3.9 regression: TS2322 error around assignment of an inferred intersection to a union

[RomainMuller]

TypeScript Version: 3.9.2, 4.0.0-dev.20200512

Search Terms: TS2322, "is not assignable to type"

Code

import * as reflect from 'jsii-reflect'; // Version ^1.5.0

export type ApiElement = reflect.Type | reflect.TypeMember | reflect.EnumMember;

export interface Data {
  readonly element: ApiElement;
}

// Note: reflect.Method and reflect.Initializer both directly or indirectly implement reflect.TypeMember
export function doSomething<T extends (reflect.Method | reflect.Initializer)>(element: T): Data {
  if (reflect.Method.isMethod(element)) {
    // Type 'T & Method' is not assignable to type 'Type'.
    return { element };
  }
  if (reflect.Initializer.isInitializer(element)) {
    // Type 'T & Initializer' is not assignable to type 'Type'.
    return { element };
  }
  // Type checking absolutely fine - why'd previous ones not?
  return { element };
}

Expected behavior:
It should work like it did in TypeScript 3.8

Actual behavior:

$ tsc --build
lib/index.ts:13:14 - error TS2322: Type 'T & Method' is not assignable to type 'ApiElement'.
  Type 'T & Method' is not assignable to type 'Type'.

12     return { element };
                ~~~~~~~

  lib/index.ts:6:12
    6   readonly element: ApiElement;
                 ~~~~~~~
    The expected type comes from property 'element' which is declared here on type 'Data'

lib/index.ts:17:14 - error TS2322: Type 'T & Initializer' is not assignable to type 'ApiElement'.
  Type 'T & Initializer' is not assignable to type 'Type'.

16     return { element };
                ~~~~~~~

  lib/index.ts:6:12
    6   readonly element: ApiElement;
                 ~~~~~~~
    The expected type comes from property 'element' which is declared here on type 'Data'

Playground Link: I stored my repro at https://github.com/RomainMuller/TypeScript39-Bug (public)

Related Issues: It appears likely related to the breaking change in #37195 (but not certain)

[RomainMuller]

Note - I have tried two build a reproduction that does NOT involve importing the separate package - to no avail. Maybe I just didn't try hard enough, or maybe this problem is triggering only when using a locally defined union of types from a dependency?

In any case, here are pointers to the relevant types within the jsii-reflect library (1.5.0 release):

• reflect.Type
• reflect.TypeMember
• reflect.EnumMember
• reflect.Callable, which is the superclass of
  • reflect.Method
  • reflect.Initializer

[ahejlsberg]

Simplified repro:

interface AB {
    kind: 'a' | 'b';
}

declare class CA { kind: 'a'; a: string; x: number };
declare class CB { kind: 'b'; b: string; y: number };

function bar<T extends CA | CB>(x: T & CA) {
    let ab: AB = x;  // Error, but shouldn't be
}

[RomainMuller]

Oh wow so the discriminators were the problem? Swoosh! Well thanks for the prompt response & quick fix!!

[ahejlsberg]

Well, the exact issue is that the new extra check in #37537 relies on empty intersections being removed (as per #36696) and that wasn't happening here.