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4578. std::start_lifetime(r) should not be able to poke objects outside the current evaluation

Section: 20.2.6 [obj.lifetime] Status: Core Submitter: Tomasz Kamiński Opened: 2026-05-15 Last modified: 2026-05-22

Priority: Not Prioritized

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Discussion:

As currently specified, the following code is well-formed: 

union U { int x = 10; float f[2]; };
U u;

consteval {
  std::start_lifetime(u.f); // changes active member
};

void foo() {
  U u;
  constexpr int x = (std::start_lifetime(u.f), 10);
}

The start_lifetime(r) when evaluated at compile time, should require that r is a subobject of the object created during the enclosing constant evaluation.

Previous resolution [SUPERSEDED]

This wording is relative to N5046.

  1. Modify 20.2.6 [obj.lifetime] as indicated:

    template<class T>
  constexpr void start_lifetime(T& r) noexcept;

    -1- Mandates: T is a complete type and an implicit-lifetime (6.9 [basic.types]) aggregate (9.5.2 [dcl.init.aggr]).

    -?- Constant When: r refers to a non-volatile object whose lifetime began within the evaluation of the core constant expression (7.7.2 [expr.const.core]) which includes the invocation of start_lifetime, or a sub-object of such an object.

    -2- Effects: If the object referenced by r is already within its lifetime (6.8.4 [basic.life]), no effects. Otherwise, begins the lifetime of the object referenced by r.

[2026-05-22; LWG telecon. Status → Core.]

Corrected example, to make it well-formed.

The recommendation is to include general prohibition in 7.7.2 [expr.const.core], that would prevent us from starting lifetime of object outside current evaluation.