`unknown` isn’t narrowed by assignment

Bug Report

🔎 Search Terms

try block

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about try-catch blocks

⏯ Playground Link

Playground link with relevant code

💻 Code

function f(request: {params: {[param: string]: unknown;}}) {
    let x = request.params.x;
    try {
        x = validate(x);
    } catch (e) {}

    try {
        takesString(x);  // Fails to compile, x is of type unknown.
    } catch (e) {}
}

function validate(x: unknown): string { return ""; }
function takesString(s: string) {}

🙁 Actual behavior

In this example, the code fails to compile because tsc has deduced x in the second block to be of type unknown

🙂 Expected behavior

Since we have successfully exited the first try-catch block, x's type should be narrowed to string in the second.

まずリンクされている TypeScript Playground の例を実行して、2 つの try-catch ブロック周辺で発生する失敗を再現します。次に、try-catch の境界をまたぐ代入に対するコンパイラーの control-flow narrowing を追跡し、示されているケースの回帰テストを追加します。最初のブロックの後で tsc が takesString(x) を受け入れ、他の箇所の narrowing を弱めていなければ完了です。