odr-use fixes for array subscripting/indirection

gistfile1.md

To 3.2p2, add:

• if e is a subscript expression (5.2.1), where one operand is of type "array of N T" or "reference to array of N T" and the other operand is a constant expression of integral or enumeration type with value n, where 0 <= n < N, the set contains the potential results of the operand of array type.
• if e is an indirection expression (5.3.1), the set contains the potential results of the operand.

Modify 3.2p3 thus:

A variable x whose name appears as a potentially-evaluated expression ex is odr-used by ex unless applying the lvalue-to-rvalue conversion (4.1) to x or, if x is of array type, an element of x yields a constant expression (5.19) that does not invoke any nontrivial functions and, if x is an object, ex is an element of the set of potential results of an expression e, where either the lvalue-to-rvalue conversion (4.1) is applied to e, or e is a discarded-value expression (Clause 5).

To 3.2p2, add:

• if e is a subscript expression (5.2.1), where one operand is of type "array of N T" or "reference to array of N T" and the other operand is a constant expression of integral or unscoped enumeration type with value n, where 0 <= n < N, the set contains the potential results of the operand of array type.

(the reference part is not needed since expressions cannot have reference type)

I think there is a problem with the modification in 3.2p3.

struct S { static const double a[2] = {0.0,1.0}; };
double d = S::a[0];

Here S::a[0] is not a constant expression, since the array is not declared constexpr.

I'm not sure, if we need to add indirection. What use cases do you have in mind?