https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97402
Bug ID: 97402
Summary: Value of dependent partial-concept-id is not usable in
a constant expression
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ed at catmur dot uk
Target Milestone: ---
#include <concepts>
template<class T> void f(T x) {
std::same_as<T> auto y = x;
}
int main() { f(1); }
In file included from <source>:1:
<source>: In instantiation of 'void f(T) [with T = int]':
<source>:5:17: required from here
.../gcc-trunk-20201013/include/c++/11.0.0/concepts:57:15: required for the
satisfaction of '__same_as<_Tp, _Up>' [with _Tp = int; _Up = T]
.../gcc-trunk-20201013/include/c++/11.0.0/concepts:57:32: error: the value of
'std::is_same_v<int, T>' is not usable in a constant expression
57 | concept __same_as = std::is_same_v<_Tp, _Up>;
| ~~~~~^~~~~~~~~~~~~~~~~~~
In file included from .../gcc-trunk-20201013/include/c++/11.0.0/concepts:44,
from <source>:1:
.../gcc-trunk-20201013/include/c++/11.0.0/type_traits:3220:25: note:
'std::is_same_v<int, T>' used in its own initializer
3220 | inline constexpr bool is_same_v = __is_same(_Tp, _Up);
| ^~~~~~~~~
<source>:3:26: error: deduced initializer does not satisfy placeholder
constraints
3 | std::same_as<T> auto y = x;
| ^
<source>:3:26: note: constraints not satisfied
https://godbolt.org/z/qGT5xe
Changing std::same_as<T> to std::same_as<int> makes it compile OK.
Feels a little like 96410.
