This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-11-11
[Accepted as a DR at the February, 2023 meeting.]
According to 7.5.8.5 [expr.prim.req.nested] paragraph 2,
A local parameter shall only appear as an unevaluated operand (7.2.3 [expr.context]) within the constraint-expression. [Example 2:
template<typename T> concept C = requires (T a) { requires sizeof(a) == 4; // OK requires a == 0; // error: evaluation of a constraint variable };—end example]
However, a can't be used in a constant expression in any event, so the restriction is meaningless, except for ruling out an expression like true ? true : a, but there seems no reason to have a special rule for such a case.
Proposed resolution (approved by CWG 2023-01-06):
Remove 7.5.8.5 [expr.prim.req.nested] paragraph 2, including its example:
A local parameter shall only appear as an unevaluated operand (7.2.3 [expr.context]) within the constraint-expression. [Example 2:template<typename T> concept C = requires (T a) { requires sizeof(a) == 4; // OK requires a == 0; // error: evaluation of a constraint variable };
Additional notes (February, 2023)
After adopting paper P2280, it is no longer accurate that any use of requirement parameters makes an expression non-constant. However, the resolution as adopted makes the treatment of examples like the following uniform in requirements and other constant expression contexts:
template<typename ArrayType> concept LargeArray =
requires (ArrayType my_array) { requires my_array.size() > 5; }