Property '[mySymbol]' is not assignable to string index type

[yortus]

TypeScript Version: 2.7.0-dev.20171118

Code

I was expecting this to work after #15473 was merged:

const mySymbol = Symbol('My Symbol');

interface TypeA {
    [mySymbol]: Function;  // OK
    [x: number]: number;
}

interface TypeB {
    [mySymbol]: Function;  // ERROR: Property '[mySymbol]' is not assignable to string index type
    [x: string]: string;
}

let a: TypeA, b: TypeB;
let a1 = a[mySymbol];   // a1 has type Function
let a2 = a[42];         // a2 has type number
let b1 = b[mySymbol];   // b1 has type Function
let b2 = b['foo'];      // b2 has type string

Expected behavior:

No compiler errors. mySymbol is a unique symbol as per #15473 and should not overlap either numeric index types or string index types.

Actual behavior:

Error in TypeB as shown in the code above. TSC rejects the unique symbol key because it is not compatible with the string index type, even though it is not a string property key.

Note that the types of a1, a2, b1 and b2 are all inferred correctly despite the compile error.