[class.mfct.non-static] p3

When an id-expression ([expr.prim.id]) that is not part of a class member access syntax ([expr.ref]) and not used to form a pointer to member ([expr.unary.op]) is used where the current class is X ([expr.prim.this]), if name lookup ([basic.lookup]) resolves the name in the id-expression to a non-static non-type member of some class C, and if either the id-expression is potentially evaluated or C is X or a base class of X, the id-expression is transformed into a class member access expression ([expr.ref]) using (*this) as the postfix-expression to the left of the . operator.

Consider this example

struct B{
    int d;
    void show(decltype(d));  // #1
};

the name d in #1 resolves to the non-static data member of B, the current class is also B, hence the id-expression is transformed into a class member access expression, however, according to [expr.prim.this] p5

The expression this shall not appear in any other context.

The transformation makes an ill-formed.