Can't destruct a Partial<T> into T that uses default values

TypeScript Version: 4.2.0-dev.20201219

Search Terms: Destructuring with partial and default value

Code

type T = {a: number};
const t: Partial<T> = {};
const {a = 10}: T = t;

Expected behavior:

This should be allowed. For each prop with a default value, the undefined type should be Exclude'd from the source object.

Actual behavior:

Throws:

Type 'Partial<T>' is not assignable to type 'T'. Types of property 'a' are incompatible. Type 'number | undefined' is not assignable to type 'number'. Type 'undefined' is not assignable to type 'number'.

Playground Link:
https://www.typescriptlang.org/play?ts=4.2.0-dev.20201219#code/C4TwDgpgBAKlC8UDeBDAXFAdgVwLYCMIAnAXwG4BYAKAGMB7TAZ2CmAwAUUjgBLFAGwA8MAHwJk5avSYtU4gIwAGEhjiJgZIA
Related Issues:

Start with the provided TypeScript Playground reproduction and compare the destructuring assignment's expected and actual behavior. The issue names no source files or tests, so trace the type-checking path for destructuring with a default value and Partial. Done means the example is accepted while preserving the intended type safety, with coverage for this case.