PR c++/84374 - diagnose invalid uses of decltype(auto).

	* decl.c (grokdeclarator): Diagnose wrong usage of decltype(auto) in
	a function declaration.

	* g++.dg/cpp1y/auto-fn57.C: New test.

From-SVN: r275557

gcc/cp/ChangeLog

+2019-09-09  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/84374 - diagnose invalid uses of decltype(auto).
+	* decl.c (grokdeclarator): Diagnose wrong usage of decltype(auto) in
+	a function declaration.
+
 2019-09-06  Nathan Sidwell  <nathan@acm.org>
 
 	PR c++/91125

gcc/cp/decl.c

@@ -11681,6 +11681,16 @@ grokdeclarator (const cp_declarator *declarator,
 			  "allowed");
 		return error_mark_node;
 	      }
+	    /* Only plain decltype(auto) is allowed.  */
+	    if (tree a = type_uses_auto (type))
+	      {
+		if (AUTO_IS_DECLTYPE (a) && a != type)
+		  {
+		    error_at (typespec_loc, "%qT as type rather than "
+			      "plain %<decltype(auto)%>", type);
+		    return error_mark_node;
+		  }
+	      }
 
 	    if (ctype == NULL_TREE
 		&& decl_context == FIELD

gcc/testsuite/ChangeLog

+2019-09-09  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/84374 - diagnose invalid uses of decltype(auto).
+	* g++.dg/cpp1y/auto-fn57.C: New test.
+
 2019-09-09  Segher Boessenkool  <segher@kernel.crashing.org>
 
 	* gcc.target/powerpc/rlwinm-0.c: Adjust expected instruction counts.

gcc/testsuite/g++.dg/cpp1y/auto-fn57.C

+// PR c++/84374 - diagnose invalid uses of decltype(auto).
+// { dg-do compile { target c++14 } }
+
+auto l = [](auto* r)->decltype(auto)* { return r; }; // { dg-error "as type rather than plain" }
+auto m = [](auto* r)->decltype(auto)& { return *r; }; // { dg-error "as type rather than plain" }
+
+decltype(auto)* f(); // { dg-error "as type rather than plain" }
+decltype(auto)& f2(); // { dg-error "as type rather than plain" }
+decltype(auto)* f3() { return 42; } // { dg-error "as type rather than plain" }
+decltype(auto)& f4() { return 42; } // { dg-error "as type rather than plain" }
+
+
+class C {
+  decltype(auto)* g(); // { dg-error "as type rather than plain" }
+  decltype(auto)& g2(); // { dg-error "as type rather than plain" }
+  decltype(auto)* g3() { } // { dg-error "as type rather than plain" }
+  decltype(auto)& g4() { } // { dg-error "as type rather than plain" }
+};