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'.