https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114354
Bug ID: 114354
Summary: std::shared_ptr constructor constraints are checked
too late
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Blocks: 88322
Target Milestone: ---
The standard says:
template<class Y> explicit shared_ptr(Y* p);
Constraints: When T is an array type, the expression delete[] 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, the expression delete p is well-formed and Y* is
convertible to T*.
We do not constrain that constructor with checks for the delete expressions
being well-formed, so this gives the wrong answer:
#include <memory>
struct A {
private:
~A();
};
static_assert( ! std::is_constructible_v<std::shared_ptr<A>, A*> );
Similarly for the constructors taking a deleter:
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. [...]
We check that with a static assert in the constructor body, which is too late,
so this fails:
struct D { };
static_assert( ! std::is_constructible_v<std::shared_ptr<A>, A*, D> );
Prior to C++20 the requirements were just preconditions, so not required to be
SFINAE-able. So we must fix this for C++20, and could do so for C++11 as QoI.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88322
[Bug 88322] Implement C++20 library features.
