[Clang]: clang rejects valid codes when it trying capturing something never occuring in lambda body

[RungeCC]

struct A {
  void operator()(int) {}
};
inline constexpr struct {
  constexpr auto operator()(A fn) const
  {
    return [f{fn}](auto &&tpl) 
        noexcept(noexcept(&f))
    { return true; };
  }
} error;

inline constexpr struct {
  constexpr auto operator()(A fn) const
  {
    return [f{fn}](auto &&tpl) 
        // noexcept(noexcept(&f))
    { return true; };
  }
} no_error;



int main() {
  (void)error( A{} )(1);    // $1
  (void)no_error( A{} )(1); // $2

}

• clang trunk with flag c++26
• https://godbolt.org/z/1EK6qf8Th
  • see no_error and error function object, if I delete the noexcept clause then everything is OK.
  • clang tells me to capture fn, which is weird (since fn is never used in function body), but if I do so, clang accepts it.
  • if I change f inside the noexcept to fn, then again, everything would be fine.

clang rejects $1 it with:

<source>:8:15: error: variable 'fn' cannot be implicitly captured in a lambda with no capture-default specified
    8 |     return [f{fn}](auto &&tpl) 
      |               ^
<source>:8:12: note: in instantiation of exception specification for 'operator()<int>' requested here
    8 |     return [f{fn}](auto &&tpl) 
      |            ^
<source>:26:21: note: in instantiation of function template specialization '(anonymous struct)::operator()(A)::(anonymous class)::operator()<int>' requested here
   26 |   (void)error( A{} )(1);
      |                     ^
<source>:6:31: note: 'fn' declared here
    6 |   constexpr auto operator()(A fn) const
      |                               ^
<source>:8:12: note: lambda expression begins here
    8 |     return [f{fn}](auto &&tpl) 
      |            ^
<source>:8:12: note: capture 'fn' by value
    8 |     return [f{fn}](auto &&tpl) 
      |            ^
      |            fn
<source>:8:12: note: capture 'fn' by reference
    8 |     return [f{fn}](auto &&tpl) 
      |            ^
      |            &fn
1 error generated.

but accepts $2.

