c++: Abbreviated function template return type [PR92187]

When updating an auto return type of an abbreviated function template in
splice_late_return_type, we should also propagate PLACEHOLDER_TYPE_CONSTRAINTS
(and cv-qualifiers) of the original auto node.

gcc/cp/ChangeLog:

	PR c++/92187
	* pt.c (splice_late_return_type): Propagate cv-qualifiers and
	PLACEHOLDER_TYPE_CONSTRAINTS from the original auto node to the new one.

gcc/testsuite/ChangeLog:

	PR c++/92187
	* g++.dg/concepts/abbrev5.C: New test.
	* g++.dg/concepts/abbrev6.C: New test.

gcc/cp/ChangeLog

+2020-04-18  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/92187
+	* pt.c (splice_late_return_type): Propagate cv-qualifiers and
+	PLACEHOLDER_TYPE_CONSTRAINTS from the original auto node to the new one.
+
 2020-04-17  Patrick Palka  <ppalka@redhat.com>
 
 	PR c++/94483

gcc/cp/pt.c

@@ -29032,10 +29032,17 @@ splice_late_return_type (tree type, tree late_return_type)
     {
       tree idx = get_template_parm_index (*auto_node);
       if (TEMPLATE_PARM_LEVEL (idx) <= processing_template_decl)
-	/* In an abbreviated function template we didn't know we were dealing
-	   with a function template when we saw the auto return type, so update
-	   it to have the correct level.  */
-	*auto_node = make_auto_1 (TYPE_IDENTIFIER (*auto_node), true);
+	{
+	  /* In an abbreviated function template we didn't know we were dealing
+	     with a function template when we saw the auto return type, so update
+	     it to have the correct level.  */
+	  tree new_auto = make_auto_1 (TYPE_IDENTIFIER (*auto_node), false);
+	  PLACEHOLDER_TYPE_CONSTRAINTS (new_auto)
+	    = PLACEHOLDER_TYPE_CONSTRAINTS (*auto_node);
+	  TYPE_CANONICAL (new_auto) = canonical_type_parameter (new_auto);
+	  new_auto = cp_build_qualified_type (new_auto, TYPE_QUALS (*auto_node));
+	  *auto_node = new_auto;
+	}
     }
   return type;
 }

gcc/testsuite/ChangeLog

+2020-04-18  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/92187
+	* g++.dg/concepts/abbrev5.C: New test.
+	* g++.dg/concepts/abbrev6.C: New test.
+
 2020-04-17  Jeff Law  <law@redhat.com>
 
 	PR rtl-optimization/90275

gcc/testsuite/g++.dg/concepts/abbrev5.C

+// PR c++/92187
+// { dg-do compile { target concepts } }
+
+template <typename>
+concept C = false;
+
+C auto f(auto)
+{
+  return 42; // { dg-error "deduced return type" }
+}
+
+void foo()
+{
+  f(0);
+}

gcc/testsuite/g++.dg/concepts/abbrev6.C

+// { dg-do compile { target concepts } }
+
+const auto &f(auto)
+{
+  static int n;
+  return n;
+}
+
+void foo()
+{
+  f(5) = 0; // { dg-error "read-only" }
+}