[ID-Prep] Type of variable is widened in declaration with !strictNullChecks

The [ID-prep] set of issues aligns Declaration Emit with the forthcoming Isolated Declarations feature.

🔎 Search Terms

const declaration widen

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground Link

💻 Code

declare function f3<T>(a: T, b: T): T;
const x3 = f3("abc", "def");  // "abc" | "def"

🙁 Actual behavior

x3 is emitted as declare const x3: string; in declaration files but it's actually "abc" | "def" in source

🙂 Expected behavior

x3 is emitted as declare const x3: "abc" | "def"

Additional information about the issue

No response

リンクされた TypeScript Playground の再現と、issue で説明されている declaration emit の動作から始めます。ジェネリック呼び出しから推論されたリテラルユニオンが、出力される declaration の型になるまでを追跡します。完了条件は、declaration が string に拡大されず、"abc" | "def" を保持することです。