Extract function parameters or change function return type in mapped types

Search Terms

extract function parameters
change function return type

Suggestion

Allow mapped types to change the return type of a function

Use Cases

I am writing a mocking framework. My system under test might be like this:

MyClass{
    myClassFunction(paramOne: string, paramTwo: number): string{}
}

but in my mocking framework I want to do this:

mockedObject
    .withFunction("myClassFunction")
    .withParameters("paramOneValue", 123)
   .wasCalled();

to do the withParameters I need to type a function with the same parameters as defined on my class but with a different return type.

Checklist

My suggestion meets these guidelines:
[ ] This wouldn't be a breaking change in existing TypeScript / JavaScript code
[ ] This wouldn't change the runtime behavior of existing JavaScript code
[ ] This could be implemented without emitting different JS based on the types of the expressions
[ ] This isn't a runtime feature (e.g. new expression-level syntax)

Start by examining TypeScript's mapped-type and function-type capabilities against the mocking-framework example. Determine how parameter extraction and return-type replacement should be expressed while preserving existing TypeScript and JavaScript behavior. Done means the requested type transformation is supported and the example's mocked function parameters are checked correctly.