Re: C++ PATCH for c++/67012, c++/86942, detect invalid cases with function return type deduction

[Jason Merrill <jason@redhat.com>]

On Fri, Aug 17, 2018 at 2:17 PM, Marek Polacek <polacek@redhat.com> wrote:
> As I promised in <https://gcc.gnu.org/ml/gcc-patches/2018-08/msg00908.html>,
> this patch fixes a couple of invalid cases we weren't detecting.  It's got
> testcases from two PRs and another case I found out; they're intertwined so
> I think it makes sense to fix them in one go.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2018-08-16  Marek Polacek  <polacek@redhat.com>
>
>         PR c++/86942
>         PR c++/67012
>         * decl.c (grokdeclarator): Disallow functions with trailing return
>         type with decltype(auto) as its type.  Also check the function if
>         it's inner declarator doesn't exist.
>
>         * g++.dg/cpp0x/auto52.C: New test.
>         * g++.dg/cpp1y/auto-fn52.C: New test.
>         * g++.dg/cpp1y/auto-fn53.C: New test.
>         * g++.dg/cpp1y/auto-fn54.C: New test.
>
> diff --git gcc/cp/decl.c gcc/cp/decl.c
> index fa58bc4d2b3..8261f8e30e5 100644
> --- gcc/cp/decl.c
> +++ gcc/cp/decl.c
> @@ -11238,7 +11238,10 @@ grokdeclarator (const cp_declarator *declarator,
>
>             /* Handle a late-specified return type.  */
>             tree late_return_type = declarator->u.function.late_return_type;
> -           if (funcdecl_p)
> +           if (funcdecl_p
> +               /* This is the case e.g. for
> +                  using T = auto () -> int.  */
> +               || inner_declarator == NULL)

Hmm, checking funcdecl_p here seems just wrong; these errors should be
the same regardless of whether this is declaring a function.  What
breaks if we just remove this condition?  The deduction guide errors
will need to be adjusted to handle the abstract declarator case, but
that looks like the only spot that would need fixing.

>               {
>                 if (tree auto_node = type_uses_auto (type))
>                   {
> @@ -11270,6 +11273,18 @@ grokdeclarator (const cp_declarator *declarator,
>                                name, type);
>                         return error_mark_node;
>                       }
> +                   else if (is_auto (type)
> +                            && (TYPE_IDENTIFIER (type)
> +                                == decltype_auto_identifier))

I think you want AUTO_IS_DECLTYPE here.

> +                     {
> +                       if (funcdecl_p)
> +                         error ("%qs function with trailing return type has "
> +                                "%<decltype(auto)%> as its type rather than "
> +                                "plain %<auto%>", name);
> +                       else
> +                         error ("invalid use of %<decltype(auto)%>");
> +                       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/cpp0x/auto52.C gcc/testsuite/g++.dg/cpp0x/auto52.C
> index e69de29bb2d..9bfe7c754b5 100644
> --- gcc/testsuite/g++.dg/cpp0x/auto52.C
> +++ gcc/testsuite/g++.dg/cpp0x/auto52.C
> @@ -0,0 +1,6 @@
> +// PR c++/86942
> +// { dg-do compile { target c++11 } }
> +
> +using T = auto() -> int;
> +using U = void() -> int; // { dg-error "function with trailing return type not declared with .auto." }
> +using W = auto(); // { dg-error "invalid use of .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" }
> diff --git gcc/testsuite/g++.dg/cpp1y/auto-fn53.C gcc/testsuite/g++.dg/cpp1y/auto-fn53.C
> index e69de29bb2d..720aeeb215d 100644
> --- gcc/testsuite/g++.dg/cpp1y/auto-fn53.C
> +++ gcc/testsuite/g++.dg/cpp1y/auto-fn53.C
> @@ -0,0 +1,4 @@
> +// PR c++/86942
> +// { dg-do compile { target c++14 } }
> +
> +using T = decltype(auto) () -> int; // { dg-error "invalid use of" }
> diff --git gcc/testsuite/g++.dg/cpp1y/auto-fn54.C gcc/testsuite/g++.dg/cpp1y/auto-fn54.C
> index e69de29bb2d..f3391ddfd75 100644
> --- gcc/testsuite/g++.dg/cpp1y/auto-fn54.C
> +++ gcc/testsuite/g++.dg/cpp1y/auto-fn54.C
> @@ -0,0 +1,3 @@
> +// { dg-do compile { target c++14 } }
> +
> +using T = int () -> decltype(auto); // { dg-error "function with trailing return type not declared with .auto." }