isolatedDeclarations causing errors in setup when using implicit slots

[PindaPixel]

Vue - Official extension or vue-tsc version

2.2.4

VSCode version

1.97.2

Vue version

3.5.13

TypeScript version

5.7.3

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (12) x64 11th Gen Intel(R) Core(TM) i5-11400H @ 2.70GHz
    Memory: 12.67 GB / 31.67 GB
  Binaries:
    Node: 22.2.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.7.0 - C:\Program Files\nodejs\npm.CMD
    bun: 1.2.4 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (130.0.2849.46)
    Internet Explorer: 11.0.22621.3527

package.json dependencies

{
    "dependencies": {
        "vue": "^3.5.13"
    },
    "devDependencies": {
        "@tsconfig/node22": "^22.0.0",
        "@types/node": "^22.13.4",
        "@vue/tsconfig": "^0.7.0",
        "typescript": "~5.7.3",
        "vue-tsc": "^2.2.2"
    }
}

Steps to reproduce

1. Clone https://github.com/PindaPixel/isolated-declarations-vue-repro
2. Install dependencies
3. Restart Vue and TS Servers?
4. Open Button.vue
5. Behold:
   
6. Do one of the following to make the error disappear:
   a. Remove any and all slots from the template
   b. Use defineSlots in setup
   c. Disable isolatedDeclarations in tsconfig.app.json
7. The error disappears

What is expected?

I don't know what the expected behaviour is of isolatedDeclarations in SFC files. If vue-tsc can benefit from it then by all means I expect helpful errors to show up in the setup script. Otherwise I don't expect any errors caused by isolatedDeclarations.

What is actually happening?

Errors are reported that probably shouldn't be.

Link to minimal reproduction

https://github.com/PindaPixel/isolated-declarations-vue-repro

Any additional comments?

Interestingly enough I don't see the same error happening in the browser editor: https://github.dev/PindaPixel/isolated-declarations-vue-repro.

Anyhow, for now the workaround is to use defineSlots everywhere.

[KazariEX]

I think this error is difficult to handle, because the concept of isolatedDeclarations conflicts with VLS: it requires explicit declaration of exported variable types, while VLS strongly depends on type inference.

[KazariEX]

The error actually persists, but sometimes VLS cannot correctly map all the error back to the source code.