SWC does not remove interface when used with parameter decorator

[8naf]

Describe the bug

When using a parameter decorator with a parameter of type interface, swc does not remove that interface, leading to errors at runtime.

Input code

// sub.ts
export interface Interface {
  p: string
}



// main.ts
import { Interface } from './sub.js'

function ParameterDecorator() {
  return function(
    target: any,
    propertyKey: string | symbol,
    parameterIndex: number,
  ) { }
}

class Class {
  public constructor(
    @ParameterDecorator()
    i: Interface,
  ) { }
}

const c = new Class({ p: '' })
console.log('success')

Config

{
	"$schema": "https://json.schemastore.org/swcrc",
	"jsc": {
		"parser": {
			"syntax": "typescript",
			"decorators": true,
			"dynamicImport": true
		},
		"transform": {
			"decoratorMetadata": true,
			"legacyDecorator": true
		},
		"target": "es2022",
		"loose": false,
		"externalHelpers": false,
		"keepClassNames": true
	},
	"sourceMaps": true,
	"module": {
		"type": "es6"
	},
	"minify": false
}

Playground link

https://replit.com/@8NAF/swc-issues-7101

Expected behavior

The interface should be removed.

Actual behavior

No response

Version

1.3.41

Additional context

I can fix this issue by using import type { Interface } from './sub.js', but I still opened the issue for the following 3 reasons:

• I am using typescript-eslint, and it will automatically remove type when importing with interfaces (specifically, with this rule).
• The interface will be removed if it is not used with a parameter decorator, but it will not be removed if it is used with one, which causes inconsistency.
• tsc can handle this case normally without any runtime errors.

[magic-akari]

This requires a cross-file type system, which is impossible for workflow like swc and Babel that compile files one by one.

When using swc, it is recommended to set importsNotUsedAsValues to error in the tsconfig.json file.
To improve other compatibilities, isolatedModules should be set to true.

In the case of typescript-eslint, if it removes type while TS considers it to be a type, then it is a bug in typescript-eslint.

[8naf]

@magic-akari

ok, I understand.

But I have a note for those who are using vscode, should not set importsNotUsedAsValues to error.

When using the value of error, vscode's auto import feature always add import type for everything, including classes. Therefore, classes are removed when compiled, leading to decorators not working correctly.

When using the other two values (remove and preserve), vscode's auto import feature doesn't add import type, but report an error when there is no import type for an interface, and not report an error when using a class as a type, which makes everything safer.

[8naf]

By the way, can you add this note somewhere in your documentation? I found someone else who encountered a similar problem (#1839), but it happened with method decorators.

[magic-akari]

When using the value of error, vscode's auto import feature always add import type for everything, including classes. Therefore, classes are removed when compiled, leading to decorators not working correctly.

Class can be used as a value, as well as a type.
Perhaps, VSCode may erroneously add 'type', however, TypeScript's type-check captures this error, avoiding it being deferred to runtime.
Conversely, without the type modifier, SWC or Babel cannot distinguish between them. The imported binding can be either a class or an interface (as with your current issue). TS's leniency may postpone errors until runtime.

tsc can eliminate implicit type imports due to its cross file type system awareness.
Therefore, having errors detected during type-checking phase facilitates compatibility when migrating to other tools.

---

@kdy1 We should document more information to assist TS users when Integrating with SWC.

[kdy1]

I made https://swc.rs/docs/migrating-from-tsc but not sure how should I describe this problem

[swc-bot]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.