c++: Fix parsing of abstract-declarator starting with ... followed by opening paren [PR115012]

The C++26 P2662R3 Pack indexing paper mentions that both GCC
and MSVC don't handle T...[10] parameter declaration when T
is a pack.  And apparently neither T...(args).
While the former will change meaning in C++26, T...(args) is still
valid even in C++26.

The following patch handles just the T...(args) case in
cp_parser_direct_declarator.

2024-06-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/115012
	* parser.cc (cp_parser_direct_declarator): Handle
	abstract declarator starting with ... followed by opening paren.

	* g++.dg/cpp0x/variadic185.C: New test.

2 files changed, 56 insertions, 2 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 7796251..3b2ad25 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -23991,7 +23991,12 @@ cp_parser_direct_declarator (cp_parser* parser,
     {
       /* Peek at the next token.  */
       token = cp_lexer_peek_token (parser->lexer);
-      if (token->type == CPP_OPEN_PAREN)
+      if (token->type == CPP_OPEN_PAREN
+	  || (first
+	      && dcl_kind != CP_PARSER_DECLARATOR_NAMED
+	      && token->type == CPP_ELLIPSIS
+	      && cxx_dialect > cxx98
+	      && cp_lexer_nth_token_is (parser->lexer, 2, CPP_OPEN_PAREN)))
 	{
 	  /* This is either a parameter-declaration-clause, or a
 	     parenthesized declarator. When we know we are parsing a
@@ -24030,6 +24035,11 @@ cp_parser_direct_declarator (cp_parser* parser,
 
 	     Thus again, we try a parameter-declaration-clause, and if
 	     that fails, we back out and return.  */
+	  bool pack_expansion_p = token->type == CPP_ELLIPSIS;
+
+	  if (pack_expansion_p)
+	    /* Consume the `...' */
+	    cp_lexer_consume_token (parser->lexer);
 
 	  if (!first || dcl_kind != CP_PARSER_DECLARATOR_NAMED)
 	    {
@@ -24173,6 +24183,7 @@ cp_parser_direct_declarator (cp_parser* parser,
 						     attrs,
 						     parens_loc);
 		  declarator->attributes = gnu_attrs;
+		  declarator->parameter_pack_p |= pack_expansion_p;
 		  /* Any subsequent parameter lists are to do with
 		     return type, so are not those of the declared
 		     function.  */
@@ -24196,7 +24207,7 @@ cp_parser_direct_declarator (cp_parser* parser,
 
 	  /* If this is the first, we can try a parenthesized
 	     declarator.  */
-	  if (first)
+	  if (first && !pack_expansion_p)
 	    {
 	      bool saved_in_type_id_in_expr_p;
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic185.C b/gcc/testsuite/g++.dg/cpp0x/variadic185.C
new file mode 100644
index 0000000..2c04afe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/variadic185.C
@@ -0,0 +1,43 @@
+// PR c++/115012
+// { dg-do compile { target { c++11 } } }
+// { dg-final { scan-assembler "_Z3fooIJidEEvDpFT_iE" } }
+// { dg-final { scan-assembler "_Z3barIiEvPFT_iE" } }
+// { dg-final { scan-assembler "_Z3bazIJidEEvDpFT_iE" } }
+
+template <typename ...T>
+void
+foo (T... x (int))
+{
+}
+
+template <typename T>
+void
+bar (T (int))
+{
+}
+
+template <typename ...T>
+void
+baz (T... (int))
+{
+}
+
+int
+f1 (int)
+{
+  return 0;
+}
+
+double
+f2 (int)
+{
+  return 0;
+}
+
+void
+corge ()
+{
+  foo <int, double> (f1, f2);
+  bar <int> (f1);
+  baz <int, double> (f1, f2);
+}