g++ and clang++ different behaviour using a using alias to define a method

Trying to respond to another question, I've found that g++ and clang++ disagree about the following simple code

template <typename>
struct foo
 { void bar (); };

template <typename T>
using fooAlias = foo<T>;

template <typename T>
void fooAlias<T>::bar ()
 { }

int main ()
 {
 }

While clang++ compile, g++ gives the following error

tmp_003-14,gcc,clang.cpp:26:24: error: invalid use of incomplete type ‘struct foo<T>’
 void fooAlias<T>::bar ()
                        ^
tmp_003-14,gcc,clang.cpp:19:8: note: declaration of ‘struct foo<T>’
 struct foo
        ^~~

The question, as usual, is: who's right? clang++ or g++?

I also observe that the disagreement is connected with the presence of template parameter.

Indeed both clang++ and g++ compile the following code

struct foo
 { void bar (); };

using fooAlias = foo;

void fooAlias::bar ()
 { }

int main ()
 {
 }