String literal type from string template

[aaronbeall]

TypeScript Version: 2.1.5

Code

I frequently run into situations where I have a computed key from another key, both of which are string literals:

type Key = "a" | "b" | "c";
type CombinedKey = "z.a" | "z.b" | "z.c";

declare const key: Key;
const combined: CombinedKey = `z.${key}`; // Can only be valid values: "z.a" | "z.b" | "z.c"
// Error: Type 'string' is not assignable to type 'CombinedKey'.

Expected behavior:

Since z.${Key} can only be valid literal values of CombinedKey it would be helpful if it compiled without error.

Actual behavior:

Error Type 'string' is not assignable to type 'CombinedKey'.

I can workaround using an assertion:

const combined: CombinedKey = `z.${key}` as CombinedKey;

But this allows Key or CombinedKey to become incompatible without a compile error:

type Key = "wrong";
type CombinedKey = "z.a" | "z.b" | "z.c";
declare const key: Key;
const combined: CombinedKey = `z.${key}` as CombinedKey; // No error for "z.wrong"

[DanielRosenwasser]

Related is #13969.

[RyanCavanaugh]

This hasn't collected much feedback and would open the door for a lot of combinatorially-explosive things we'd rather avoid