Full name of submitter (unless configured in github; will be published with the issue): Hubert Tong

Reference (section label): [class.temporary]

Link to reflector thread (if any): N/A

Issue description:

P2748R5, "Disallow Binding a Returned Glvalue to a Temporary" only adds an error for functions that return references but entirely strikes the provision that prevents lifetime-extension of temporaries bound to the return value of a function. It thus creates situations whereby temporaries created within a function can be lifetime-extended to lifetimes that are opaque to the function.

Consider:

struct B { ~B(); };
struct A { const B &b; };
A foo() { return {{}}; }
void bar();
int main() {
  A a = foo();
  bar();
}

Based on guaranteed copy elision, the reference that the B temporary is bound to is the a.b from main. [class.temporary] says that the lifetime of the B temporary is extended. This requirement is not practical to implement.

Suggested resolution:

Restore [class.temporary] bullet 6.11 from C++23.

• The lifetime of a temporary bound to the returned value in a function return statement ([stmt.return]) is not extended; the temporary is destroyed at the end of the full-expression in the return statement.