non-null assertion not working on type parameter with constraint

[ajafff]

TypeScript Version: 2.7.0-dev.20171230

Code

function takeString(p: string) {}

function foo<T extends string | undefined>(param: T) {
 takeString(param!); // error on this line
}

Expected behavior:
Because of the constraint the compiler knows the type of param can only be string | undefined. Asserting the type with a non-null assertion should result in string and the code should compile without error.

If this is working as intended, I'll write a lint rule to warn about non-null assertions on type parameters.

Actual behavior:

Argument of type 'T' is not assignable to parameter of type 'string'.
 Type 'string | undefined' is not assignable to type 'string'.
 Type 'undefined' is not assignable to type 'string'.

[RyanCavanaugh]

Asserting the type with a non-null assertion should result in string

We would need subtraction types to do this correctly. The correct type isn't string (because T might be instantiated with some type like "foo" | "bar"), it's T - undefined.

[ajafff]

Ryan Cavanaugh (@RyanCavanaugh) makes sense. So this is basically a duplicate of #4183?

[RyanCavanaugh]

Not a duplicate since this case wouldn't start working automatically

[ajafff]

Wesley Wigham (@weswigham) Is this fixed by #22096 or #22348?

[weswigham]

#22096 should cover it.

[weswigham]

Oh, in case anyone happens to hit this again in the future: The new (more correct) behavior is dependent on the presence of the global NonNullable<T> type. If that's not there (because your compilation doesn't include the standard lib or includes an old one), it'll fall back to the older less-correct behavior (and consequently have the same problems). So if you still encounter this issue in TS 2.8+, double check that you've got the latest lib in your build!