Expression.Equals and/or == fails

[ewoutkramer]

The code for Equals in Expression (FhirPath in common) looks like this:

   public override bool Equals(object obj) => Equals(obj as Expression);
        public bool Equals(Expression other) => other != null && EqualityComparer<TypeSpecifier>.Default.Equals(ExpressionType, other.ExpressionType);
        public override int GetHashCode() => -28965461 + EqualityComparer<TypeSpecifier>.Default.GetHashCode(ExpressionType);
        public static bool operator ==(Expression left, Expression right) => EqualityComparer<Expression>.Default.Equals(left, right);
        public static bool operator !=(Expression left, Expression right) => !(left == right);

However, when we test a subclass of Expression on equality like so:

    Expression x = new SomeSubClassExpression1();
    Expression y = new SomeSubClassExpression2();
    x == y;

this always return true.