Non-null assertion should work on possibly-uninitialized auto-typed `let` variables

Bug Report

🔎 Search Terms

non-null postfix bang implicit any

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

This is the link

💻 Code

let foo;
if (Math.random() > 0.5) {
  foo = "hello";
}
// bar: string | undefined, seems fine
let bar = foo;
/* -- pretend I know foo is defined for other reasons -- */
// Error, foo is implicitly any
let baz = foo!;

🙁 Actual behavior

Implicit any on foo!

🙂 Expected behavior

Since the inferred type of foo (as seen in bar) is string | undefined, foo! should trivially have the type string

Start with the linked TypeScript Playground and the foo, bar, and baz example, then trace the compiler's inference and non-null assertion handling for an uninitialized let. Done means foo! is accepted with type string rather than producing an implicit-any error, while bar continues to infer string | undefined.