In a class, an unique symbol attribute is sometime lost during incremental build

[denis-migdal]

🔎 Search Terms

• unique property
• incremental build
• interface declaration merging

🕗 Version & Regression Information

• Tested with 6.0.2 and 6.0.3

⏯ Playground Link

Playground Link

💻 Code

I do not have a minimal reproductible exemple as the issue seems to come from the incremental build.

I had a class with an unique symbol property, on which I was doing interface declaration merging:

const SYMBOL: unique symbol = Symbol();

class X {

    protected attr = "ok";

    readonly [SYMBOL] = "ok";

    protected foo() {
        // use it
        this[SYMBOL];
    }
}

type T<N extends string> = Record<`add${N}`, () => void>;

interface X extends T<"hoc"> {}

const x = {} as X;
x.addhoc

🙁 Actual behavior

The following error first appeared when I set some of the public attributes as protected (the error below is edited to match the provided exemple) :

ERROR in [...]
TS7053: Element implicitly has an 'any' type because expression of type 'unique symbol' can't be used to index type 'X'.
  Property '[????.SYMBOL]' does not exist on type 'X'.
    92 |
    93 |     protected foo() {
  > 94 |         this[SYMBOL];
       |                 ^^^^^^^^^^^^^^^^
    95 |     }

I noticed that :

• The error disappear if I delete all of TS cache files.
• The error reappear if I rebuild without any changes to the files.
• The error reappear if I edit the files that defines X.
• The error does not reappear if I change other files.
• In watch mode, the error sometimes disappear and reappear when I change the files that defines X.
• The error doesn't appear in my VSC editor.

🙂 Expected behavior

No TS errors.

Additional information about the issue

I also reproduced the error with tsc --noEmit, so it doesn't seem to come from my Webpack build system.