c++: context completion in lookup_template_class [PR105982]

c++: context completion in lookup_template_class [PR105982]

The below testcase demonstrates that completion of the substituted
context during lookup_template_class can end up registering the desired
specialization for us in more cases than r13-1045-gcb7fd1ea85feea
anticipated.  In particular this can happen for a non-dependent
specialization of a nested class as well.

For this testcase, during overload resolution with A's guides, we
substitute the deduced argument T=int into the TYPENAME_TYPE B::C,
during which we call lookup_template_class for A<T>::B with T=int,
which completes A<int> for the first time, which recursively registers
the desired specialization of B already.  The parent call to
lookup_template_class then tries to register the same specialization,
triggering an ICE.

This patch fixes this by making lookup_template_class determine more
directly whether we need to recheck the specializations table after
completion of the context -- when and only when the call to complete_type
had an effect.

	PR c++/105982

gcc/cp/ChangeLog:

	* pt.cc (lookup_template_class): After calling complete_type for
	the substituted context, check the table again iff the type was
	previously incomplete and complete_type made it complete.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction111.C: New test.