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

[trippels at gcc dot gnu.org]

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

--- Comment #4 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
I'm not sure this is a compiler bug at all.

Even clang warns:

foo.ii:10:32: warning: class template partial specialization contains a
template parameter that cannot be deduced; this partial specialization will
never be used
  template <typename U> struct predicate<typename outer<T>::template inner<U>>
{
                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.ii:10:22: note: non-deducible template parameter 'U'
  template <typename U> struct predicate<typename outer<T>::template inner<U>>
{
                     ^
1 warning generated.