This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-08-11
2386. tuple_size requirements for structured bindingSection: 9.7 [dcl.struct.bind] Status: CD5 Submitter: Stephan T. Lavavej Date: 2018-09-19[Accepted as a DR at the February, 2019 meeting.]
According to 9.7 [dcl.struct.bind] paragraph 4,
Otherwise, if the qualified-id std::tuple_size<E> names a complete type, the expression std::tuple_size<E>::value shall be a well-formed integral constant expression and the number of elements in the identifier-list shall be equal to the value of that expression.
However, the common idiom in the library is that SFINAE tests the presence or absence of a value member rather than the completeness of the class; see 22.4.7 [tuple.helper] paragraph 4. The core language requirement should be changed to match the common library practice.
Proposed resolution (December, 2018):
Change 9.6 [dcl.fct.def] paragraph 4 as follows:
Otherwise, if the qualified-id std::tuple_size<E> names a complete class type with a member value, the expression std::tuple_size<E>::value shall be a well-formed integral constant expression and the number of elements in the identifier-list shall be equal to the value of that expression...
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