Function return type is not implied conditionally based on type of parameter

[joshuafairchild1]

TypeScript Version: 2.9.2

Search Terms:
typescript return type based on parameter
Code

type StringOrNull<B extends Boolean> = B extends true ? string : (string | null)
type StringNotNull = StringOrNull<true>

function foo<B extends boolean>(returnString: B): StringOrNull<B> {
  if (returnString) {
    return '' as StringNotNull // type error: "Type 'string' is not assignable to type 'StringOrNull<B>'"
  } else {
    return null
  }
}

Expected behavior:

Typecast should be valid, with the return type of foo being:

• string when returnString is passed as true
• string | null when returnString is passed as false

Actual behavior:
Cast cannot succeed, and return type is not dynamic based on the boolean passed to foo

Playground Link:
https://www.typescriptlang.org/play/#src=type%20StringOrNull%3CB%20extends%20Boolean%3E%20%3D%20B%20extends%20true%20%3F%20string%20%3A%20(string%20%7C%20null)%0D%0Atype%20StringNotNull%20%3D%20StringOrNull%3Ctrue%3E%0D%0A%0D%0Afunction%20foo%3CB%20extends%20boolean%3E(throwError%3A%20B)%3A%20StringOrNull%3CB%3E%20%7B%0D%0A%20%20if%20(throwError)%20%7B%0D%0A%20%20%20%20throw%20Error()%0D%0A%20%20%7D%0D%0A%20%20return%20''%20as%20StringNotNull%0D%0A%7D

Related Issues:

[weswigham]

StringNotNull expressly passes true to your helper which expressly resolves to just string (it has no generics left so is no longer actually a conditional; it's already "executed"). You should probably just cast to StringOrNull<typeof returnString> to retain the dependence on B instead.

[joshuafairchild1]

StringNotNull expressly passes true to your helper which expressly resolves to just string (it has no generics left so is no longer actually a conditional; it's already "executed"). You should probably just cast to StringOrNull<typeof returnString> to retain the dependence on B instead.

Casting to StringOrNull<typeof returnString> will still result in the same type error that 'string' is not assignable to type 'StringOrNull<B>'. I was, however, able to make the cast succeed by casting to StringOrNull<B>, though that route doesn't fix the root issue that the return type of foo will then always be string, independent of the passed boolean.

[poseidonCore]

The problem is that the conditional type does not collapse to a basic type.

This is a reduced example.

TS Playground

[jack-williams]

This is tracked by #26933.