Please see https://gcc.gnu.org/bugs for the requested info.

The testcase from the godbolt link is:

// { dg-options "-std=gnu++17" }
#include <functional>

template <typename T>
struct CallableTrait;

template <typename R, typename... Args>
struct CallableTrait<std::function<R(Args...)>>
{
    using ReturnType = R;
};

template <typename Callable>
using CallableTraitT = CallableTrait<decltype(std::function{std::declval<Callable>()})>;

template <typename Callable>
auto test(Callable&&)
{
    using CallableInfo = CallableTraitT<Callable>;
    static_assert(!std::is_void_v<typename CallableInfo::ReturnType>);
}

int main()
{
    test([]() { return 42; });
}


It requires at least C++17.

Reduced:


template<typename T> T&& declval();

template<typename R> struct function
{
  template<typename T> function(T) { }
};

template<typename T> function(T) -> function<decltype(declval<T>()())>;

template<typename T> constexpr bool is_void_v = false;
template<> constexpr bool is_void_v<void> = true;

template <typename T>
struct CallableTrait;

template <typename R>
struct CallableTrait<function<R>>
{
    using ReturnType = R;
};

template <typename Callable>
using CallableTraitT = CallableTrait<decltype(function{declval<Callable>()})>;

template <typename Callable>
auto test(Callable&&)
{
    using CallableInfo = CallableTraitT<Callable>;
    static_assert(!is_void_v<typename CallableInfo::ReturnType>);
}

int main()
{
    test([]() { return 42; });
}



98077.C: In instantiation of ‘auto test(Callable&&) [with Callable = main()::<lambda()>]’:
98077.C:34:29:   required from here
98077.C:29:20: error: invalid use of incomplete type ‘struct CallableTrait<main()::<lambda()> >’
   29 |     static_assert(!is_void_v<typename CallableInfo::ReturnType>);
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
98077.C:14:8: note: declaration of ‘struct CallableTrait<main()::<lambda()> >’
   14 | struct CallableTrait;
      |        ^~~~~~~~~~~~~

This is essentially a dup of the older 91911, I think

*** This bug has been marked as a duplicate of bug 91911 ***