Weird inference bug where a union is expanded with "never" in the position of generics

[calebmer]

TypeScript Version: 3.5.0-dev.20190423

This code works in 3.3.4000 and is broken in 3.4.5

Code

function MyComponent({a, b}: {a: number; b: number}) {
  return a + b;
}

function test<Props>(
  _getComponent: () => Promise<(props: Props) => number | string>,
) {}

test(() => Promise.resolve({MyComponent}).then(m => m.MyComponent));

Of course this is a simplified example. The code I found this in uses React and lazy loading with import().

Expected behavior: This example should pass type checking without any problems.

Actual behavior:

test.ts:9:12 - error TS2322: Type 'Promise<(({ a, b }: { a: number; b: number; }) => number) | ((props: never) => string | number)>' is not assignable to type 'Promise<(props: { a: number; b: number; }) => string | number>'.
  Type '(({ a, b }: { a: number; b: number; }) => number) | ((props: never) => string | number)' is not assignable to type '(props: { a: number; b: number; }) => string | number'.
    Type '(props: never) => string | number' is not assignable to type '(props: { a: number; b: number; }) => string | number'.
      Types of parameters 'props' and 'props' are incompatible.
        Type '{ a: number; b: number; }' is not assignable to type 'never'.

9 test(() => Promise.resolve({MyComponent}).then(m => m.MyComponent));
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  test.ts:6:18
    6   _getComponent: () => Promise<(props: Props) => number | string>,
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The expected type comes from the return type of this signature.

Notably here the actual type is inferred to be…

Promise<(({ a, b }: { a: number; b: number; }) => number) | ((props: never) => string | number)>

…which has an extra union variant…

(props: never) => string | number

…coming out of nowhere.

Playground Link:

You need to enable strictFunctionTypes.

http://www.typescriptlang.org/play/index.html#src=function%20MyComponent(%7Ba%2C%20b%7D%3A%20%7Ba%3A%20number%3B%20b%3A%20number%7D)%20%7B%0A%20%20return%20a%20%2B%20b%3B%0A%7D%0A%0Afunction%20test%3CProps%3E(%0A%20%20_getComponent%3A%20()%20%3D%3E%20Promise%3C(props%3A%20Props)%20%3D%3E%20number%20%7C%20string%3E%2C%0A)%20%7B%7D%0A%0Atest(()%20%3D%3E%20Promise.resolve(%7BMyComponent%7D).then(m%20%3D%3E%20m.MyComponent))%3B%0A