Destructing an optional prop of a Readonly generic with a default, the type is narrowed incorrectly

[agubler]

TypeScript Version: 2.7.2 & 2.8.0-insiders.20180320

Search Terms:

Description

For a generic (that is being mapped with Readonly<>), it's properties are being narrowed to the value that is assigned as a default during destructuring with strictNullChecks set to true. I have seen this for boolean and enum types.

Code

interface Props {
	foo?: boolean;
}

class Foo<P extends Props = Props> {
	props: Readonly<P> = {} as P;

	myFunc() {
		const { foo = false } = this.props;
                
                // errors because the type of `foo` is narrowed to `false` and cannot be 
                // compared to `true`
		if (foo === true) {}
	}
}

Removing Readonly from the type of props "fixes" the error, so it seems like it is related to mapped types.

Expected behavior:

foo is narrowed to the defaulted value false but should keep the type boolean.

Actual behavior:

foo should default to false but keep the base type of boolean.

Playground Link (use strictNullCheck): https://www.typescriptlang.org/play/index.html#src=interface%20Props%20%7B%0D%0A%09foo%3F%3A%20boolean%3B%0D%0A%7D%0D%0A%0D%0Aclass%20Foo%3CP%20extends%20Props%20%3D%20Props%3E%20%7B%0D%0A%20%20%20%20props%3A%20Readonly%3CP%3E%20%3D%20%7B%7D%20as%20P%3B%0D%0A%0D%0A%09myFunc()%20%7B%0D%0A%09%09const%20%7B%20foo%20%3D%20false%20%7D%20%3D%20this.props%3B%0D%0A%09%09if%20(foo%20%3D%3D%3D%20true)%20%7B%7D%0D%0A%09%7D%0D%0A%7D

Related Issues:

#17005

[agubler]

Thanks @ahejlsberg!