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CWG2784 Unclear definition of member-designator for offsetof #1616

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jensmaurer opened this issue Aug 23, 2023 · 3 comments
Open

CWG2784 Unclear definition of member-designator for offsetof #1616

jensmaurer opened this issue Aug 23, 2023 · 3 comments
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EWG Evolution paper needed An issue needs a paper to describe its solution

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@jensmaurer
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  • Is a qualified-id allowed to appear in a member-designator?
  • Is a template-id allowed to appear in a member-designator?
  • Is an offsetof expression required to be a constant expression (7.7 [expr.const])?

See CWG2784 for details.

This issue lands right on the boundary of core and library specification, so EWG is asked to weigh in first, possibly deciding to involve L(E)WG.
@jensmaurer jensmaurer added the EWG Evolution label Aug 23, 2023
@cor3ntin cor3ntin mentioned this issue Aug 23, 2023
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@hubert-reinterpretcast
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In C, it seems everything that can be applied after the . token to get at a subobject is allowed. The questions may therefore extend to chains of .s and []s.

For C++, a question is whether .*s with sufficiently-constant pointers-to-member is a bridge too far.

@erichkeane
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EWG discussed CWG2784 during the Tuesday Morning session in Kona. The following poll was taken.

CWG2784 "Unclear definition of member-designator for offsetof" this is an issue, we would like to see a paper which thoroughly explores the design space from and C++.

SF F N A SA
6 14 1 1 0

Result: Consensus

@erichkeane erichkeane added the paper needed An issue needs a paper to describe its solution label Mar 18, 2024
@hanickadot
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EWG St. Louis: Alisdair Meredith volunteered to write a paper to turn offsetof to keyword

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@hubert-reinterpretcast @hanickadot @erichkeane @jensmaurer