3.9 regression: TS2322 error around assignment of an inferred intersection to a union #38542
Description
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)