c++: Fix default argument parsing in non-comma variadic methods [PR121539]

While the non-comma variadic functions/methods were deprecated in C++26,
they are still valid and they are valid without deprecation in C++98 to
C++23.
We parse default arguments followed by ...) outside of classes or
for out of class definitions of methods, but I think since C++11 support
in GCC 4.9 or so we consider ... to be a part of a default argument and
error on it.
I think a default argument can't validly contain a pack expansion
that ends the expression with ..., so I think we can simply handle
...) if at depth 0 as not part of the default argument.

2025-08-15  Jakub Jelinek  <jakub@redhat.com>

	PR c++/121539
	* parser.cc (cp_parser_cache_defarg): Set done to true for
	CPP_ELLIPSIS followed by CPP_CLOSE_PAREN in !nsdmi at depth 0.

	* g++.dg/parse/defarg20.C: New test.

2 files changed, 84 insertions, 1 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index d733b72..d66b658 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -37174,7 +37174,11 @@ cp_parser_cache_defarg (cp_parser *parser, bool nsdmi)
 	case CPP_CLOSE_SQUARE:
 	  if (depth == 0
 	      /* Handle correctly int n = sizeof ... ( p );  */
-	      && token->type != CPP_ELLIPSIS)
+	      && (token->type != CPP_ELLIPSIS
+		  /* For int n = 42 ...) handle ... as variadic arguments.  */
+		  || (!nsdmi
+		      && cp_lexer_nth_token_is (parser->lexer, 2,
+						CPP_CLOSE_PAREN))))
 	    done = true;
 	  /* Update DEPTH, if necessary.  */
 	  else if (token->type == CPP_CLOSE_PAREN
diff --git a/gcc/testsuite/g++.dg/parse/defarg20.C b/gcc/testsuite/g++.dg/parse/defarg20.C
new file mode 100644
index 0000000..577c093
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/defarg20.C
@@ -0,0 +1,79 @@
+// PR c++/121539
+// { dg-do run }
+
+#include <cstdarg>
+
+#if __cplusplus >= 201103L
+#define I {}
+#else
+#define I 0
+#endif
+
+void foo (int = I...);				// { dg-warning "omission of ',' before varargs '...' is deprecated" "" { target c++26 } }
+struct S {
+  void foo (int = I...);			// { dg-warning "omission of ',' before varargs '...' is deprecated" "" { target c++26 } }
+  void bar (int...);				// { dg-warning "omission of ',' before varargs '...' is deprecated" "" { target c++26 } }
+};
+
+void
+foo (int a, ...)
+{
+  if (a == 42)
+    {
+      va_list ap;
+      va_start (ap, a);
+      if (va_arg (ap, double) != 15.0)
+	__builtin_abort ();
+      va_end (ap);
+    }
+  else if (a != 0)
+    __builtin_abort ();
+}
+
+void
+S::foo (int a, ...)
+{
+  if (a == 43)
+    {
+      va_list ap;
+      va_start (ap, a);
+      if (va_arg (ap, double) != 16.0)
+	__builtin_abort ();
+      va_end (ap);
+    }
+  else if (a != 0)
+    __builtin_abort ();
+}
+
+void
+S::bar (int a = I...)				// { dg-warning "omission of ',' before varargs '...' is deprecated" "" { target c++26 } }
+{
+  if (a == 44)
+    {
+      va_list ap;
+      va_start (ap, a);
+      if (va_arg (ap, double) != 17.0)
+	__builtin_abort ();
+      va_end (ap);
+    }
+  else if (a != 0)
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  S s;
+  foo ();
+  foo (0);
+  foo (42, 15.0);
+  foo (42, 15.0, 128LL);
+  s.foo ();
+  s.foo (0);
+  s.foo (43, 16.0);
+  s.foo (43, 16.0, 129ULL);
+  s.bar ();
+  s.bar (0);
+  s.bar (44, 17.0);
+  s.bar (44, 17.0, 130L);
+}