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.
