Double error message when referencing variable of type union of string literals from ambient declaration

[devanshj]

🔎 Search Terms

double error, same message twice, ambient declaration, string literal union

🕗 Version & Regression Information

This changed between versions 3.9.7 and 4.0.5

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/CYUwxgNghgTiAEYD2A7AzgF3gDwFzwCIoD4AfQgIwIChrl0sBPfFAVwFsKQZ4BeHIA

💻 Code

declare const x: "a" | "b"

const y: number = x
//    ~
//    Type 'string' is not assignable to type 'number'.
//      Type 'string' is not assignable to type 'number'. (2322)

🙁 Actual behavior

You see same error message twice

🙂 Expected behavior

The error message should be the one in 3.9.7 ie...

Type '"a" | "b"' is not assignable to type 'number'.
  Type '"a"' is not assignable to type 'number'. (2322)

Or the following message just once also works...

Type 'string' is not assignable to type 'number'. (2322)

Additional information about the issue

No response

[Andarist]

The source gets generalized here:
https://github.com/microsoft/typescript-go/blob/2ca325f7c6968077053fbc26302a2dfd44e4903b/internal/checker/relater.go#L4691-L4695

The code is not union-aware and it comes from #38049

[AdyaTech]

Hey! I think I figured out why you’re seeing the duplicate error messages.

From what I can tell:

• x is a union of string literals ("a" | "b").
• y is a number.

Now, in updated TypeScript versions, the compiler simplifies "a" | "b" to string internally, which triggers two error messages:

1. Type 'string' is not assignable to type 'number' (general)
2. Type '"a" | "b"' is not assignable to type 'number' (specific literal)

However, in TS 3.9.7 it only showed the more specific one, which is why it looked clean.