Type error with 'extends' constraint and inheritance

[remagpie]

TypeScript Version: 2.4.1

Code

interface A {
  a: number;
}

interface B extends A {
  b: number;
}

function x(b: B): void {}
const y: <C extends A>(c: C) => void = x;

Expected behavior:
No Type Error

Actual behavior:
Type error with message :

Type '(b: B) => void' is not assignable to type '<C extends A>(c: C) => void'.
  Types of parameters 'b' and 'c' are incompatible.
    Type 'C' is not assignable to type 'B'.
      Type 'A' is not assignable to type 'B'.
        Property 'b' is missing in type 'A'.

Comment
Weirdly, it doesn't show any errors on Playground, but it shows errors when compiled on local project.

[jcalz]

This is intended behavior.

Playground is currently using TypeScript 2.3.3. Stricter checking for generic functions was introduced in TypeScript 2.4.

The error message is a good one. If I change your implementation of x() to actually use the b parameter:

function x(b: B): void { 
  console.log(b.b.toString()); // expects b.b to be defined
} 
const y: <C extends A>(c: C) => void = x;  

Then you can see why assigning x to y is flagged. You can apparently call y() on any A, but this will blow up at runtime unless the parameter is a B:

y({'a': 2});  // no error in TS, but TypeError at runtime

---

EDIT: of course, your next question might be: given that argument, why doesn't the following throw a type error?

const y: (a: A) => void = x;  // just fine apparently

The answer is: 🤷. Parameter bivariance is unsound and makes me sad, but I guess it allows for some useful programming patterns. See #14973 for some discussion and pointers to docs.

[remagpie]

@jcalz
I'm still confused about your EDIT part, but I think my problem is answered.
Thank you for your help.