C++ PATCH for c++/89024 - ICE with incomplete enum type

[Tim Song]

On Thu, Jan 24, 2019 at 4:14 PM Jason Merrill <jason@redhat.com> wrote:
>
> On 1/24/19 2:16 PM, Marek Polacek wrote:
> > This test ICEs since r159006 which added
> >
> >     type = ENUM_UNDERLYING_TYPE (type);
> >
> > to type_promotes_to.  In this test ENUM_UNDERLYING_TYPE is null because we
> > haven't yet parsed '}' of the enum and the underlying type isn't fixed, and
> > so checking TYPE_UNSIGNED crashed.
> >
> > I've added some checks to the test to see if the types seem to be OK; clang++
> > agrees.
> >
> > Bootstrapped/regtested on x86_64-linux, ok for trunk/8/7?
> >
> > 2019-01-24  Marek Polacek  <polacek@redhat.com>
> >
> >       PR c++/89024 - ICE with incomplete enum type.
> >       * cvt.c (type_promotes_to): Check if prom is non-null.
>
> 9.6/6: An enumeration whose underlying type is not fixed is an
> incomplete type from its point of declaration to immediately after the
> closing } of its enum-specifier, at which point it becomes a complete type.
>
> So the conversion is ill-formed.
>
> Jason

But the conversion in the example (in
decltype(__test_aux<_To1>(declval<_From1>())))
is in a SFINAE context, so shouldn't it gracefully fall back to the
`(...)` overload?