Can't `#include <llvm/ADT/FloatingPointMode.h>` with `-std=c++23` with clang++ 21.1.0-rc1

[chriselrod]

#include <llvm/ADT/FloatingPointMode.h>

int main(){
  return 0;
}

I get

❯ clang++ llvminclude.cpp

~/Documents/progwork/cxx/test
❯ clang++ -std=c++23 llvminclude.cpp
In file included from llvminclude.cpp:1:
In file included from /usr/include/llvm/ADT/FloatingPointMode.h:19:
In file included from /usr/include/llvm/ADT/StringSwitch.h:16:
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:193:14: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<1UL>' requested here
  193 |       .Cases("", "ieee", DenormalMode::IEEE)
      |              ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr StringLiteral(const char (&Str)[N])
      |                                          ^
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:193:18: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<5UL>' requested here
  193 |       .Cases("", "ieee", DenormalMode::IEEE)
      |                  ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr= StringLiteral(const char (&Str)[N])
      |                                          ^
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:194:13: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<14UL>' requested here
  194 |       .Case("preserve-sign", DenormalMode::PreserveSign)
      |             ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr StringLiteral(const char (&Str)[N])
      |                                          ^
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:196:13: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<8UL>' requested here
  196 |       .Case("dynamic", DenormalMode::Dynamic)
      |             ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr StringLiteral(const char (&Str)[N])
      |                                          ^
4 errors generated.

This works with clang++ 20, as well as with

clang version 21.0.0git (git@github.com:llvm/llvm-project d4331344ac45979f5bc58508d7e186f86fe907bd)

This is

commit d4331344ac45979f5bc58508d7e186f86fe907bd
Author: Nilanjana Basu <nilanjana.basu87@gmail.com>
Date:   Wed Jul 2 14:41:30 2025 -0700

    [Clang][Driver][SamplePGO] Introduce -fno_sample_profile_use_profi flag for SamplePGO (#145957)
    
    This flag allows opting out of using profile inference pass for SamplePGO.

so the regression was some time in the past couple of weeks.

[chriselrod]

Note that I can build clang just fine, the problem is with including this header in a project that uses c++23 and depends on LLVM (where including the header file results in the above compile-time error).

[zwuis]

Sorry for misreading the issue title. The patch has not been backported.

[frederick-vs-ja]

This IS duplicate of #149188. Backport PR is opened but not merged yet, though.