CWG Issue 2490 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-11-05

2490. Restrictions on destruction in constant expressions

Section: 7.7 [expr.const]Status: CD6Submitter: Jiang AnDate: 2021-05-04

[Accepted as a DR at the October, 2021 meeting.]

According to 7.7 [expr.const] paragraph 6,

For the purposes of determining whether an expression E is a core constant expression, the evaluation of a call to a member function ofstd::allocator as defined in 20.2.10.2 [allocator.members], where T is a literal type, does not disqualify E from being a core constant expression, even if the actual evaluation of such a call would otherwise fail the requirements for a core constant expression. Similarly, the evaluation of a call tostd::destroy_at, std::ranges::destroy_at,std::construct_at, or std::ranges::construct_at does not disqualify E from being a core constant expression unless:

for a call to std::construct_at orstd::ranges::construct_at, the first argument, of type T*, does not point to storage allocated withstd::allocator or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying constructor call disqualifies E from being a core constant expression, or

for a call to std::destroy_at orstd::ranges::destroy_at, the first argument, of type T*, does not point to storage allocated withstd::allocator or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying destructor call disqualifies E from being a core constant expression.

There are, however, no specific restrictions in 7.7 [expr.const] regarding destructor or pseudo-destructor calls. In particular, a constexpr destructor can be called for any object, regardless of how it was constructed or the start of its lifetime, and similarly for pseudo-destructor calls. This seems inconsistent.

If those restrictions are added, would the specific restrictions on library destruction facilities still be needed?

Notes from the August, 2021 teleconference:

CWG agreed that since trivial destructors and pseudo-destructors are now considered to end the lifetime of the object for which they are called, they should be prohibited from being invoked for a runtime object in a constant expression.

Proposed resolution (August, 2021):

Change 7.7 [expr.const] paragraph 5 as follows:

An expression E is a _core constant expression_unless the evaluation of E, following the rules of the abstract machine (6.10.1 [intro.execution]), would evaluate one of the following:

...

a modification of an object (7.6.19 [expr.assign], 7.6.1.6 [expr.post.incr], 7.6.2.3 [expr.pre.incr]) unless it is applied to a non-volatile lvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;

an invocation of a destructor (11.4.7 [class.dtor]) or a function call whose_postfix-expression_ names a pseudo-destructor (7.6.1.3 [expr.call]), in either case for an object whose lifetime did not begin within the evaluation of E;

a new-expression (7.6.2.8 [expr.new]), unless...

Change 7.7 [expr.const] paragraph 6 as follows, merging the single remaining bulleted item into the running text of the paragraph:

For the purposes of determining whether an expression E is a core constant expression, the evaluation of a call to a member function ofstd::allocator as defined in 20.2.10.2 [allocator.members], where T is a literal type, does not disqualify E from being a core constant expression, even if the actual evaluation of such a call would otherwise fail the requirements for a core constant expression. Similarly, the evaluation of a call to~~std::destroy_at, std::ranges::destroy_at,~~ std::construct_at,or std::ranges::construct_at does not disqualify E from being a core constant expression unless:

~~for a call to std::construct_at orstd::ranges::construct_at,~~ the first argument, of type T*, does not point to storage allocated withstd::allocator or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying constructor call disqualifies E from being a core constant expression~~, or~~

for a call to std::destroy_at orstd::ranges::destroy_at, the first argument, of type T*, does not point to storage allocated withstd::allocator or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying destructor call disqualifies E from being a core constant expression.