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

[dragomirtitian]

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

[fatcerberus]

I think this behavior might be connected with automatic literal widening because
Playground

declare function f3<T>(a: T, b: T): T;
const x4_1 = f3("abc", "def");  // "abc" | "def"
const x4_2 = f3("abc" as const, "def" as const);  // "abc" | "def"
let x5_1 = x4_1;
//  ^? string
let x5_2 = x4_2;
//  ^? "abc" | "def"

The behavior on x5_1 might seem weird but past input from maintainers has suggested it's intentional.

btw your playground link in the OP doesn't seem to work properly - for me it just loads whatever I had last entered into the playground.