[expr.const.core] Using this in a default member initializer during constant evaluation

[t3nsor]

Full name of submitter: Brian Bi

Issue description: [expr.const.core]/2.1 permits the use of this during constant evaluation only in a constexpr function or as the postfix-expression of a class member access expression. This rule appears to prohibit, e.g.

constexpr struct A {
    A* p = this;
} a;
static_assert(a.p == &a);

Suggested resolution: Edit [expr.const.core]/2.1 as follows:

• this ([expr.prim.this]), except
  • in a constexpr function ([dcl.constexpr]) that is being evaluated as part of E or ,
  • in a default member initializer for an object that is being initialized as part of E, or
  • when appearing as the postfix-expression of an implicit or explicit class member access expression ([expr.ref])