3233. Broken requirements for shared_ptr converting constructors

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++20 Submitter: Casey Carter Opened: 2019-07-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

Issue 2875 added 20.3.2.2.2 [util.smartptr.shared.const] paragraph 13:

Remarks: When T is an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*. When T is not an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and Y* is convertible to T*.
which pertains to the four constructor overloads: 
template<class Y, class D> shared_ptr(Y* p, D d);
template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
template<class D> shared_ptr(nullptr_t p, D d);
template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
which is fine (ignoring for now that two occurrences of "this constructor" should read "these constructors") for the two overloads with a template parameter Y, but not so much for the two with no such template parameter.

MSVC ignores the constraints on Y for the overloads with no such template parameter, whereas libstdc++ and libc++ seem to ignore all of the constraints for those overloads (See Compiler Explorer). We should fix the broken wording, ideally by requiring the MSVC interpretation - the nullptr_t constructors participate in overload resolution only when is_movable_v<D> is true and d(p) is well-formed - so concepts and traits that check constructibility are correct.

[2019-11-17 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4835.

  1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

    template<class Y, class D> shared_ptr(Y* p, D d);
template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
template<class D> shared_ptr(nullptr_t p, D d);
template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

    -?- Constraints: is_move_constructible_v<D> is true, and d(p) is a well-formed expression. For the first two overloads:

    • If T is an array type, then either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*.

    • If T is not an array type, then Y* is convertible to T*.

    -9- RequiresExpects: Construction of d and a deleter of type […]

    […]

    -13- Remarks: When T is an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*. When T is not an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and Y* is convertible to T*.