Re: [PATCH] c++: DR2397 - auto specifier for * and & to arrays [PR100975]

[Jason Merrill <jason@redhat.com>]

On 6/29/21 6:01 PM, Marek Polacek wrote:
> On Tue, Jun 29, 2021 at 03:50:27PM -0400, Jason Merrill wrote:
>> On 6/29/21 3:25 PM, Marek Polacek wrote:
>>> --- a/gcc/testsuite/g++.dg/cpp0x/auto3.C
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/auto3.C
>>> @@ -10,7 +10,7 @@ auto x;				// { dg-error "auto" }
>>>    auto i = 42, j = 42.0;		// { dg-error "auto" }
>>>    // New CWG issue
>>
>> Let's at least update this comment to quote [dcl.type.auto.deduct]/2: "T
>> shall not be an array type".  I guess "unable to deduce" is a suitable
>> diagnostic for that error.
> 
> Fixed.
> 
>>> diff --git a/gcc/testsuite/g++.dg/diagnostic/auto1.C b/gcc/testsuite/g++.dg/diagnostic/auto1.C
>>> index ee2eefd59aa..9d9979e3fdc 100644
>>> --- a/gcc/testsuite/g++.dg/diagnostic/auto1.C
>>> +++ b/gcc/testsuite/g++.dg/diagnostic/auto1.C
>>> @@ -1,4 +1,5 @@
>>>    // PR c++/86915
>>>    // { dg-do compile { target c++17 } }
>>> +// Allowed since DR2397.
>>
>> Well, not really; any attempt to use this template should hit the same
>> problem as above of trying to do auto deduction where T is an array type.
>> Please add to the testcase to get the error.
> 
> Hmm, this
> 
> template<auto [1]> struct S { };
> static int arr[1];
> S<arr> s;
> 
> won't give an error: I think it's because we coerce the auto tparm into 'auto*'
> before deducing and so don't get the type mismatch error.  That seems to be
> in line with how 'template<int [1]>' works, though.

Ah, good point.

> So I think we don't need to change this in the patch.  Do you agree?

Yes, the patch is OK with the earlier comment tweak.

Jason