[basic.compound] p5 A deallocation cannot happen before itself

[xmh0511]

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

[basic.compound] p5 says:

A pointer value P is valid in the context of an evaluation E if P is a pointer to function or a null pointer value, or if it is a pointer to or past the end of an object O and E happens before the end of the duration of the region of storage for O. If a pointer value P is used in an evaluation E and P is not valid in the context of E, then the behavior is undefined if E is an indirection ([expr.unary.op]) or an invocation of a deallocation function ([basic.stc.dynamic.deallocation])

[basic.stc.dynamic.deallocation] p5 says:

If the argument given to a deallocation function in the standard library is a pointer that is not the null pointer value ([basic.compound]), the deallocation function shall deallocate the storage referenced by the pointer, ending the duration of the region of storage.

Consider this example:

int main(){
  auto p = operator new(sizeof(char)); // #0
  operator delete(p); // #1
}

#1 is an invocation of a deallocation function, and it ends the duration of the region of storage referenced by p. However, according to [intro.races] note 7

An evaluation does not happen before itself.

So, p is not valid in #1, therefore, the program has UB?

[safocl]

#1 is an invocation of a deallocation function, and it ends the duration of the region of storage referenced by p. However, according to [intro.races] note 7

An evaluation does not happen before itself.

So, p is not valid in #1, therefore, the program has UB?

[intro.races] defines the order of operations, and therefore it is impossible to make an operation "happen before" itself (as indicated in note 7). This does not determine that the pointer can be invalid.

[safocl]

in other words, if we imagine a hypothetical check operation is_happens_before(op1, op1), it can never return true

[frederick-vs-ja]

The use of the pointer value in this case, i.e. initialization of the parameter of the deallocation, is sequenced before the execution of the function body. Maybe we can just clarify that "ending the duration of the region of storage" can only happen after entering the function body and happen before leaving it.

[xmh0511]

The use of the pointer value in this case, i.e. initialization of the parameter of the deallocation, is sequenced before the execution of the function body.

IIUC, these evaluations all occur within the function invocation, see #847 (comment). [basic.compound] p5 uses invocation of a deallocation function. So, the function invocation can also comprise the evaluations within the function's body.