Mark templates users shouldn't specialize with no_specializations attributes

[localspook]

This PR marks templates users shouldn't specialize with [[msvc::no_specializations]] or [[clang::no_specializations]], depending on the compiler.

Resolves #5179.

All citations in this PR description are to WG21-N5014.

Class Templates

[namespace.std]/2 allows class template specializations by default. Exceptions:

Specializations forbidden	By paragraph
initializer_list	[initializer.list.syn]/2
compare_three_way_result	[cmp.result]/1
allocator_traits	[allocator.traits.general]/1
coroutine_handle	[coroutine.handle.general]/2
is_execution_policy	[execpol.type]/3
basic_format_arg	[format.arg]/2
basic_format_context	[format.context]/2
basic_format_parse_context	[format.parse.ctx]/2
is_clock	[time.traits.is.clock]/2
generator	[coro.generator.class]/4
range_adaptor_closure	[range.adaptor.object]/5
variant	[variant.variant.general]/4
tuple	[tuple.tuple.general]/1
unary_function	WG21-N4140 [depr.base]/1
binary_function	WG21-N4140 [depr.base]/1

Variable Templates

[namespace.std]/3 forbids variable template specializations by default.

Specializations allowed	By paragraph
format_kind	[format.range.fmtkind]/3
disable_sized_sentinel_for	[iterator.concept.sizedsentinel]/3
enable_borrowed_range	[range.range]/6
disable_sized_range	[range.sized]/3
enable_view	[range.view]/7
enable_nonlocking_formatter_optimization	[format.formatter.locking]/1
e_v	[math.constants]/2
log2e_v	[math.constants]/2
log10e_v	[math.constants]/2
pi_v	[math.constants]/2
inv_pi_v	[math.constants]/2
inv_sqrtpi_v	[math.constants]/2
ln2_v	[math.constants]/2
ln10_v	[math.constants]/2
sqrt2_v	[math.constants]/2
sqrt3_v	[math.constants]/2
inv_sqrt3_v	[math.constants]/2
egamma_v	[math.constants]/2
phi_v	[math.constants]/2

Type Traits

[meta.rqmts]/4 forbids specializing any template in <type_traits> by default.

Specializations allowed	By paragraph
common_type	[meta.trans.other]/5
basic_common_reference	[meta.trans.other]/7

Function Templates

Specializing function templates is forbidden, but, following the decision made in #24, this PR doesn't touch any of them (except for is_pointer_interconvertible_with_class and is_corresponding_member in <type_traits>, as specializing those has always been forbidden).

Notes

Relevant: llvm/llvm-project#118167 implemented this warning in libc++.

[StephanTLavavej]

Click to expand usage example:

D:\GitHub\STL\out\x64>type meow.cpp

#include <chrono>
#include <type_traits>

template <class T>
struct Kitty {};

namespace std {
    namespace chrono {
        template <class T>
        struct is_clock<Kitty<T>> : true_type {};
    } // namespace chrono
} // namespace std

int main() {
    static_assert(std::chrono::is_clock<Kitty<int>>::value);
}

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W1 /std:c++latest meow.cpp
meow.cpp
meow.cpp(10): warning C5285: cannot declare a specialization for 'std::chrono::is_clock': Specializing this standard library template is forbidden by N5014 [time.traits.is.clock]/2
D:\GitHub\STL\out\x64\out\inc\__msvc_chrono.hpp(72): note: see declaration of 'std::chrono::is_clock'

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W1 /std:c++latest /wd5285 meow.cpp
meow.cpp

D:\GitHub\STL\out\x64>clang-cl /EHsc /nologo /W1 /std:c++latest meow.cpp
meow.cpp(10,16): error: 'is_clock' cannot be specialized: Specializing this standard library template is forbidden by
      N5014 [time.traits.is.clock]/2 [-Winvalid-specialization]
   10 |         struct is_clock<Kitty<T>> : true_type {};
      |                ^
D:\GitHub\STL\out\x64\out\inc\__msvc_chrono.hpp(72,12): note: marked 'no_specializations' here
   72 |     struct _NO_SPECIALIZATIONS_CITING("N5014 [time.traits.is.clock]/2") is_clock : bool_constant<is_clock_v<_Clock>> {};
      |            ^
D:\GitHub\STL\out\x64\out\inc\yvals_core.h(748,5): note: expanded from macro '_NO_SPECIALIZATIONS_CITING'
  748 |     _NO_SPECIALIZATIONS_MSG("Specializing this standard library template is forbidden by " _Standardese)
      |     ^
D:\GitHub\STL\out\x64\out\inc\yvals_core.h(742,41): note: expanded from macro '_NO_SPECIALIZATIONS_MSG'
  742 | #define _NO_SPECIALIZATIONS_MSG(_Msg) [[_Clang::__no_specializations__(_Msg)]]
      |                                         ^
1 error generated.

D:\GitHub\STL\out\x64>clang-cl /EHsc /nologo /W1 /std:c++latest -Wno-invalid-specialization meow.cpp

D:\GitHub\STL\out\x64>

It was difficult to search the Standard but I didn't find any other occurrences of missed enforcement.

Now all we have to do is remember to keep this current as we implement new features! (Oh yeah, and deal with any fallout in the internal builds and tests, which I will take care of.)

[StephanTLavavej]

Thanks to @MattStephanson for reminding me about <mdspan>'s mapping types! I pushed a commit to enforce WG21-N5014 [namespace.std]/4.3 "The behavior of a C++ program is undefined if it declares [...] an explicit or partial specialization of any member class template of a standard library class or class template" for mapping and allocator rebinds:

Click to expand member class template example:

D:\GitHub\STL\out\x64>type meow.cpp

#include <mdspan>
#include <memory>

template <typename T>
struct Kitty {};

namespace std {
    template <typename T>
    struct layout_left::mapping<Kitty<T>> {};

    template <typename T>
    template <typename U>
    struct allocator<T>::rebind<Kitty<U>> {};
} // namespace std

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /c /D_HAS_DEPRECATED_ALLOCATOR_MEMBERS=1 meow.cpp
meow.cpp
meow.cpp(9): warning C5285: cannot declare a specialization for 'std::layout_left::mapping': Specializing this standard library template is forbidden by N5014 [namespace.std]/4.3
D:\GitHub\STL\out\x64\out\inc\mdspan(480): note: see declaration of 'std::layout_left::mapping'
meow.cpp(13): warning C5285: cannot declare a specialization for 'std::allocator<_Ty>::rebind': Specializing this standard library template is forbidden by N5014 [namespace.std]/4.3
D:\GitHub\STL\out\x64\out\inc\xmemory(968): note: see declaration of 'std::allocator<_Ty>::rebind'

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for helping users write more conformant code! 😻 🪄 📈