C++ PATCH for c++/67012, diagnose decltype(auto) f() -> int;

[Marek Polacek]

I think that [dcl.fct] says that a function with trailing-return-type should
have plain auto as the return type, so this test should not compile.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-08-15  Marek Polacek  <pola...@redhat.com>

        PR c++/67012
        * decl.c (grokdeclarator): Disallow functions with trailing return
        type with decltype(auto) as its type.

        * g++.dg/cpp1y/auto-fn52.C: New test.

diff --git gcc/cp/decl.c gcc/cp/decl.c
index fa58bc4d2b3..05f4991c184 100644
--- gcc/cp/decl.c
+++ gcc/cp/decl.c
@@ -11263,7 +11263,10 @@ grokdeclarator (const cp_declarator *declarator,
                            virtualp = false;
                          }
                      }
-                   else if (!is_auto (type) && sfk != sfk_conversion)
+                   else if ((!is_auto (type) && sfk != sfk_conversion)
+                            || (is_auto (type)
+                                && (TYPE_IDENTIFIER (type)
+                                    == decltype_auto_identifier)))
                      {
                        error ("%qs function with trailing return type has"
                               " %qT as its type rather than plain %<auto%>",
diff --git gcc/testsuite/g++.dg/cpp1y/auto-fn52.C 
gcc/testsuite/g++.dg/cpp1y/auto-fn52.C
index e69de29bb2d..e239bc27dc2 100644
--- gcc/testsuite/g++.dg/cpp1y/auto-fn52.C
+++ gcc/testsuite/g++.dg/cpp1y/auto-fn52.C
@@ -0,0 +1,4 @@
+// PR c++/67012
+// { dg-do compile { target c++14 } }
+
+decltype(auto) f() -> int; // { dg-error "function with trailing return type 
has" }