c++: alias of decltype(lambda) is opaque [PR116714, PR107390]

c++: alias of decltype(lambda) is opaque [PR116714, PR107390]

Here for

  using type = decltype([]{});
  static_assert(is_same_v<type, type>);

we strip the alias ahead of time during template argument coercion
which effectively transforms the template-id into

  is_same_v<decltype([]{}), decltype([]{})>

which is wrong because later substitution into the template-id will
produce two new lambdas with distinct types and cause is_same_v to
return false.

This demonstrates that such aliases should be considered opaque (a
notion that we recently introduced in r15-2331-g523836716137d0).
(An alternative solution might be to consider memoizing lambda-expr
substitution rather than always producing a new lambda, but this is
much simpler.)

	PR c++/116714
	PR c++/107390

gcc/cp/ChangeLog:

	* pt.cc (dependent_opaque_alias_p): Also return true for a
	decltype(lambda) alias.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-uneval18.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>