https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122494
Bug ID: 122494
Summary: gcc rejects valid C++ codes
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rungecc at gmail dot com
Target Milestone: ---
Reproducer(See online compiler: https://godbolt.org/z/bPnaMoY48):
```c++
template <typename Ts>
struct L {};
template <typename T>
struct S;
template <typename T>
concept C = requires { T::v; };
struct A {};
static_assert(!C<A>);
template <typename T> requires (!C<T>)
struct S<T> {
constexpr static unsigned v = 0;
};
template <typename T>
struct S<L<T>> {
// unsigned int or long or long long
constexpr static unsigned v = S<T>::v;
// generic lambda
static_assert([](auto) {
// "v == 0"
if constexpr (v == 0) {}
return true;
}(1));
};
S<L<A>> _ = {};
int main() {}
```
Actual result:
g++15.2.0 with flat `-std=c++23`complains that
```
<source>: In instantiation of 'constexpr const unsigned int S<L<A> >::v':
<source>:25:23: required from 'struct S<L<A> >'
25 | if constexpr (v == 0) {}
| ^
<source>:30:9: required from here
30 | S<L<A>> _ = {};
| ^
<source>:21:41: error: incomplete type 'S<A>' used in nested name specifier
21 | constexpr static unsigned v = S<T>::v;
| ^
<source>: In instantiation of 'struct S<L<A> >':
<source>:30:9: required from here
30 | S<L<A>> _ = {};
| ^
<source>:27:6: error: non-constant condition for static assertion
23 | static_assert([](auto) {
| ~~~~~~~~~~
24 | // "v == 0"
| ~~~~~~~~~~~
25 | if constexpr (v == 0) {}
| ~~~~~~~~~~~~~~~~~~~~~~~~
26 | return true;
| ~~~~~~~~~~~~
27 | }(1));
| ~^~~
<source>:27:6: error: 'S<L<A> >::<lambda(auto:1)> [with auto:1 = int]' called
in a constant expression
<source>:23:19: note: 'S<L<A> >::<lambda(auto:1)> [with auto:1 = int]' is not
usable as a 'constexpr' function because:
23 | static_assert([](auto) {
| ^
Compiler returned: 1
```
Expected result, compiled successfully with no warnings, as msvc and clang++
do.
