Type guard cannot remove `undefined` from `Partial<T>[keyof T] | undefined`

Bug Report

When writing a function to merge object A with non-undefined values from object B into C where A is of type T and B is of type Partial<T>, I came across a situation where the type guard seems to be failing.

🔎 Search Terms

• type guard
• Partial

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about common-bugs-that-arent-bugs

⏯ Playground Link

Playground link with relevant code

💻 Code

function merge<T>(a: T, b: Partial<T>, keys: (keyof T)[]): T {
    const v: T = { ...a };
    for (const key of keys) {
        const value = b[key];
        if (value !== undefined) {
            v[key] = value;
        }
    }
    return v;
}

🙁 Actual behavior

There is an error at v[key] = value;:

Type 'T[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
  Type 'undefined' is not assignable to type 'T[keyof T]'.

Clearly value is NOT undefined, yet the compiler complains that it is.

🙂 Expected behavior

I would expect that the code compiled.

Start with the linked TypeScript Playground and reproduce the error in the generic merge function at v[key] = value after the value !== undefined guard. Trace the checker behavior for Partial[keyof T] and indexed assignment; done means the example compiles without weakening the intended type checking.