C++ PATCH for c++/34758 (unclear error message about circular dependency in default arg)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 5953e35..499ed03 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -5746,10 +5746,17 @@  cxx_type_promotes_to (tree type)
 }
 
 /* ARG is a default argument expression being passed to a parameter of
-   the indicated TYPE, which is a parameter to FN.  Do any required
-   conversions.  Return the converted value.  */
+   the indicated TYPE, which is a parameter to FN.  PARMNUM is the
+   zero-based argument number.  Do any required conversions.  Return
+   the converted value.  */
 
 static GTY(()) VEC(tree,gc) *default_arg_context;
+void
+push_defarg_context (tree fn)
+{ VEC_safe_push (tree, gc, default_arg_context, fn); }
+void
+pop_defarg_context (void)
+{ VEC_pop (tree, default_arg_context); }
 
 tree
 convert_default_arg (tree type, tree arg, tree fn, int parmnum)
@@ -5757,15 +5764,8 @@  convert_default_arg (tree type, tree arg, tree fn, int parmnum)
   int i;
   tree t;
 
-  /* If the ARG is an unparsed default argument expression, the
-     conversion cannot be performed.  */
-  if (TREE_CODE (arg) == DEFAULT_ARG)
-    {
-      error ("the default argument for parameter %d of %qD has "
-	     "not yet been parsed",
-	     parmnum, fn);
-      return error_mark_node;
-    }
+  /* See through clones.  */
+  fn = DECL_ORIGIN (fn);
 
   /* Detect recursion.  */
   FOR_EACH_VEC_ELT (tree, default_arg_context, i, t)
@@ -5774,7 +5774,17 @@  convert_default_arg (tree type, tree arg, tree fn, int parmnum)
 	error ("recursive evaluation of default argument for %q#D", fn);
 	return error_mark_node;
       }
-  VEC_safe_push (tree, gc, default_arg_context, fn);
+
+  /* If the ARG is an unparsed default argument expression, the
+     conversion cannot be performed.  */
+  if (TREE_CODE (arg) == DEFAULT_ARG)
+    {
+      error ("call to %qD uses the default argument for parameter %P, which "
+	     "is not yet defined", fn, parmnum);
+      return error_mark_node;
+    }
+
+  push_defarg_context (fn);
 
   if (fn && DECL_TEMPLATE_INFO (fn))
     arg = tsubst_default_argument (fn, type, arg);
@@ -5814,7 +5824,7 @@  convert_default_arg (tree type, tree arg, tree fn, int parmnum)
     }
   pop_deferring_access_checks();
 
-  VEC_pop (tree, default_arg_context);
+  pop_defarg_context ();
 
   return arg;
 }
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 4b49046..6ef6e6e 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -4638,6 +4638,8 @@  extern bool can_convert				(tree, tree);
 extern bool can_convert_arg			(tree, tree, tree, int);
 extern bool can_convert_arg_bad			(tree, tree, tree, int);
 extern bool enforce_access			(tree, tree, tree);
+extern void push_defarg_context			(tree);
+extern void pop_defarg_context			(void);
 extern tree convert_default_arg			(tree, tree, tree, int);
 extern tree convert_arg_to_ellipsis		(tree);
 extern tree build_x_va_arg			(tree, tree);
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 4260f6d..a9fd201 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -20521,6 +20521,8 @@  cp_parser_late_parsing_default_args (cp_parser *parser, tree fn)
   saved_local_variables_forbidden_p = parser->local_variables_forbidden_p;
   parser->local_variables_forbidden_p = true;
 
+  push_defarg_context (fn);
+
   for (parm = TYPE_ARG_TYPES (TREE_TYPE (fn)),
 	 parmdecl = DECL_ARGUMENTS (fn);
        parm && parm != void_list_node;
@@ -20579,6 +20581,8 @@  cp_parser_late_parsing_default_args (cp_parser *parser, tree fn)
       cp_parser_pop_lexer (parser);
     }
 
+  pop_defarg_context ();
+
   /* Make sure no default arg is missing.  */
   check_default_args (fn);
 
diff --git a/gcc/testsuite/g++.dg/parse/pr34758.C b/gcc/testsuite/g++.dg/parse/pr34758.C
new file mode 100644
index 0000000..d694853
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/pr34758.C
@@ -0,0 +1,28 @@ 
+// PR 34758 Bad diagnostic for circular dependency in constructor default argument
+// { dg-do compile }
+// { dg-options "" }
+struct A
+{
+  A (const A& = A()); // { dg-error "recursive evaluation of default argument" }
+};
+
+
+struct S {
+  S(const S& = f()); // { dg-error "default argument\[^\n\]*which is not yet defined" }
+  static const S& f(int i = 3);
+};
+
+struct J {
+  J(const J& = f(2)); // { dg-error "default argument.*which is not yet defined" }
+  static const J& f(int i = 3, int j = 4);
+};
+
+struct Z {
+  Z(const Z& = f(4));
+  static const Z& f(int i = 3);
+};
+
+struct X {
+  X(const X& = g());
+  static const X& g(void);
+};