Assignment of argument to argument loses deduced typing

Could be a duplicate but difficult to construct an issue search to discern.

Problem shows with typescript@latest version 3.0.1.

Given nature of problem suspect that it would not have been fixed in typescript@next. Apologies for not testing.

type inference error argument

Code

// A *self-contained* demonstration of the problem follows...
// Just paste this code into your favorite TypeScript IDE

function foobar_good(arg: unknown): void | number {

  if (typeof arg != 'number') {
    return
  }

  return arg;
}

function foobar_bad(arg: unknown): void | number {

  if (typeof arg != 'number') {
    return
  }

  arg = arg;

  return arg;
}

[ts] Type 'unknown' is not assignable to type 'number | void'.
Type 'unknown' is not assignable to type 'void'.

Expected behavior:
Should compile.

Actual behavior:
Does not compile.

issue にある自己完結型の TypeScript の例から始め、TypeScript 3.0.1 でコンパイルし、foobar_good と foobar_bad を比較します。arg 自身への代入が unknown から number への絞り込みにどのような影響を与えるかを調査します。示されたコードがコンパイルでき、期待される型チェックの動作が維持されれば完了です。