| Issue |
150426
|
| Summary |
extent_v in static_assert not detecting inferred array length with rest argument expansion before use
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
mstange
|
https://godbolt.org/z/c61WhKbaz
The `static_assert` below fails in clang 20.1.0, but passes in gcc 15.1 and msvc 19:
```cpp
#include <stdio.h>
#include <type_traits>
template <int... ArgValues>
struct MyTemplateStruct {
static constexpr int values[] = {ArgValues...};
};
int main() {
using ConcreteStruct = MyTemplateStruct<5>;
// printf("test: %d\n", ConcreteStruct::values[0]);
// This assertion starts passing if you uncomment the above printf
static_assert(std::extent_v<decltype(ConcreteStruct::values)> == 1,
"should detect one element in values member");
return 0;
}
```
Error message with clang:
```
<source>:15:19: error: static assertion failed due to requirement 'std::extent_v<const int[], 0> == 1': should detect one element in values member
15 | static_assert(std::extent_v<decltype(ConcreteStruct::values)> == 1,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:15:67: note: _expression_ evaluates to '0 == 1'
15 | static_assert(std::extent_v<decltype(ConcreteStruct::values)> == 1,
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
1 error generated.
```
The first template argument to `extent_v` is `const int[]` without the printf, and `const int[1]` with the printf.
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