Error: the type of variable template specialization declared with deduced type depends on itself

[Eczbek]

https://godbolt.org/z/eWjrxjeds
GCC and MSVC accept this

template<int>
struct A {
	template<int x>
	static constexpr auto foo = []<int y> {
		return A<(x + y)>();
	};

	template<int x>
	using bar = decltype(foo<0>.template operator()<x>());

	template<int x>
	using baz = decltype(foo<0>.template operator()<x>());
};

A<0> x;

int main() {}

<source>:9:29: error: member reference base type 'const auto' is not a structure or union
    9 |         using bar = decltype(foo<0>.template operator()<x>());
      |                              ~~~~~~^         ~~~~~~~~
<source>:8:2: note: in instantiation of template type alias 'bar' requested here
    8 |         template<int x>
      |         ^
<source>:15:6: note: in instantiation of template class 'A<0>' requested here
   15 | A<0> x;
      |      ^
<source>:12:23: error: the type of variable template specialization 'foo<0>' declared with deduced type 'const auto' depends on itself
   12 |         using baz = decltype(foo<0>.template operator()<x>());
      |                              ^
<source>:11:2: note: in instantiation of template type alias 'baz' requested here
   11 |         template<int x>
      |         ^
<source>:15:6: note: in instantiation of template class 'A<0>' requested here
   15 | A<0> x;
      |      ^
2 errors generated.
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Eczbek)

https://godbolt.org/z/eWjrxjeds GCC and MSVC accept this

template<int>
struct A {
	template<int x>
	static constexpr auto foo = []<int y> {
		return A<(x + y)>();
	};

	template<int x>
	using bar = decltype(foo<0>.template operator()<x>());

	template<int x>
	using baz = decltype(foo<0>.template operator()<x>());
};

A<0> x;

int main() {}

<source>:9:29: error: member reference base type 'const auto' is not a structure or union
    9 |         using bar = decltype(foo<0>.template operator()<x>());
      |                              ~~~~~~^         ~~~~~~~~
<source>:8:2: note: in instantiation of template type alias 'bar' requested here
    8 |         template<int x>
      |         ^
<source>:15:6: note: in instantiation of template class 'A<0>' requested here
   15 | A<0> x;
      |      ^
<source>:12:23: error: the type of variable template specialization 'foo<0>' declared with deduced type 'const auto' depends on itself
   12 |         using baz = decltype(foo<0>.template operator()<x>());
      |                              ^
<source>:11:2: note: in instantiation of template type alias 'baz' requested here
   11 |         template<int x>
      |         ^
<source>:15:6: note: in instantiation of template class 'A<0>' requested here
   15 | A<0> x;
      |      ^
2 errors generated.
Compiler returned: 1