[Bug c++/102881] gcc totally broken when trailing return type combine with decltype lambda

["hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102881

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to 康桓瑋 from comment #0)
> template<int N>
> using R = decltype([] { return 0; }());
> 
> template <int N>
> auto you_can_see_me() -> R<N> {
>   return {};
> }
> 
> int main() {
>   you_can_see_me<0>();
> }
> 
> GCC rejects with:
> 
> <source>:10:3: error: 'you_can_see_me' was not declared in this scope
>    10 |   you_can_see_me<0>();
>       |   ^~~~~~~~~~~~~~~~~
> 
> https://godbolt.org/z/7WEsjjscT

Even broken when there is no trailing return type:

#include <concepts>

template<class>
struct O {};

template<std::size_t>
using R = decltype([] { return O<int>{}; }());

template <int N>
constexpr R<N> get_variant() { return {}; }

static_assert(std::same_as<R<0>, O<int>>);
auto x = get_variant<0>();
static_assert(!std::same_as<decltype(x), O<int>>);
static_assert(std::same_as<decltype(x), int>);

https://godbolt.org/z/4bKccTEe5