Infer literal types for string concatenations and similar expressions

TypeScript Version: 2.1.5

Expected behavior:

const foo = 'world'; // value type: 'world'
const bar = `hello ${foo}`; // value type: 'hello world'

Actual behavior:

const foo = 'world'; // value type: 'world'
const bar = `hello ${foo}`; // type: 'string'

Motivation

For constants which are created from other constants the IntelliSense is often more useful, when we see its value type instead of the “real” type.

More practical example:

const fooBreakpoint = '800px';
const fooMediaQuery = `@media(min-width: ${fooBreakpoint})`;

When fooMediaQuery is used I’d like to see @media(min-width: 800px) as its type instead of string.

[bradzacher]

Worth noting that if you add as const this should work now with template literal types!

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZiEMC8MDkB3EAnANgEzQG4YB6UmANwENcBXAUxigE8AHBgLnSz0IChQkWACNq2FDAAGACwa5cSACQBvBCAC+UmNQgwh0Iv3IxTpgHoB+IA

[fjmorel]

I’d like to be able to do something like this, building a key using string literals:

type Obj = Record<"a" | "a1" |  "b" | "b1", string>;

function getKey(useA: boolean, useOne: boolean): keyof Obj {
  // [ts] Type 'string' is not assignable to type '"a" | "a1" | "b" | "b1"'.
  return (useA ? "a" : "b") + (useOne ? "1" : "");
}

Adding a cast after the string concatenation makes the error go away, but doesn’t actually check that my key generation is still valid if I change Obj.