| Issue |
149188
|
| Summary |
[Clang] Error on an `enable_if` attribute in `ADT/StringRef.h` in C++23 mode
|
| Labels |
clang:frontend,
regression,
rejects-valid
|
| Assignees |
|
| Reporter |
Sirraide
|
Attempting to create an `llvm::StringLiteral` in C++23 mode currently causes an error. Specifically, the code we complain about is this constructor in `ADT/StringRef.h`:
```c++
template <size_t N>
constexpr StringLiteral(const char (&Str)[N])
#if defined(__clang__) && __has_attribute(enable_if)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgcc-compat"
__attribute((enable_if(__builtin_strlen(Str) == N - 1,
"invalid string literal")))
#pragma clang diagnostic pop
#endif
: StringRef(Str, N - 1) {
}
```
which can be reduced to (https://godbolt.org/z/xa3zs5qj6):
```c++
template <__SIZE_TYPE__ N>
constexpr void foo(const char (&Str)[N])
__attribute((enable_if(__builtin_strlen(Str) == N - 1,
"invalid string literal")))
{}
void x() {
foo("1234");
}
```
which causes the following error:
```console
<source>:3:14: error: 'enable_if' attribute _expression_ never produces a constant _expression_
3 | __attribute((enable_if(__builtin_strlen(Str), ""))) {}
| ^
<source>:6:5: note: in instantiation of function template specialization 'foo<5UL>' requested here
6 | foo("1234");
| ^
<source>:3:41: note: read of variable 'Str' whose value is not known
3 | __attribute((enable_if(__builtin_strlen(Str), ""))) {}
| ^
<source>:2:33: note: declared here
2 | constexpr void foo(const char (&Str)[N])
|
```
The `enable_if` attribute has been there since 2016. Interestingly, we compile this just fine in C++20 mode, and Clang 20 doesn’t complain about it at all even in C++23 mode.
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