

src/core/include/mp-units/framework/unit_concepts.h

@@ -62,38 +62,13 @@ template<typename T>
 inline constexpr bool is_derived_from_specialization_of_named_unit =
   requires(T* t) { to_base_specialization_of_named_unit(t); };
 
-template<typename T>
-inline constexpr bool is_specialization_of_named_unit = false;
-
-template<symbol_text Symbol, auto... Args>
-inline constexpr bool is_specialization_of_named_unit<named_unit<Symbol, Args...>> = true;
-
-/**
- * @brief A concept matching all units with special names
- *
- * Satisfied by all unit types derived from the specialization of `named_unit`.
- */
-template<typename T>
-concept NamedUnit =
-  Unit<T> && detail::is_derived_from_specialization_of_named_unit<T> && (!detail::is_specialization_of_named_unit<T>);
-
 }  // namespace detail
 
-/**
- * @brief Prevents assignment of a prefix to specific units
- *
- * By default all named units allow assigning a prefix for them. There are some notable exceptions like
- * `hour` or `degree_Celsius`. For those a partial specialization with the value `false` should be
- * provided.
- */
-MP_UNITS_EXPORT template<Unit auto V>
-inline constexpr bool unit_can_be_prefixed = true;
-
 /**
  * @brief A concept to be used to define prefixes for a unit
  */
 MP_UNITS_EXPORT template<typename T>
-concept PrefixableUnit = detail::NamedUnit<T> && unit_can_be_prefixed<T{}>;
+concept PrefixableUnit = Unit<T> && detail::is_derived_from_specialization_of_named_unit<T>;
 
 namespace detail {
 

src/systems/include/mp-units/systems/si/units.h

@@ -125,15 +125,6 @@ using namespace non_si;
 
 }  // namespace si
 
-template<>
-inline constexpr bool unit_can_be_prefixed<si::degree_Celsius> = false;
-template<>
-inline constexpr bool unit_can_be_prefixed<non_si::minute> = false;
-template<>
-inline constexpr bool unit_can_be_prefixed<non_si::hour> = false;
-template<>
-inline constexpr bool unit_can_be_prefixed<non_si::day> = false;
-
 template<>
 inline constexpr bool space_before_unit_symbol<non_si::degree> = false;
 template<>

test/static/concepts_test.cpp

@@ -155,31 +155,6 @@ static_assert(!Unit<int>);
 static_assert(!Unit<std::chrono::seconds>);
 #endif
 
-// NamedUnit
-static_assert(detail::NamedUnit<struct si::metre>);
-static_assert(detail::NamedUnit<struct natural::electronvolt>);
-static_assert(!detail::NamedUnit<decltype(si::kilogram)>);
-static_assert(!detail::NamedUnit<decltype(si::kilo<si::gram>)>);
-static_assert(!detail::NamedUnit<decltype(si::metre / si::second)>);
-static_assert(!detail::NamedUnit<decltype(inverse(si::second))>);
-static_assert(!detail::NamedUnit<decltype(mag<10> * si::second)>);
-static_assert(!detail::NamedUnit<decltype(square(si::metre))>);
-static_assert(!detail::NamedUnit<decltype(pow<2>(si::metre))>);
-static_assert(detail::NamedUnit<struct si::standard_gravity>);
-static_assert(!detail::NamedUnit<scaled_unit<mag<10>, struct si::second>>);
-static_assert(!detail::NamedUnit<derived_unit<struct si::metre, per<struct si::second>>>);
-static_assert(!detail::NamedUnit<struct one>);
-static_assert(!detail::NamedUnit<named_unit<"?", kind_of<isq::length>>>);
-static_assert(!detail::NamedUnit<named_unit<"?">>);
-static_assert(!detail::NamedUnit<named_unit<"?", si::metre / si::second>>);
-static_assert(!detail::NamedUnit<named_unit<"?", si::metre, kind_of<isq::length>>>);
-static_assert(!detail::NamedUnit<prefixed_unit<"?", mag<10>, si::second>>);
-static_assert(!detail::NamedUnit<struct isq::dim_length>);
-static_assert(!detail::NamedUnit<int>);
-#if MP_UNITS_HOSTED
-static_assert(!detail::NamedUnit<std::chrono::seconds>);
-#endif
-
 // PrefixableUnit
 static_assert(PrefixableUnit<struct si::metre>);
 static_assert(PrefixableUnit<struct natural::electronvolt>);

test/static/si_test.cpp

@@ -63,10 +63,6 @@ static_assert(1 * Qm == 1'000'000'000'000'000'000 * Tm);
 template<template<typename U> typename prefix, auto V1>
 concept can_not_be_prefixed = Unit<decltype(V1)> && !requires { typename prefix<decltype(V1)>; };
 
-static_assert(can_not_be_prefixed<si::milli_, si::degree_Celsius>);
-static_assert(can_not_be_prefixed<si::milli_, si::minute>);
-static_assert(can_not_be_prefixed<si::milli_, si::hour>);
-static_assert(can_not_be_prefixed<si::milli_, si::day>);
 static_assert(can_not_be_prefixed<si::milli_, si::kilogram>);
 static_assert(can_not_be_prefixed<si::milli_, si::hectare>);
 static_assert(can_not_be_prefixed<si::milli_, si::kilo<si::metre>>);

test/static/unit_test.cpp

@@ -106,18 +106,18 @@ static_assert(Unit<decltype(metre / second)>);
 static_assert(Unit<decltype(nu_second / nu_second)>);
 static_assert(Unit<decltype(kilometre)>);
 
-static_assert(detail::NamedUnit<metre_>);
+static_assert(PrefixableUnit<metre_>);
-static_assert(detail::NamedUnit<hertz_>);
+static_assert(PrefixableUnit<hertz_>);
-static_assert(detail::NamedUnit<newton_>);
+static_assert(PrefixableUnit<newton_>);
-static_assert(detail::NamedUnit<minute_>);
+static_assert(PrefixableUnit<minute_>);
-static_assert(detail::NamedUnit<radian_>);
+static_assert(PrefixableUnit<radian_>);
-static_assert(!detail::NamedUnit<decltype(kilogram)>);
+static_assert(!PrefixableUnit<decltype(kilogram)>);
-static_assert(!detail::NamedUnit<decltype(kilojoule)>);
+static_assert(!PrefixableUnit<decltype(kilojoule)>);
-static_assert(!detail::NamedUnit<decltype(si::kilo<gram>)>);
+static_assert(!PrefixableUnit<decltype(si::kilo<gram>)>);
-static_assert(!detail::NamedUnit<decltype(square(metre))>);
+static_assert(!PrefixableUnit<decltype(square(metre))>);
-static_assert(!detail::NamedUnit<decltype(cubic(metre))>);
+static_assert(!PrefixableUnit<decltype(cubic(metre))>);
-static_assert(!detail::NamedUnit<decltype(mag<60> * second)>);
+static_assert(!PrefixableUnit<decltype(mag<60> * second)>);
-static_assert(!detail::NamedUnit<decltype(kilometre)>);
+static_assert(!PrefixableUnit<decltype(kilometre)>);
 
 // named unit
 static_assert(is_of_type<metre, metre_>);