Typecast string to string literal

[stochris]

TypeScript Version: 2.1.4

Code

// in 2.1.4  iShouldBeStringLiteral is now a string
let iShouldBeStringLiteral = type('This is a string')
export function type<T>(label: T | ''): T {
  return <T>label;
}

Expected behavior:
Before updating to typescript 2.1.4 the function transformed string to string literals with the same value
Actual behavior:
Now, even though the function returns a string literal type, assigning the return value to a variable / object property, will downcast it to string

In the second image, I would expect x to be of type 'String literal'

This is especially in redux like scenarions, where you have to switch based on a constant and need the exhaustive checking in order to get type safety inside every case branch

edit: I know the solution is a bit hackish, but it helps a lot. If there is any other way of doing this, I'm more than open to suggestions

[stochris]

Also, the non templated version the function works as intended

function type(val: any){
    return <'this is string literal'>val;
}

// stringLiteral appears as ' let stringLiteral: "this is string literal" '
let stringLiteral = type('this is string literal');

[ahejlsberg]

In 2.1 we preserve literal types of expressions and variables for a long as possible, but when we infer a literal type for a mutable location such as a let or var variable, we widen to the base primitive type. However, if you declare iShouldBeStringLiteral using const you shouldn't need the type function at all.

For more details see #10676.

[stochris]

Thank you very much. Problem solved