[PATCH] c++: Diagnose when "requires" is used instead of "requires requires" [PR94306]

[Patrick Palka via Gcc-patches]

This adds support to detect and recover from the case where an opening brace
immediately follows the start of a requires-clause.  So rather than emitting the
error

  error: expected primary-expression before '{' token

followed by a slew of irrevelant errors, we now assume the user had intended to
write "requires requires {" and diagnose and recover accordingly.

Tested on x86_64-pc-linux-gnu, does this look OK?

gcc/cp/ChangeLog:

        PR c++/94306
        * parser.c (cp_parser_requires_clause_opt): Diagnose and recover from
        "requires {" when "requires requires {" was probably intended.

gcc/testsuite/ChangeLog:

        PR c++/94306
        * g++.dg/concepts/diagnostic8.C: New test.
---
 gcc/cp/parser.c                             | 17 ++++++++++++++++-
 gcc/testsuite/g++.dg/concepts/diagnostic8.C |  6 ++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/concepts/diagnostic8.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 05363653691..73c2c2cb010 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -27639,7 +27639,22 @@ cp_parser_requires_clause_opt (cp_parser *parser, bool 
lambda_p)
        }
       return NULL_TREE;
     }
-  cp_lexer_consume_token (parser->lexer);
+
+  cp_token *tok2 = cp_lexer_peek_nth_token (parser->lexer, 2);
+  if (tok2->type == CPP_OPEN_BRACE)
+    {
+      /* An opening brace following the start of a requires-clause is
+        ill-formed; the user likely forgot the second `requires' that
+        would start a requires-expression.  */
+      gcc_rich_location richloc (tok2->location);
+      richloc.add_fixit_insert_before (" requires");
+      error_at (&richloc, "missing additional %<requires%> to start "
+               "a requires-expression");
+      /* Don't consume the `requires', so that it's reused as the start of a
+        requires-expression.  */
+    }
+  else
+    cp_lexer_consume_token (parser->lexer);
 
   if (!flag_concepts_ts)
     return cp_parser_requires_clause_expression (parser, lambda_p);
diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic8.C 
b/gcc/testsuite/g++.dg/concepts/diagnostic8.C
new file mode 100644
index 00000000000..70d7e4a9cc1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/diagnostic8.C
@@ -0,0 +1,6 @@
+// PR c++/94306
+// { dg-do compile { target c++2a } }
+
+template<typename T> struct S { };
+template<typename T> requires { typename T::type; } struct S<T> { };
+// { dg-error "missing additional .requires." "" { target *-*-* } .-1 }
-- 
2.26.0.rc1.11.g30e9940356