Re: [PATCH v3] c++: P2448 - Relaxing some constexpr restrictions [PR106649]

[Jason Merrill <jason@redhat.com>]

On 11/16/22 15:27, Jason Merrill wrote:
> On 11/16/22 11:06, Marek Polacek wrote:
>> On Wed, Nov 16, 2022 at 08:41:53AM -0500, Jason Merrill wrote:
>>> On 11/15/22 19:30, Marek Polacek wrote:
>>>> @@ -996,19 +1040,26 @@ register_constexpr_fundef (const 
>>>> constexpr_fundef &value)
>>>>      **slot = value;
>>>>    }
>>>> -/* FUN is a non-constexpr function called in a context that requires a
>>>> -   constant expression.  If it comes from a constexpr template, 
>>>> explain why
>>>> -   the instantiation isn't constexpr.  */
>>>> +/* FUN is a non-constexpr (or, with -Wno-invalid-constexpr, a 
>>>> constexpr
>>>> +   function called in a context that requires a constant expression).
>>>> +   If it comes from a constexpr template, explain why the 
>>>> instantiation
>>>> +   isn't constexpr.  */
>>>
>>> The "if it comes from a constexpr template" wording has needed an 
>>> update for
>>> a while now.
>>
>> Probably ever since r178519.  I've added "Otherwise, explain why the 
>> function
>> cannot be used in a constexpr context."  Is that acceptable?
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit15.C
>>>> @@ -0,0 +1,43 @@
>>>> +// PR c++/106649
>>>> +// P2448 - Relaxing some constexpr restrictions
>>>> +// { dg-do compile { target c++23 } }
>>>> +// { dg-options "-Winvalid-constexpr" }
>>>> +// A copy/move assignment operator for a class X that is defaulted and
>>>> +// not defined as deleted is implicitly defined when it is odr-used,
>>>> +// when it is needed for constant evaluation, or when it is explicitly
>>>> +// defaulted after its first declaration.
>>>> +// The implicitly-defined copy/move assignment operator is constexpr.
>>>> +
>>>> +struct S {
>>>> +  constexpr S() {}
>>>> +  S& operator=(const S&) = default; // #1
>>>> +  S& operator=(S&&) = default; // #2
>>>> +};
>>>> +
>>>> +struct U {
>>>> +  constexpr U& operator=(const U&) = default;
>>>> +  constexpr U& operator=(U&&) = default;
>>>> +};
>>>> +
>>>> +/* FIXME: If we only declare #1 and #2, and default them here:
>>>> +
>>>> +   S& S::operator=(const S&) = default;
>>>> +   S& S::operator=(S&&) = default;
>>>> +
>>>> +then they aren't constexpr.  This sounds like a bug:
>>>> +<https://gcc.gnu.org/PR107598>.  */
>>>
>>> As I commented on the PR, I don't think this is actually a bug, so let's
>>> omit this FIXME.
>>
>> I'm glad I didn't really attempt to "fix" it (the inform message is 
>> flawed
>> and should be improved).  Thanks for taking a look.
>>
>> Here's a version with the two comments updated.
>>
>> Ok?
> 
> OK.

Since this patch I'm seeing these failures:

FAIL: g++.dg/cpp0x/constexpr-ex1.C  -std=c++23 -fimplicit-constexpr  at 
line 91 (test for errors, line 89)
FAIL: g++.dg/cpp23/constexpr-nonlit10.C  -std=gnu++23 
-fimplicit-constexpr  (test for warnings, line 14)
FAIL: g++.dg/cpp23/constexpr-nonlit10.C  -std=gnu++23 
-fimplicit-constexpr  (test for warnings, line 20)
FAIL: g++.dg/cpp23/constexpr-nonlit11.C  -std=gnu++23 
-fimplicit-constexpr  (test for warnings, line 28)
FAIL: g++.dg/cpp23/constexpr-nonlit11.C  -std=gnu++23 
-fimplicit-constexpr  (test for warnings, line 31)
FAIL: g++.dg/cpp2a/spaceship-eq3.C  -std=c++23 -fimplicit-constexpr 
(test for excess errors)

Jason