Inference of constrained generic causes open generic to "escape"

[ddurschlag6river]

TypeScript Version: 3.5.0-dev.20190514

Search Terms:
constrained generic inference infer keyword
infer open generic
infer constrained generic

Code

interface Synthetic<A, B extends A> {}
type SyntheticDestination<T, U> = U extends Synthetic<T, infer V> ? V : never;
type TestSynthetic = // Resolved to T
 SyntheticDestination<number, Synthetic<number, number>>;

const w: TestSynthetic = undefined // ✓
const x: TestSynthetic = null; // ✓

const y: TestSynthetic = 3; // Type '3' is not assignable to type 'T'.
const z: TestSynthetic = '3'; // Type '"3""' is not assignable to type 'T'.

Expected behavior:

• TestSynthetic resolved as number
• w, x, and z invalid with strict
• y valid

Actual behavior:

• TestSynthetic resolved as T -- an open generic from the underlying conditional type
• w and x valid, even with strict
• y and z error with commented errors

Playground Link:
https://bit.ly/30lTSqk

Related Issues:
#31326

Notes:

• Removing the extends A constraint causes expected behavior, so that's likely involved somehow.
• Turning on all strict options in typescript playground causes all four example cases to be errors -- unclear why this is stricter than strict: true in my local tsconfig with @next.
• I'm not 100% sure if a conditional type that resolves to an open generic is allowed -- I was certainly surprised to see it. If it is, this still seems to be a bug in conditional type resolution.

[jack-williams]

Related #31337. I think this issue still exists with that fix