Cannot narrow the optional properties to T properly through (T | undefined) => T if --strictOptionalProperties=true

[tetsuharuohzeki]

Bug Report

🔎 Search Terms

• --strictOptionalProperties, Strict optional properties
  • Strict optional properties #43947
  • Distinguish missing and undefined #13195

🕗 Version & Regression Information

• typescript@4.4.0-dev.20210602.
  • I seem this contains Strict optional properties #43947

• This changed between versions typescript@4.4.0-dev.20210601 and typescript@4.4.0-dev.20210602
  • Of course, this will not be happen if --strictOptionalProperties=false. But I could not think this is reasonable behavior.

⏯ Playground Link

Playground link with relevant code

At this moment which I filed this issue, typescript playground does not prepare to use 4.4.0-dev.20210602.

💻 Code

type Undefinable<T> = T | undefined;

function expectNotUndefined<T>(value: Undefinable<T>): T {
    if (value === undefined) {
        throw new TypeError('value is undefined');
    }

    return value;
}

interface Bar {
    bar?: number;
}

function a(input: Bar): void {
    // This variable should be expected as `number`
    // but typescript compiler with `--strictOptionalProperties=true` infer this variable as `number | undefined`.
    const notUndefinedVal = expectNotUndefined(input.bar);

    // TypeScript compiler says:
    //  > Argument of type 'number | undefined' is not assignable to parameter of type 'number'.
    //  > Type 'undefined' is not assignable to type 'number'.ts(2345)
    b(notUndefinedVal);
}

function b(input: number) {
    // something
}

🙁 Actual behavior

The above code will be compile error as I commented.

🙂 Expected behavior

• Pass to compile.
  • typescript compiler should infer notUndefinedVal as number.
• I think this would be a bug of strictOptionalProperties and this behavior is not reasonable.

...or are there any workaround?