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Section: 126.96.36.199 [variant.ctor] Status: New Submitter: Louis Dionne Opened: 2019-06-04 Last modified: 2020-09-06
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In 188.8.131.52 [variant.ctor] p5, we say:
Remarks: This function shall be constexpr if and only if the value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function. […]
First of all, I find it confusing that we say "This function shall be constexpr if […]", when the declaration of the function clearly has the constexpr keyword on it unconditionally. Instead, I would use the wording "This function shall be usable in a constexpr context if […]".Secondly, I think we shouldn't be using if-and-only-if since it restricts whether implementations can be constexpr-friendly as an extension. Instead, it seems better to just say "if". Finally, I think the condition under which the function must be constexpr-friendly is not something we can test for because it says "value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function", which doesn't imply the value initialization can actually be be performed inside a constexpr context (for example the default constructor could be constexpr friendly but not marked with the constexpr keyword).
[2017-06-17, Tim Song comments]
This issue is related to LWG 2833.
[2019-07 Issue Prioritization]
Priority to 2 after discussion on the reflector.Previous resolution from Daniel [SUPERSEDED]:
This wording is relative to N4810.
Modify 184.108.40.206 [variant.ctor] as indicated:constexpr variant() noexcept(see below);
-2- Effects: […]-3- Ensures: […] -4- Throws: […] -5- Remarks: This function shall be
constexpr if and only if the value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function. […]
[2020-06-08 Nina Dinka Ranns comments]
The revised wording provided by LWG 2833 should resolve this issue as well.