c++: Fix up C++23 [] <...> requires primary -> type {} parsing [PR99850]

c++: Fix up C++23 [] <...> requires primary -> type {} parsing [PR99850]

The requires clause parsing has code to suggest users wrapping
non-primary expressions in (), so if it e.g. parses a primary expression
and sees it is followed by ++, --, ., ( or -> among other things it
will try to reparse it as assignment expression or what and if that works
suggests wrapping it inside of parens.
When it is requires-clause that is after <typename T> etc. it already
has an exception from that as ( can occur in valid C++20 expression there
- starting the parameters of the lambda.
In C++23 another case can occur, as the parameters with the ()s can be
omitted, requires C can be followed immediately by -> which starts a
trailing return type.  Even in that case, we don't want to parse that
as C->...

2021-04-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99850
	* parser.c (cp_parser_constraint_requires_parens) <case CPP_DEREF>:
	If lambda_p, return pce_ok instead of pce_maybe_postfix.

	* g++.dg/cpp23/lambda-specifiers2.C: New test.