Experiment with always using parameters from base types for derived methods

Note some related issues for doing this with properties rather than methods: #3667, #6118, #1373.

Goal

We want to make it easier for users to derive types without rewriting the same parameter signature every time

class Base {
  method(x: number) {
    // ...
  }
}

class Derived extends Base {
  method(x) {
    // 'x' should have the type 'number' here.
  }
}

Potential ideas

• Only enable in noImplicitAny (doesn't work for default initializers 😞)
• Revert to any in all locations, opt in with another strictness flag (😞)
• Something else? 😕

Potential issues

Default initializers with more capable derived types

class A {
  a = 1;
}

class B extends A {
  b = 2;
}

class Base {
  method(x: A) {
    // ...
  }
}

class Derived extends Base {
  method(x = new B) {
    x.b;
    // Today, 'x' has type 'B' which is technically unsound
    // but that's just what we do. Does changing this to 'A' break things?
  }
}

Default initializers that become less-capable via contextual types

class Base {
  method(x: "a" | "b") {
    // ...
  }
}

class Derived extends Base {
  method(x = "a") {
    // We have to make sure 'x' doesn't have type '"a"'
    // which is both unsound and less useful.
  }
}

Distinction between properties and methods

Would this work?

class Base {
  method = (x: number) => {
    // ...
  }
}

class Derived extends Base {
  method(x) {
    // Does 'x' have the type 'number' here?
  }
}

What about this?

class Base {
  method(x: number) {
    // ...
  }
}

class Derived extends Base {
  method = (x) => {
    // Does 'x' have the type 'number' here?
  }
}

Keywords: base type derived contextual contextually inherit methods implicit any

Start with the goal and examples in this issue, then compare the related property issues #3667, #6118, and #1373. Investigate how derived methods, default initializers, and method properties should be contextualized; done means the behavior and strictness implications are resolved for all examples.