All versions of GCC struggle with a template that has the default type in a definition

What the standard says ([1] page 350):

The set of default template-arguments available for use with a
template declaration or definition is obtained by merging the default
arguments from the definition (if in scope) and all declarations in
scope in the same way default function arguments are (8.3.6). [
Example:

template<class T1, class T2 = int> class A;
template<class T1 = int, class T2> class A;
is equivalent to
template<class T1 = int, class T2 = int> class A;

— end example ]

So GCC is wrong here. It ignores default template arguments in declarations.

Not all declarations, only function template declarations. Class template declarations are okay:

#include <type_traits>

template <typename T, typename>
struct Foo;

template <typename T, typename = typename std::enable_if<1>::type>
struct Foo
{
    T t;
};

int main()
{
    Foo<int> foo;
    return 0;
}

Live example on godbolt.org

Probably it is due to the nature of how non-default arguments are deduced. In the function template they are deducted from function arguments. In the class template we have to specify them explicitly.

Anyway, I have created a bug report.