Created on 2023-12-18.00:00:00 last changed 12 months ago
Proposed resolution:
This wording is relative to N4971.
Modify [meta.rel] as indicated:
Table 49 — Type relationship predicates [tab:meta.rel] Template Condition Comments […] […] […] template<class From, class To> struct is_convertible;see below From and To shall be
complete types, cv void, or arrays of
unknown bound. template<class From, class To> struct is_nothrow_convertible;is_convertible_v<From,
To> is true andthe
conversion, as defined by
is_convertible, is known
not to throw any exceptions
either both From and To
are cv void, or the function call
expression used for specifying is_convertible
is non-throwing ([expr.unary.noexcept])From and To shall be
complete types, cv void, or arrays of
unknown bound.[…] […] […] -5- The predicate condition for a template specialization is_convertible<From, To> shall be satisfied if and only if
the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:To test() { return declval<From>(); }
[Note 2: This requirement gives well-defined results for reference types, array types, function types, and cv void. — end note] Access checking is performed in a context unrelated to To and From. Only the validity of the immediate context of the expression of the
(?.1) — either both From and To are cv void, or
(?.2) — To is neither array nor function type and the function call expression conv-dest(declval<From>()) would be well-formed when treated as an unevaluated operand, where conv-dest is a hypothetical function declared as
void conv-dest(To) noexcept;.
return statement ([stmt.return])function call expression ([expr.call]) (including initialization of thereturnedparameter object or reference) is considered.
The current specification for std::is_convertible is sensitive to the requirements for the return statements. As a result, the requirements were accidentally changed by P0135R1 and then changed back by CWG issue 2426. The current revision of P2748 also plans to change the wording for std::is_convertible to avoid actual behavioral changing.
IMO it's better to specify std::is_convertible in a such way that is independent to return statements. The proposed resolution matches what mainstream implementations do, and should resolve LWG 3400 together.History | |||
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Date | User | Action | Args |
2023-12-22 12:45:23 | admin | set | messages: + msg13898 |
2023-12-18 00:00:00 | admin | create |