Possible type inference regression (from v3.2.4 onwards)

[tlaitinen]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms: type inference regression

Code

export function f<A>() {
  let a:undefined extends A | undefined ? A | undefined : undefined = undefined;
  return a;
}

Expected behavior: No type error. TypeScript compiler version 3.2.2 gives no error.

Actual behavior:
Starting version 3.2.4 TypeScript compiler gives the following type error:

$ tsc --strict error.ts
error.ts:2:7 - error TS2322: Type 'undefined' is not assignable to type 'undefined extends A | undefined ? A | undefined : undefined'.

2   let a:undefined extends A | undefined ? A | undefined : undefined = undefined;

Playground Link: could not reproduce in TypeScript Playground.

Related Issues:

[jack-williams]

Looks related to #29662

[DanielRosenwasser]

Looks to be fixed in 4.3.

export function f<A>() {
    let a: undefined extends A | undefined ? A | undefined : undefined = undefined;
    return a;
}

[typescript-bot]

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of this repro running against the nightly TypeScript.

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Comment by @DanielRosenwasser

👍 Compiled

Historical Information

Version	Reproduction Outputs
4.3.2, 4.4.2, 4.5.2, 4.6.2, 4.7.2	👍 Compiled