https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116952
--- Comment #4 from Giel <giel+gcc at mortis dot eu> ---
Also taking `decltype(<lambda-expr>)` to convert it to a type-param doesn't
help (Clang still accepts it):
```
template <typename, typename F>
concept A = F()();
template <A<decltype([]{ return true; })> T>
constexpr auto fun(T) {}
```
