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CWG2565 Invalid types in the parameter-declaration-clause of a requires-expression #1695

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CWG2565 Invalid types in the parameter-declaration-clause of a requires-expression#1695
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EWGEvolutionpaper neededAn issue needs a paper to describe its solution

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@jensmaurer
jensmaurer
opened on Nov 9, 2023

These two examples are very similar, but the second one is ill-formed per the wording despite implementations accepting it due to user demand:

  template <typename T>
  concept C = requires (typename T::type x) {
    x + 1;
  };

  static_assert(!C<int>);

vs.

  template <typename T>
  constexpr bool b = requires (T::type x) { x + 1; };

  static_assert(!b<int>);  // de-jure ill-formed, but widely accepted by implementations

In order to make the second case well-formed, the user needs to write a nested requires that checks for the presence of T::type first.

See CWG2565 for details.

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  1. jensmaurer
    added
    EWGEvolution
    on Nov 9, 2023

  2. erichkeane commented on Mar 18, 2024

    @erichkeane
    erichkeane
    on Mar 18, 2024
    Collaborator

    This was discussed during the Morning AM session in Tokyo, the following poll was taken:

    CWG2565 EWG agrees that this is an issue worth considering, but would like to see a paper to propose a change.

    SF F N A SA
    1 17 7 1 0

    Result: Consensus

    Paper author not identified.

  3. hanickadot
    added
    paper neededAn issue needs a paper to describe its solution
    on Mar 18, 2024
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