I built a tiny library that makes mixins typesafe, including detecting method collisions at compile time. The implementation is really simple, and it's a nicer API than the one in the TS handbook, I think.

I'm having trouble extending it to support more than a few parameters passed to mixin(A, B, C), because I have to test all permutations for Extract to work:

• 2-ary mixin():
  • Extract<keyof A, keyof B>
• 3-ary mixin():
  • Extract<keyof A, keyof B | keyof C>
  • Extract<keyof A | keyof C, keyof B>
• 4-ary mixin():
  • Extract<keyof A, keyof B | keyof C | keyof D>
  • Extract<keyof A | keyof D, keyof B | keyof C>
  • Extract<keyof A | keyof C, keyof B | keyof D>
  • Extract<keyof A | keyof C | keyof D, keyof B | keyof D>

This quickly explodes and gets really branchy, making the PreventCollisions type (below) really unwieldy.

Any suggestions for how to avoid this branchieness, to support more params? The thing I'd like to express is "What are the keys that shapes A, B, C ... Z have in common?"

The issue tracker is not the best place for questions like these, but I thought TS maintainers would be interested to see this solution. Feel free to close if you feel otherwise :)

Code is here: https://github.com/bcherny/magical-mixin/blob/master/src/index.ts.

Relevant section is:

type PreventCollisions<A, B, C = never> = Extract<keyof A, keyof B | keyof C> extends never
  ? Extract<keyof A | keyof C, keyof B> extends never
    ? Ctor<A & B & C>
    : 'Error' & Methods<Extract<keyof A | keyof C, keyof B>>
  : 'Error' & Methods<Extract<keyof A, keyof B | keyof C>>