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

[Jason Merrill]

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.

Jason