Re: [PATCH] replace ICE with error for failed template deduction (PR 84355)

[Martin Sebor <msebor@gmail.com>]

On 02/23/2018 07:32 PM, Jason Merrill wrote:
> On Sun, Feb 18, 2018 at 11:39 PM, Jason Merrill <jason@redhat.com> wrote:
>> On Fri, Feb 16, 2018 at 4:33 PM, Martin Sebor <msebor@gmail.com> wrote:
>>> On 02/16/2018 07:04 AM, Jason Merrill wrote:
>>>>
>>>> On Thu, Feb 15, 2018 at 6:36 PM, Martin Sebor <msebor@gmail.com> wrote:
>>>>>
>>>>> A failed template deduction in template member of a template
>>>>> triggers an ICE with -std=c++17 due to what seems like
>>>>> a missing handling of invalid input.  Replacing
>>>>> the gcc_unreachable() call that causes the ICE with a return
>>>>> statement indicating the deduction failure eliminates the ICE
>>>>> and restores sane diagnostics.
>>>>
>>>>
>>>> Hmm, we really shouldn't have gotten there; that assert is checking
>>>> that when we see a TEMPLATE_*_PARM node in the template signature, it
>>>> corresponds to one of the actual parms of the template.  Sounds like
>>>> something is going wrong in build_deduction_guide.
>>>
>>>
>>> Are you suggesting that build_deduction_guide should fail somehow
>>> (it's not expected to fail right now) or that the guide it creates
>>> is wrong?
>>
>> The latter.  Maybe we're handling T wrong somehow?  We shouldn't be
>> trying to deduce it.  In fact, we probably shouldn't be trying to
>> deduce arguments for 'b' until we instantiate A.
>
> Looks like the problem is that when we substitute into the
> TEMPLATE_TYPE_PARM representing 'B' in the function, we don't touch
> CLASS_PLACEHOLDER_TEMPLATE:
>
>                     else if (tree pl = CLASS_PLACEHOLDER_TEMPLATE (t))
>                       {
>                         if (DECL_TEMPLATE_TEMPLATE_PARM_P (pl))
>                           pl = tsubst (pl, args, complain, in_decl);
>                         CLASS_PLACEHOLDER_TEMPLATE (r) = pl;
>                       }
>
> This code is failing to replace A<T>::B with A<int>::B.

I don't know what to do here/what you're suggesting.  Before
the call to tsubst() pl is a TEMPLATE_DECL of struct A<T>::B.
Calling tsubst() works and replaces the ICE with a reasonable
error but then causes an ICE in cpp1z/class-deduction19.C.
There, pl is also TEMPLATE_DECL, and I'm not sure how to
differentiate between the two at this level.

I was hoping I could fix this ICE quickly but I've spent too
much time on it with no progress so unless I'm just being
dense by missing your hint I think I'm going to have to give
up on this bug.

Martin