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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: 2024-08-11

Priority: Not Prioritized

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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.

Proposed resolution:

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 (6.7.3 [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.