Expected behavior

Accepts both of them.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Runge (RungeCC)

```c+++

struct A {
void operator()(int) {}
};
inline constexpr struct {
constexpr auto operator()(A fn) const
{
return [f{fn}](auto &&tpl)
noexcept(noexcept(&f))
{ return true; };
}
} error;

inline constexpr struct {
constexpr auto operator()(A fn) const
{
return [f{fn}](auto &&tpl)
// noexcept(noexcept(&f))
{ return true; };
}
} no_error;

int main() {
(void)error( A{} )(1);
(void)no_error( A{} )(1);

}

- clang trunk with flag `c++26`
- https://godbolt.org/z/hMacPx4a6
  - see `no_error` and `error` function object, if I delete the `noexcept` clause then everything is OK.
  - clang tells me to capture `fn`, which is weird (since `fn` is never used in function body), but if I do so, clang accepts it.
  - if I change `f` inside the `noexcept` to `fn`, then again, everything would be fine.

</details>

[cor3ntin]

Confirmed https://godbolt.org/z/vP8d1sW6r

[kamrann]

Don't currently have any simple repro to add I'm afraid so not sure how much use this will be, but just wanted to add that I'm seeing this error all over my codebase and STL headers after updating to 18.1.8 (from pre 18.0 source build) and starting to convert my code to C++20 modules. I believe the latter is more likely the trigger.

The interesting thing is that while the above repro is very specific, requiring the conditional noexcept, the locations I'm seeing the same error include innocuous lambdas (e.g. no parameters or qualifiers, but defined within a member function template), and even in one case, in MSVC's format header, on a line in a member function that was not even in a lambda:

In module 'kdeps' imported from C:\Kantan\root\kdeps\kdeps\kdeps/include\kdeps/kdeps_no_sys.h:9:
In module 'kdeps:ref' imported from C:\Kantan\root\kdeps\kdeps\kdeps\include\kdeps\kdeps.ipp:10:
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:3494:27: error: variable '_Specs' cannot be implicitly captured in a lambda with no capture-default specified
 3494 |     int _Width          = _Specs._Precision;
      |                           ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:2956:16: note: in instantiation of function template specialization 'std::__p2286::_Write_escaped<char, std::_Phony_fmt_iter_for<char>>' requested here
 2956 |         return _Write_escaped(_STD move(_Out), basic_string_view<_CharT>{&_Value, 1}, _Specs, '\'');
      |                ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:2824:16: note: in instantiation of function template specialization 'std::__p2286::_Fmt_write<char, std::_Phony_fmt_iter_for<char>>' requested here
 2824 |         return _Fmt_write(_STD move(_Out), static_cast<_CharT>(_Value), _Specs, _Locale);
      |                ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:2913:12: note: in instantiation of function template specialization 'std::__p2286::_Write_integral<char, std::_Phony_fmt_iter_for<char>, int>' requested here
 2913 |     return _Write_integral(_STD move(_Out), _Value, _Specs, _Locale);
      |            ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:3572:16: note: in instantiation of function template specialization 'std::__p2286::_Fmt_write<char, std::_Phony_fmt_iter_for<char>, int>' requested here
 3572 |         return _Fmt_write(_Ctx->out(), _Val, *_Specs, _Ctx->_Get_lazy_locale());
      |                ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:773:20: note: in instantiation of function template specialization 'std::__p2286::_Arg_formatter<std::_Phony_fmt_iter_for<char>, char>::operator()<int>' requested here
  773 |             return _STD forward<_Visitor>(_Vis)(_Int_state);
      |                    ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\yvals_core.h:1894:17: note: expanded from macro '_STD'
 1894 | #define _STD    ::std::
      |                 ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\__msvc_formatter.hpp:162:16: note: (skipping 13 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
  162 |         return _Formatter_base_format(_Specs, _Val, _FormatCtx);
      |                ^
C:\Kantan\root\kdeps\kdeps\kdeps\include\kdeps\detail\kdeps.sys.external_intf.ipp:215:18: note: in instantiation of function template specialization 'kdeps::detail::DepSystemBase::push_external<(lambda at C:\Kantan\root\kdeps\kdeps\kdeps\include\kdeps\detail\kdeps.sys.external_intf.ipp:221:25)>' requested here
  215 |             sys->push_external(std::forward< F >(update), tag);
      |                  ^
C:\Kantan\root\kdeps\kdeps\kdeps\include\kdeps\detail\kdeps.sys.external_intf.ipp:221:13: note: in instantiation of function template specialization 'kdeps::ExternalInterface::push_update<(lambda at C:\Kantan\root\kdeps\kdeps\kdeps\include\kdeps\detail\kdeps.sys.external_intf.ipp:221:25)>' requested here
  221 |             push_update([update = std::forward< F >(update)](auto intf) mutable {
      |             ^
C:\Kantan\root\kdeps\kdeps\kdeps/include\kdeps/nodes/collection/detail/dep_graph_source_base.h:43:27: note: in instantiation of function template specialization 'kdeps::ExternalInterface::execute_update<(lambda at C:\Kantan\root\kdeps\kdeps\kdeps\include\kdeps\nodes\collection\detail\dep_graph_source_base.h:43:42)>' requested here
   43 |             this->system->execute_update([node_id = this->node_id](auto intf, UpdateResult& result) mutable {
      |                           ^
C:\Kantan\root\kdeps\kdeps\kdeps/include\kdeps/sources/source_graph.h:51:18: note: in instantiation of member function 'kdeps::GraphSourceBase<kdeps::DepNodeId, std::tuple<kdeps::TypedDepNodeId<std::basic_string<char>>, kdeps::TypedDepNodeId<std::basic_string<char>>, kdeps::TypedDepNodeId<std::basic_string<char>>, kdeps::TypedDepNodeId<kdeps::dbg::Debugger::NodeData::MetaState>, bool, std::optional<kdeps::DepNodeId>, kdeps::TypedDepNodeId<std::basic_string<char>>>>::temp_create_storage' requested here
   51 |                         immediate_src.temp_create_storage();
      |                                       ^
C:\Kantan\root\kdeps\kdeps\kdeps\include\kdeps\dbg\kdeps.debugger.ipp:188:25: note: in instantiation of function template specialization 'kdeps::detail::SourceSystemMap<kdeps::GraphSourceBase<kdeps::DepNodeId, std::tuple<kdeps::TypedDepNodeId<std::basic_string<char>>, kdeps::TypedDepNodeId<std::basic_string<char>>, kdeps::TypedDepNodeId<std::basic_string<char>>, kdeps::TypedDepNodeId<kdeps::dbg::Debugger::NodeData::MetaState>, bool, std::optional<kdeps::DepNodeId>, kdeps::TypedDepNodeId<std::basic_string<char>>>>, kdeps::OwnedGraphSource<kdeps::DepNodeId, kdeps::dbg::Debugger::NodeData, kdeps::dbg::Debugger::NodeData::Projection>>::request_source_node<kdeps::ExternalInterface>' requested here
  188 |                         return dep_graph_src.request_source_node(meta_sys());
      |                                              ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:2779:83: note: '_Specs' declared here
 2779 |     _OutputIt _Out, basic_string_view<_CharT> _Value, _Basic_format_specs<_CharT> _Specs, char _Delim);
      |                                                                                   ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:3497:79: note: lambda expression begins here
 3497 |     return _Write_aligned(_STD move(_Out), _Width, _Specs, _Fmt_align::_Left, [&_Temp, _Last](_OutputIt _Out) {
      |                                                                               ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:3497:93: note: capture '_Specs' by value
 3497 |     return _Write_aligned(_STD move(_Out), _Width, _Specs, _Fmt_align::_Left, [&_Temp, _Last](_OutputIt _Out) {
      |                                                                                             ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:3497:93: note: capture '_Specs' by reference
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:3497:80: note: default capture by value
 3497 |     return _Write_aligned(_STD move(_Out), _Width, _Specs, _Fmt_align::_Left, [&_Temp, _Last](_OutputIt _Out) {
      |                                                                                ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.33923\include\format:3497:80: note: default capture by reference