Constant-folding error with negative constant on RHS of shift

[eefriedman]

fn main() {
    let a = -1+2isize;
    let _b = a<<(1isize-1);
    let _c = a<<(a+-1);
    let _d = a<<(1isize+-1);
}

Produces:

<anon>:5:25: 5:27 error: unary negation of unsigned integers may be removed in the future
<anon>:5     let _d = a<<(1isize+-1);
                                 ^~
error: aborting due to previous error

I would expect no error.

I think the error message is coming out of eval_const_expr_with_substs.