Type narrowing on properties of object not respected when destructuring into new object

[tubocurarin]

Bug Report

It appears, that when a type is narrowed, the narrowing is ignored when assigning to a technically compatible type.

I skimmed the issue tracker rather quickly, I hope I haven't missed an already known issue with a ticket.

🔎 Search Terms

• narrow(ed)
• destructuring / spread

🕗 Version & Regression Information

• TypeScript v4.3.2 & v.4.5.2

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about 10 minutes

⏯ Playground Link

Playground link with relevant code

💻 Code

Happens when:

type X = {
    a: string
    b: string
    c: string | null
}

type Y = {
    a: string
    b: string
    c: string
}

const x: X = {
    a: 'a',
    b: 'b',
    c: 'c',
}

if (x.c !== null) {
    const y: Y = {
        ...x,  // ERROR
    }
}

error TS2322: Type '{ a: string; b: string; c: string | null; }' is not assignable to type 'Y'.
  Types of property 'c' are incompatible.
    Type 'string | null' is not assignable to type 'string'.
      Type 'null' is not assignable to type 'string'.

20     const y: Y = {
             ~

When explicitly passing the field again, it works:

type X = {
    a: string
    b: string
    c: string | null
}

type Y = {
    a: string
    b: string
    c: string
}

const x: X = {
    a: 'a',
    b: 'b',
    c: 'c',
}

if (x.c !== null) {
    const y: Y = {
        ...x,
        c: x.c,  // WORKS!
    }
}

🙁 Actual behavior

See above, error is thrown although the "problematic" property was narrowed down to be compatible with the target type in the assignment.

🙂 Expected behavior

Should not throw a type error and should not require the workaround in the above code example with the explicit assignment from the narrowed-down field: c: x.c

[fatcerberus]

Narrowing individual properties of an object won’t narrow the containing type except in certain situations, because it’s generally difficult (from a technical standpoint) to do so; if this were possible in general we wouldn’t need to have discriminated unions.

duplicate of #30506.

[RyanCavanaugh]

See #26645