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

[Jason Merrill]

On 1/25/19 12:09 PM, Marek Polacek wrote:
> On Fri, Jan 25, 2019 at 10:55:55AM -0600, Tim Song wrote:
>> 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?
> 
> I think so, and clang++ and icc also compile the testcase fine (and we used to
> too, before r159006).

Absolutely, the conversion being ill-formed means substitution fails, 
and we reject that candidate.  I meant that we shouldn't get as far as 
type_promotes_to for an incomplete type.

Jason