Re: C++ PATCH for c++/84374 - diagnose invalid uses of decltype(auto)

[Marek Polacek]

On Mon, Sep 09, 2019 at 04:59:57PM -0400, Jason Merrill wrote:
> Maybe check this in one place, after splice_late_return_type?

That works!  So like this?

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

2019-09-09  Marek Polacek  <pola...@redhat.com>

        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.

diff --git gcc/cp/decl.c gcc/cp/decl.c
index 88e2c3beb2b..dfcd7b16e6e 100644
--- gcc/cp/decl.c
+++ 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
diff --git gcc/testsuite/g++.dg/cpp1y/auto-fn57.C 
gcc/testsuite/g++.dg/cpp1y/auto-fn57.C
new file mode 100644
index 00000000000..e58df187df5
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1y/auto-fn57.C
@@ -0,0 +1,18 @@
+// 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" }
+};