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

[Jason Merrill <jason@redhat.com>]

On 12/2/22 14:48, Marek Polacek wrote:
> On Fri, Nov 18, 2022 at 09:26:26PM -0500, Jason Merrill wrote:
>> 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)
> 
> Ah, sorry.  The following patch fixes those fails.
> 
> Ok?

OK, thanks.

> -- >8 --
> Some of the new tests were failing with -fimplicit-constexpr.  This
> patch adjusts the expected diagnostic.  Tested with
> 
> GXX_TESTSUITE_STDS=98,11,14,17,20,23 make check-c++ RUNTESTFLAGS="--target_board=unix\{,-fimplicit-constexpr\} dg.exp=spaceship-eq3.C"
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/constexpr-ex1.C: Adjust dg-error.
> 	* g++.dg/cpp23/constexpr-nonlit10.C: Adjust dg-warning.
> 	* g++.dg/cpp23/constexpr-nonlit11.C: Likewise.
> 	* g++.dg/cpp2a/spaceship-eq3.C: Add dg-error.
> ---
>   gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C      | 6 +++---
>   gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C | 4 ++--
>   gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C | 4 ++--
>   gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C      | 1 +
>   4 files changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C
> index 48281a47784..383d38a42d4 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C
> @@ -87,8 +87,8 @@ struct resource {
>     }
>   };
>   constexpr resource f(resource d)
> -{ return d; }                  // { dg-error "non-.constexpr." "" { target { { ! implicit_constexpr } && c++20_down } } }
> -// { dg-error "non-.constexpr." "" { target c++23 } .-2 }
> -constexpr resource d = f(9);   // { dg-message ".constexpr." "" { target { ! implicit_constexpr } } }
> +{ return d; }                  // { dg-error "non-.constexpr." "" { target { { { ! implicit_constexpr } && c++20_down } || c++11_only } } }
> +// { dg-error "non-.constexpr." "" { target { c++23 && { ! implicit_constexpr } } } .-2 }
> +constexpr resource d = f(9);   // { dg-message ".constexpr." "" { target { { ! implicit_constexpr } || c++11_only } } }
>   
>   // 4.4 floating-point constant expressions
> diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C
> index 48706f7b66e..31d4b873bbf 100644
> --- a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C
> +++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C
> @@ -11,13 +11,13 @@ struct NonLiteral {
>   // C++23: It is possible to write a constexpr function for which no
>   // invocation satisfies the requirements of a core constant expression.
>   constexpr NonLiteral
> -fn0 (int) // { dg-warning "invalid return type" }
> +fn0 (int) // { dg-warning "invalid return type" "" { target { ! implicit_constexpr } } }
>   {
>     return NonLiteral{};
>   }
>   
>   constexpr int
> -fn1 (NonLiteral) // { dg-warning "invalid type" }
> +fn1 (NonLiteral) // { dg-warning "invalid type" "" { target { ! implicit_constexpr } } }
>   {
>     return 42;
>   }
> diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C
> index a7114bc66cb..e08809f873c 100644
> --- a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C
> +++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C
> @@ -25,10 +25,10 @@ struct X {
>   struct S {
>     X x;
>     // Calls a non-constexpr constructor X::X(int).
> -  constexpr S(int i) : x(i) { } // { dg-warning "call to" }
> +  constexpr S(int i) : x(i) { } // { dg-warning "call to" "" { target { ! implicit_constexpr } } }
>     S(int, int) { }
>     // Target constructor isn't constexpr.
> -  constexpr S() : S(42, 42) { } // { dg-warning "call to" }
> +  constexpr S() : S(42, 42) { } // { dg-warning "call to" "" { target { ! implicit_constexpr } } }
>   };
>   
>   namespace N1 {
> diff --git a/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C b/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C
> index 69eaa7b9b20..246839f1f95 100644
> --- a/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C
> +++ b/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C
> @@ -9,6 +9,7 @@ struct D
>     A i;
>     bool operator==(const D& x) const = default; // { dg-error "A::operator==" "" { target c++20_down } }
>     bool operator!=(const D& z) const = default; // { dg-error "D::operator==" "" { target c++20_down } }
> +// { dg-error "called" "" { target { c++23 && implicit_constexpr } } .-1 }
>   };
>   
>   constexpr D d{A()};
> 
> base-commit: d19aa6af6634b1e97f38431ad091f3b3f12baf2f