clang-21 regression: "enable_if attribute expression never produces a constant expression" when compiling LLVM in C++23 #150131
Closed as duplicate of#149188
Description
The command
echo '#include "llvm/ADT/StringExtras.h"' | clang++-21 -x c++ - -std=c++23
fails with the compilation error
In file included from <stdin>:1:
In file included from ./llvm/include/llvm/ADT/StringExtras.h:19:
In file included from ./llvm/include/llvm/ADT/SmallString.h:18:
./llvm/include/llvm/ADT/StringRef.h:871:22: error: 'enable_if' attribute
expression never produces a constant expression
871 | __attribute((enable_if(__builtin_strlen(Str) == N - 1,
| ^
./llvm/include/llvm/ADT/StringExtras.h:150:34: note: in instantiation of
function template specialization 'llvm::StringLiteral::StringLiteral<33UL>' requested here
150 | static constexpr StringLiteral Punctuations =
| ^
./llvm/include/llvm/ADT/StringRef.h:871:49: note: read of variable 'Str'
whose value is not known
871 | __attribute((enable_if(__builtin_strlen(Str) == N - 1,
| ^
./llvm/include/llvm/ADT/StringRef.h:867:42: note: declared here
867 | constexpr StringLiteral(const char (&Str)[N])
| ^
1 error generated.
Both clang-21 and the used LLVM source were taken from the 21.1.0-rc1 tag.
The same command works against clang++ version 20, as such this is a regression compared to clang-20.
(The issue also only appears for C++23, using C++20 on clang-21 works)