'Using Type Parameters in Generic Constraints' code error

[leebenson]

TypeScript Version: 2.1.5

Code

Taken from 'Using Type Parameters in Generic Constraints' in the official docs

function copyFields<T extends U, U>(target: T, source: U): T {
    for (let id in source) {
        target[id] = source[id];
    }
    return target;
}

let x = { a: 1, b: 2, c: 3, d: 4 };
copyFields(x, { b: 10, d: 20 });

Expected behavior:

Compiles.

Actual behavior:

error TS2322: Type 'U[keyof U]' is not assignable to type 'T[keyof U]'.
Type 'U' is not assignable to type 'T'.

[gcnew]

I raised the documentation issue in #13023