[expr.const]/2.11 CWG 2186: "Preceding" is unclear #2307
Description
[expr.const]/2.11:
An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine, would evaluate one of the following expressions:
- ...
- an id-expression that refers to a variable or data member of reference type unless the reference has a preceding initialization and either
- it is initialized with a constant expression or
- its lifetime began within the evaluation of e;
It's unclear what "preceding" means. It may either mean the initialization happens before the evaluation of the id-expression, or mean the initialization occurs before the id-expression in lexical order.
According to this example in cppreference:
struct S {
static const int c;
};
const int d = 10 * S::c; // not a constant expression: S::c has no preceding
// initializer, this initialization happens after const
const int S::c = 5; // constant initialization, guaranteed to happen first
It seems "preceding" means the latter, but consider:
constexpr int f(const int &i)
{
return i;
}
constexpr int j = f(0);
Here f(0) is obviously a constant expression, but the initialization of i does not occur before return i; in lexical order.
I think it makes more sense to say:
- an id-expression that refers to a variable or data member of reference type unless either
- the reference has a preceding initialization, initialized with a constant expression or
- its lifetime began within the evaluation of e;