Simple ternary inference issue with generics

[jeremybparagon]

TypeScript Version: 3.2.0-dev.20181004

Code

const f = <T>(a: number) => {
  const b: (T extends { } ? number : number) = a;
};

Expected behavior: Successful compilation

Actual behavior: I get the following error:

error TS2322: Type 'number' is not assignable to type 'T extends {} ? number : number'.

   const b: (T extends { } ? number : number) = a;
         ~

Playground Link: Link

Use case:
I wouldn't expect anybody to write the code above. Here's something more like my original example (link):

const f = <B extends true | false>(b: B, n: number): (true extends B ? number | null : number) => {
  if (b) {
    return null as true extends B ? null : never;
  }
  return n;
};

This gives the error

error TS2322: Type 'number' is not assignable to type 'true extends B ? number : number'.

   return n;
   ~~~~~~~~~

In this practical example, there is a workaround, changing the return type to number | (true extends B ? null : never). But it was hard for me to figure out what was making TypeScript unhappy, since this seemed too simple an inference for TypeScript to miss.

[jack-williams]

The first example is due to this #27530 I think.

The second example is covered by this #26933 I think.

[jeremybparagon]

Both should be covered by #26933. Thanks.