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[meta.rel] is_­nothrow_­convertible does not clearly require nothrow destruction LWG 3400, LWG 2116 #3576

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[meta.rel] is_­nothrow_­convertible does not clearly require nothrow destruction LWG 3400, LWG 2116#3576
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lwgIssue must be reviewed by LWG.not-editorialIssue is not deemed editorial; the editorial issue is kept open for tracking.

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@frederick-vs-ja
frederick-vs-ja
opened on Dec 22, 2019

By @Dani-Hub , is_nothrow_convertible considers destruction of the converted object (if To is an object type) or the temporary (if To is a reference type and the temporary to bind would be created) by design. And known implementations (msvc, libc++, libstdc++) behave correctly.

For example, given a type Weird defined as

struct Weird {
  Weird(int) noexcept {}
  ~Weird() noexcept(false) {}
};

Then std::is_nothrow_convertible_v<int, Weird> is false.

However, I suspect that wording in [meta.rel]/5 only requires the well-formedness, and whether the destruction after the initialization of To is considered as part of the conversion is not mentioned.
So "the conversion, as defined by is_­convertible" in the specification of is_nothrow_convertible might not be precisely defined, and may lead to unintended comprehension, e.g. std::is_nothrow_convertible_v<int, Weird> can be true since the converting constructor itself is noexcept, although destruction of Weird is not.

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  1. jensmaurer commented on Dec 22, 2019

    @jensmaurer
    jensmaurer
    on Dec 22, 2019
    Member

    Could you please be a bit more explicit why you believe a change in this area would be editorial, i.e. the desired effects are already implied by other normative wording?

  2. frederick-vs-ja commented on Dec 23, 2019

    @frederick-vs-ja
    frederick-vs-ja
    on Dec 23, 2019
    ContributorAuthor

    It looks like an LWG issue and is related to LWG 2116 IMO, so I mailed to LWG at first.
    But Daniel Krügler replied me with

    Since existing compilers do not violate that expectation there is IMO
    currently no reason to open an issue for this.

    It seems that the issue can be treated as editorial.

  3. jensmaurer commented on Dec 23, 2019

    @jensmaurer
    jensmaurer
    on Dec 23, 2019
    Member

    Just because no implementation disagrees doesn't mean an issue is editorial. The question is what the normative words of the standard say (and what they possibly neglect to say).

  4. jensmaurer
    added
    decision-requiredA decision of the editorial group (or the Project Editor) is required.
    on Dec 26, 2019

  5. jensmaurer commented on Feb 10, 2020

    @jensmaurer
    jensmaurer
    on Feb 10, 2020
    Member

    Editorial meeting: Should become an LWG issue.

  6. jensmaurer
    added
    lwgIssue must be reviewed by LWG.
    and removed
    decision-requiredA decision of the editorial group (or the Project Editor) is required.
    on Feb 10, 2020

  7. Dani-Hub commented on Feb 22, 2020

    @Dani-Hub
    Dani-Hub
    on Feb 22, 2020
    Member

    A corresponding library issues exists now: LWG 3400

  8. jensmaurer
    changed the title [-][meta.rel] Wording for is_­nothrow_­convertible might not cleary enough to indicate the destruction[/-] [+][meta.rel] Wording for is_­nothrow_­convertible might not cleary enough to indicate the destruction LWG 3400, LWG 2116[/+] on Feb 5, 2021
  9. jensmaurer
    added
    not-editorialIssue is not deemed editorial; the editorial issue is kept open for tracking.
    on Feb 5, 2021
  10. jensmaurer
    changed the title [-][meta.rel] Wording for is_­nothrow_­convertible might not cleary enough to indicate the destruction LWG 3400, LWG 2116[/-] [+][meta.rel] is_­nothrow_­convertible does not clearly require nothrow destruction LWG 3400, LWG 2116[/+] on Feb 5, 2021
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