Using enums in Typescript's generics with strictFunctionTypes - function

Question

I have the following code (TS playground link):
const enum Enum { A, B, C }
interface Args {
e: Enum.A;
}
interface GenericClass<A> {
new (args: A) : void;
}
class TestClass {
constructor(args: Args) {}
}
function func<A>(C: GenericClass<A>, args: A) {
return new C(args);
}
func(TestClass, { e: Enum.A });
The last line [1] throws an error with strictFunctionTypes enabled:
Argument of type 'typeof TestClass' is not assignable to parameter of type 'GenericClass<{ e: Enum; }>'.
Types of parameters 'args' and 'args' are incompatible.
Type '{ e: Enum; }' is not assignable to type 'Args'.
Types of property 'e' are incompatible.
Type 'Enum' is not assignable to type 'Enum.A'.
That's strange because I accept exact enum value Enum.A and I pass the exactly same value Enum.A into function.
I know I can use type casting { e: <Enum.A>Enum.A }, but it looks strange for me. Is here a way to fix this problem without type casting?

Answer 1

I am not 100% sure why this happens, but I belive that when inferring A the compiler will consider both places where A appears and decide that the widest possible type is { e:Enum} based on the fact that object literals don't usually infer literal types for their fields. After the inference it will see that under strict functions the type is not compatible with the class. Under this theory, if we decrees the priority of the second inference site, we should get the correct type for A,. We can do this using an intersection type A&{} (I am not sure where exactly I read this but it was in a github issue and a member of the compiler team mentioned that this way of decreasing inference priority is probably going to work for the foreseeable future) .
Again that is mostly an educated guess, but the solution works :
const enum Enum { A, B, C }
interface Args {
e: Enum.A;
}
interface GenericClass<A> {
new (args: A) : void;
}
class TestClass {
constructor(args: Args) {}
}
function func<A>(C: GenericClass<A>, args: A & {}) {
return new C(args);
}
func(TestClass, { e: Enum.A });
playground link