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

[Marek Polacek]

[dcl.type.auto.deduct]/5 "If the placeholder-type-specifier is of the
form type-constraintopt decltype(auto), T shall be the placeholder alone."

So, only plain decltype(auto) is allowed.  But we aren't diagnosing it in
function declarations, because do_auto_deduction is never called for those.

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

2019-09-08  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..b1777730934 100644
--- gcc/cp/decl.c
+++ gcc/cp/decl.c
@@ -11560,6 +11560,14 @@ grokdeclarator (const cp_declarator *declarator,
                                      "cannot have deduced return type");
                            virtualp = false;
                          }
+                       else if (is_auto (auto_node)
+                                && AUTO_IS_DECLTYPE (auto_node)
+                                && type != auto_node)
+                         {
+                           error_at (typespec_loc, "%qT as type rather than "
+                                     "plain %<decltype(auto)%>", type);
+                           return error_mark_node;
+                         }
                      }
                    else if (!is_auto (type) && sfk != sfk_conversion)
                      {
@@ -11580,6 +11588,16 @@ grokdeclarator (const cp_declarator *declarator,
                                    "invalid use of %<decltype(auto)%>");
                        return error_mark_node;
                      }
+                   else if (tree a = type_uses_auto (late_return_type))
+                     {
+                       if (AUTO_IS_DECLTYPE (a) && a != late_return_type)
+                         {
+                           error_at (typespec_loc, "%qT as type rather than "
+                                     "plain %<decltype(auto)%>",
+                                     late_return_type);
+                           return error_mark_node;
+                         }
+                     }
                    tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node);
                    if (!tmpl)
                      if (tree late_auto = type_uses_auto (late_return_type))
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" }
+};