Please explain what the issue is and how this fix solves it.

In the process of doing control flow analysis for a, we replace it with it's filtered constraints in either branch of the conditional following the destructuring pattern, then union them back together. In this way, T["a"] becomes undefined | number | object. We fire off a used before defined check because we don't see that undefined is in the domain of T["a"]. By checking it's apparent type for falsy flags, we check against the constraint instead, which yields the correct result.

This seems to fix the problem, but I don't think it's fixing the actual problem, which is the "use before assignment" assertion on something that has been assigned. Regardless of what the type of T["a"] is, after let { a } = anything, a is definitely assigned. If T["a"]'s domain includes undefined, that just makes so that a's type can include undefined.

(NOTE: I'm not very familiar with how the checking works internally, so I'm commenting based on the explanation above)

@Kovensky the way that we do use before assignment checks in strict null checks mode is all about the type. You can only use before assignment a type without undefined in it's domain, so if the type at a specific location still has undefined in it (which is introduced by a declaration without an assignment) but didn't have it in the declared type, then it must still be there because it hasn't been assigned yet. At least, that's the assumption we currently make to issue the error.

LGTM. @ahejlsberg any comments?

@ahejlsberg This now uses the fix we talked about in person.

@ahejlsberg done

@weswigham I think @Kovensky is right that fixing the issue mentioned above could be achieved with a smaller change.
Identifiers whose declaration is a BindingElement are always initialized. That means you could just add another condition to the initializer of assumeInitialized inside checkIdentifier (currently line 13043).

@ajafff This also undoes an older bugfix which caused the issue in the first place, and instead fixes that bug differently. That's why the change is large.