[gcc r11-8210] c++: Fix up C++23 [] <...> requires primary -> type {} parsing [PR99850]

[gcc r11-8210] c++: Fix up C++23 [] <...> requires primary -> type {} parsing [PR99850]

[Jakub Jelinek]

https://gcc.gnu.org/g:784de5292c34e287c848b382b431599b818ea76e

commit r11-8210-g784de5292c34e287c848b382b431599b818ea76e
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Apr 16 09:34:26 2021 +0200

    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.

Diff:
---
 gcc/cp/parser.c                                 | 13 +++++++++++++
 gcc/testsuite/g++.dg/cpp23/lambda-specifiers2.C |  7 +++++++
 2 files changed, 20 insertions(+)

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 940751b5f05..dfc9b8251a7 100644
--- a/gcc/cp/parser.c
+++ b/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;
    }
diff --git a/gcc/testsuite/g++.dg/cpp23/lambda-specifiers2.C b/gcc/testsuite/g++.dg/cpp23/lambda-specifiers2.C
new file mode 100644
index 00000000000..0cc69bebc64
--- /dev/null
+++ b/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 {};