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4138. is_within_lifetime should mandate is_object

Section: 21.3.11 [meta.const.eval] Status: New Submitter: Corentin Opened: 2024-08-09 Last modified: 2026-06-10

Priority: 3

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Discussion:

int f();
std::is_within_lifetime<int()>(f);

This is currently well-formed, and only fails when evaluated because is_within_lifetime is not constrained. However talking of lifetime of a non-object does not make sense, and the lack of constraint makes the implementation and use of that function and underlying built-in more convoluted than necessary.

[2025-10-14; Reflector poll]

Set priority to 3 after reflector poll.

"What about void*, should that be allowed? The compiler should know which union member the void* came from, as this is only using during constant evaluation."

"NAD, a function could have its lifetime constrained by a dynamic loader, so it seems wrong to assume that taling of function lifetimes makes no sense."

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

  1. Modify 21.3.3 [meta.type.synop], header <type_traits> synopsis, as indicated:

    […]
// 21.3.11 [meta.const.eval], constant evaluation context
constexpr bool is_constant_evaluated() noexcept;
template<class T>
consteval bool is_within_lifetime(const Tauto*) noexcept;

  2. Modify 21.3.11 [meta.const.eval] as indicated:

    template<class T>
consteval bool is_within_lifetime(const Tauto* p) noexcept;

    -?- Mandates: is_object_v<T> is true.

    -3- Returns: true if p is a pointer to an object that is within its lifetime (6.8.4 [basic.life]); otherwise, false.

    -4- Remarks: During the evaluation of an expression E as a core constant expression, a call to this function is ill-formed unless p points to an object that is usable in constant expressions or whose complete object's lifetime began within E.

[2026-06-09; Tim rebases and provides new wording]

Functions do not have lifetimes as far as the standard is concerned. However, there is no reason to ban void.

Proposed resolution:

This wording is relative to N5046.

  1. Modify 21.3.11 [meta.const.eval] as indicated:

    template<class U = void, class T>
  consteval bool is_within_lifetime(const T* p) noexcept;

    -3- Mandates: static_cast<const volatile U*>(p) is well-formed. T is an object type or cv void.

    -4- Returns: true if p is a pointer to an object that is within its lifetime (6.8.4 [basic.life]) and static_cast<const volatile U*>(p) is a constant subexpression; otherwise, false.

    -4- Remarks: During the evaluation of an expression E as a core constant expression, a call to this function is ill-formed unless p points to an object that is usable in constant expressions or whose complete object's lifetime began within E.