Mark templates users shouldn't specialize with `no_specializations` attributes #5536

localspook · 2025-05-24T00:57:53Z

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++.

.clang-format

stl/inc/xmemory

stl/inc/initializer_list

tests/libcxx/expected_results.txt

… it unconditional.

This marks the definition of coroutine_handle, and the declarations of generator, variant, and allocator_traits, to match their counterparts.

…at_context, which were missing.

…ts>` and `<xtr1common>`.

…ype traits I checked that everything in `<xtr1common>` is depicted by the Standard in `<type_traits>`.

stl/inc/yvals_core.h

tests/libcxx/expected_results.txt

stl/inc/__msvc_ranges_tuple_formatter.hpp

stl/inc/yvals_core.h

stl/inc/initializer_list

stl/inc/yvals_core.h

StephanTLavavej · 2025-09-18T20:39:40Z

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.)

…ype traits.

StephanTLavavej · 2025-09-19T17:26:19Z

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 · 2025-09-19T18:43:36Z

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

StephanTLavavej · 2025-09-22T16:04:27Z

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

Mark templates users shouldn't specialize with _NO_SPECIALIZATIONS

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localspook requested a review from a team as a code owner May 24, 2025 00:57

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localspook changed the title ~~Mark templates users shouldn't specialize with _NO_SPECIALIZATIONS~~ Mark templates users shouldn't specialize with _NO_SPECIALIZATIONS May 24, 2025

localspook added 3 commits May 23, 2025 18:37

Silence Clang's -Wunknown-warning-option warning

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Guard against evil macros

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Oops; accidentally removed one _NO_SPECIALIZATIONS

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Disable tests properly

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localspook added 2 commits June 5, 2025 20:33

Also mark unary_function and binary_function

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Move expected test failures into bogus section

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Mark more templates in <system_error>...

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Realign with main

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StephanTLavavej added 9 commits September 18, 2025 09:50

Add citations, part 2: Update attributes.

f039e81

Add citations, part 3: Update attribute for initializer_list and make…

6ddd383

… it unconditional.

Add citations, part 4: Mark declarations and definitions consistently.

8cd9ea8

This marks the definition of coroutine_handle, and the declarations of generator, variant, and allocator_traits, to match their counterparts.

Add citations, part 5: Mark basic_format_parse_context and basic_form…

8f6cf3c

…at_context, which were missing.

Add citations, part 6: Mark variable templates outside of `<type_trai…

69c2fd2

…ts>` and `<xtr1common>`.

Add citations, part 7: Mark <type_traits> (and <xtr1common>) as t…

3dfe3bf

…ype traits I checked that everything in `<xtr1common>` is depicted by the Standard in `<type_traits>`.

Add defenses to __msvc_formatter.hpp.

568f620

Improve comment clarity.

969e218

Update common_reference with dual citations.

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Comment when specializations are allowed for variable templates and t…

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…ype traits.

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StephanTLavavej moved this from Initial Review to Ready To Merge in STL Code Reviews Sep 18, 2025

Mark member class templates mapping and rebind.

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Oops, guard <mdspan> and <scoped_allocator>.

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StephanTLavavej changed the title ~~Mark templates users shouldn't specialize with _NO_SPECIALIZATIONS~~ Mark templates users shouldn't specialize with no_specializations attributes Sep 19, 2025

StephanTLavavej merged commit b49a10b into microsoft:main Sep 22, 2025
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Add is_implicit_lifetime trait (for Clang) #5445

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This was referenced Oct 2, 2025

Implement P2255R2 Type Traits To Detect References Binding To Temporaries for Clang #5537

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<type_traits>: Partially unblock P0466R5 for Clang #5621

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Mark templates users shouldn't specialize with no_specializations attributes #5536

Mark templates users shouldn't specialize with no_specializations attributes #5536

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Class Templates

Variable Templates

Type Traits

Function Templates

Notes

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Mark templates users shouldn't specialize with `no_specializations` attributes #5536

Mark templates users shouldn't specialize with `no_specializations` attributes #5536

localspook commented May 24, 2025 •

edited by StephanTLavavej

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