[Bug c++/70141] [6.0 regression] template parameter not deducible in partial specialization of template inside template

["kholdstare0.0 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70141

--- Comment #7 from Alexander Kondratskiy <kholdstare0.0 at gmail dot com> ---
To add some color, maybe this is related to non-deduced contexts from
14.8.2.5p5 in the standard:

The non-deduced contexts are:
â The nested-name-specifier of a type that was specified using a qualified-id.

So I agree that this should not work for code like the following, where the T
in outer<T> is being deduced:

        template <typename T>
        struct outer
        {
                template <typename U>
                struct inner
                {

                };
        };

        // doesn't work, because non-deduced context
        template <typename T, typename U>
        void foo(typename outer<T>::template inner<U> val)
        { }

        void bar()
        {
                foo(typename outer<int>::template inner<double>{});
        }

But in the situation mentioned here, outer<T> is a concrete type, so anything
within it should be deducible. If I get rid of the template parameter on outer,
everything works fine, and the following code compiles:

        // no outer template!
        struct outer
        {
                template <typename U>
                struct inner
                {

                };
        };


        struct is_inner_for
        {
                template <typename Whatever>
                struct predicate
                {
                        static constexpr bool value = false;
                };

                template <typename U>
                struct predicate<outer::inner<U>>
                {
                        static constexpr bool value = true;
                };
        };

        static_assert(
                is_inner_for::predicate<
                        outer::inner<double>
                >::value,
                "Yay!"
        );

This simplified case where outer has no template parameter, in my mind should
not be any different from the original problem - it's just that we had a
template parameter but it was fully specified.