Intersection type of abstract classes does not throw error if same member exists in multiple abstract classes

TypeScript Version: 3.0.1

Search Terms: "Intersection", "abstract"

Code

export type Constructor<T = {}> = new (...args: any[]) => T;

abstract class Outer1 {
  abstract same: number;
  abstract notSame: string;
}

abstract class Outer2 {
  abstract same: number;
}

function Mixin1<TBase extends Constructor>(Base: TBase) {
  abstract class Inner1 extends Base {   // Same as Outer1
    abstract same: number;
  }
  return Inner1;
}

function Mixin2<TBase extends Constructor>(Base: TBase) {
  abstract class Inner2 extends Base {   // Same as Outer2
    abstract same: number;
  }
  return Inner2;
}

function rollUp<T>(mixins): new (...args) => T {
  return mixins.reduce((acc, mixin) => mixin(acc), class Seed {});
}

class Example extends rollUp<Outer1 & Outer2>([Mixin1, Mixin2]) {
  // Compiler will throw error when "notSame" is unimplemented
  // Compiler DOES NOT throw error about non-implemented "same" member
}

Expected behavior:
Compiler should say that same is not implemented in regular class

Actual behavior:
Compiler does not throw error saying that same is not present in example class, despite throwing an error about notSame. It will throw an error, however, if the types of same are incompatible.

Playground Link: https://www.typescriptlang.org/play/#src=export%20type%20Constructor%3CT%20%3D%20%7B%7D%3E%20%3D%20new%20(...args%3A%20any%5B%5D)%20%3D%3E%20T%3B%0D%0A%0D%0Aabstract%20class%20Outer1%20%7B%0D%0A%20%20abstract%20same%3A%20number%3B%0D%0A%20%20abstract%20notSame%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Aabstract%20class%20Outer2%20%7B%0D%0A%20%20abstract%20same%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Afunction%20Mixin1%3CTBase%20extends%20Constructor%3E(Base%3A%20TBase)%20%7B%0D%0A%20%20abstract%20class%20Inner1%20extends%20Base%20%7B%20%20%20%2F%2F%20Same%20as%20Outer1%0D%0A%20%20%20%20abstract%20same%3A%20number%3B%0D%0A%20%20%7D%0D%0A%20%20return%20Inner1%3B%0D%0A%7D%0D%0A%0D%0Afunction%20Mixin2%3CTBase%20extends%20Constructor%3E(Base%3A%20TBase)%20%7B%0D%0A%20%20abstract%20class%20Inner2%20extends%20Base%20%7B%20%20%20%2F%2F%20Same%20as%20Outer2%0D%0A%20%20%20%20abstract%20same%3A%20number%3B%0D%0A%20%20%7D%0D%0A%20%20return%20Inner2%3B%0D%0A%7D%0D%0A%0D%0Afunction%20rollUp%3CT%3E(mixins)%3A%20new%20(...args)%20%3D%3E%20T%20%7B%0D%0A%20%20return%20mixins.reduce((acc%2C%20mixin)%20%3D%3E%20mixin(acc)%2C%20class%20Seed%20%7B%7D)%3B%0D%0A%7D%0D%0A%0D%0Aclass%20Example%20extends%20rollUp%3COuter1%20%26%20Outer2%3E(%5BMixin1%2C%20Mixin2%5D)%20%7B%0D%0A%20%20%2F%2F%20Compiler%20will%20throw%20error%20when%20%22notSame%22%20is%20unimplemented%0D%0A%20%20%2F%2F%20Compiler%20DOES%20NOT%20throw%20error%20about%20non-implemented%20%22same%22%20member%0D%0A%7D

Start by running the linked TypeScript Playground reproduction and compare the diagnostics for same and notSame in the Example class. Trace the compiler's handling of abstract members produced by the intersection and mixins; done means the missing same member is reported while the existing notSame diagnostic remains correct.