Mixin abstract class: union of abstract propeties is not abstract.

TypeScript Version: 3.9.2

Search Terms: mixin abstract union

Expected behavior:
Union of abstract propeties is abstract property.

Actual behavior:
Union of abstract propeties is not abstract.

Related Issues:

Code

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

class BaseClass {}

function AB<TBase extends Constructor>(Base: TBase) {
  abstract class AB extends Base {
    abstract a: number
    abstract b: number
  };
  return AB
}
function AC<TBase extends Constructor>(Base: TBase) {
  abstract class AC extends Base {
    abstract a: number
    abstract c: number
  };
  return AC
}

// should throw "Non-abstract class 'ShouldHavePropA' does not implement inherited abstract member 'a' from class ..."
class ShouldHavePropA extends AB(AC(BaseClass)) { 
    b = 1;
    c = 2;
}

"use strict";
class BaseClass {
}
function AB(Base) {
    class AB extends Base {
    }
    ;
    return AB;
}
function AC(Base) {
    class AC extends Base {
    }
    ;
    return AC;
}
class ShouldHavePropA extends AB(AC(BaseClass)) {
    constructor() {
        super(...arguments);
        this.b = 1;
        this.c = 2;
    }
}

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "ES2017",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

Start with the TypeScript 3.9.2 Playground link and reduce the provided mixin example to a focused compiler test. Trace how abstract properties are collected through the union of the AB and AC mixins. Done means the example reports that ShouldHavePropA fails to implement inherited abstract member a, with a regression test covering the behavior.