Cannot pass optional property _a_ to the other type's optional property _b_ 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.

⏯ 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

interface A {
    a?: number;
}

function outer(input: A) {
    inner({
        // typescript compiler with `--strictOptionalProperties=true` will say here is error as
        //  > Type 'number | undefined' is not assignable to type 'number'.
        //  > Type 'undefined' is not assignable to type 'number'.ts(2322)
        a: input.a,
    });
}

function inner(input: A) {
}

🙁 Actual behavior

TypeScript compiler fail to compile the the above example with the reason I commented.
After I read #43947, I could not conclude the current behavior is expected design. So I filed this as a bug.

🙂 Expected behavior

I thinks this should be pass to compile.

Remarks

If we pass outer's input to inner directly like the following example, typescript compile success to compile.

interface A {
    a?: number;
    b: string;
}

interface B {
    a?: number;
}

function outer(input: A) {
    inner(input);
}

function inner(input: B) {
}

[fatcerberus]

strictOptionalProperties means, roughly, that an optional property must either exist with the specified type, or not exist at all. Allowing the assignment would mean you've possibly created an explicit property with the value undefined, which is counter to the purpose of the flag.

[tetsuharuohzeki]

Ah, I see what #13195 aims. By that, this would not be a bug. Thank you.