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CWG2565 Invalid types in the parameter-declaration-clause of a requires-expression #1695
Description
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.
Activity
erichkeane commentedon Mar 18, 2024
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.
Result: Consensus
Paper author not identified.