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.

gcc/cp/parser.c

@@ -28530,7 +28530,20 @@ cp_parser_constraint_requires_parens (cp_parser *parser, bool lambda_p)
       case CPP_PLUS_PLUS:
       case CPP_MINUS_MINUS:
       case CPP_DOT:
+	/* Unenclosed postfix operator.  */
+	return pce_maybe_postfix;
+
       case CPP_DEREF:
+	/* A primary constraint that precedes the lambda-declarator of a
+	   lambda expression is followed by trailing return type.
+
+	      []<typename T> requires C -> void {}
+
+	   Don't try to re-parse this as a postfix expression in
+	   C++23 and later.  In C++20 ( needs to come in between but we
+	   allow it to be omitted with pedwarn.  */
+	if (lambda_p)
+	  return pce_ok;
 	/* Unenclosed postfix operator.  */
 	return pce_maybe_postfix;
    }

gcc/testsuite/g++.dg/cpp23/lambda-specifiers2.C

@@ -0,0 +1,7 @@
+// PR c++/99850
+// P1102R2 - Down with ()!
+// { dg-do compile { target c++23 } }
+
+auto l = []<auto> requires true -> void {};
+template <typename...> concept C = true;
+auto m = []<typename... Ts> requires (C<Ts> && ...) -> void {};