The function signature for func_returnsInput is as follows:
const func_returnsInput: (input: string) => Promise<string>
This means that it specifically takes a string input and returns a Promise<string>. You could tweak the implementation slightly while still adhering to this type signature:
const func_returnsInput = (input: string): Promise<string> =>
Promise.resolve(input.toUpperCase());
The compiler only focuses on the call signature, not the specific implementation. If someone provides me with func_returnsInput, all I know is that it expects a string input and will return a Promise<string>. Feeding it anything other than a string would be incorrect.
Now let's compare that to the type ReturnsInput_T:
type ReturnsInput_T = <T>(data: T) => Promise<T>;
This is a generic function signature with a type parameter T. The caller, not the implementer, can specify the type parameter. So, ReturnsInput_T is a function that can accept any value chosen by the caller and will return a Promise of the same type.
You can think of a generic call signature as an infinite intersection of all possible types for T, even though it cannot be explicitly written out.
From this perspective, it becomes clear why you cannot assign func_returnsInput to a variable of type ReturnsInput:
const test: ReturnsInput_T = func_returnsInput; // error!
// Type 'T' is not assignable to type 'string'.
test(123); // no error
If test truly has the type ReturnsInput_T, then I should be able to call it with any type of data and get back a Promise of the same type. However, the compiler cannot infer this behavior from func_returnsInput.
Thus, an error occurs!
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