Fix boolean literal types

[ahejlsberg]

Fixes #10432.

This PR introduces two changes:

• When an expression has the contextual type boolean we now consider it a literal type context. Previously we specifically excluded this case.
• When a binding element has a default value, we now give it a union type of the destructured type and the default type. Previously we'd give it only the destructured type and require the default type to be assignable to the destructured type.

Some background on the changes.

Since boolean is identical to the union type true | false, when an expression has the contextual type boolean we really should consider it a literal type context (because the contextual type is a union of unit types). Before this PR we specifically excluded that case in the name of backwards compatibility. With the fix we no longer do. However, that causes another issue to appear:

let { x = true } = { x: cond ? false : undefined };

In the example above, the expression cond ? false : undefined is contextually typed by type boolean because of the default value on the left hand side. That causes us to consider the expression a literal type context, and we therefore use type false for the false literal. So, the type of x on the right hand side becomes false | undefined. Now, when we destructure x we require the default value to be of a type assignable to false | undefined. But boolean isn't.

The issue really is that when a binding element has a default value, we ought to give it a union type of the destructuring type and the default type. With this PR we do. The downside is that you might sometimes end up with a union type when an error might have been more appropriate.

@mhegazy @DanielRosenwasser You might want to take a look.

[DanielRosenwasser]

The initializer is completely ignoring the type annotation in this case. I would say this is undesirable.

[ahejlsberg]

@mhegazy Want to take a look? I think we need this one in 2.0.

[mhegazy]

👍

[mhegazy]

merged in release-2.0 in 2cd4d20