Full name of submitter: Vincent X

Reference: [expr.prim.id.general]

Issue description: P1787R6 effectively relaxed the constraints on the transformation of id-expressions (to non-static non-type members) in class scope into class member access expressions, such that they may be transformed even if the expression this is not allowed in the context. Consider:

struct A {
  int x;
  int a[sizeof(x)]; // (1)
  decltype(x) f();  // (2)
};

Under the current rules of [expr.prim.id.general]/2, both expressions of x in (1) and (2) are transformed into (*this).x, making the declarations ill-formed. This is because P1787R6 removed the old constraint (as referenced in #[class.mfct.non-static]):

[...] in a context where this can be used [...]

Suggested resolution: Reintroduce the constraint where appropriate.