Fix importHelpers incorrectly requiring tslib for native #private class members (#63728)

[Youssef Mansour (YoussefMansour9)]

Summary

The check for private field helpers was incorrectly gated by ClassAndClassElementDecorators (pinned to ESNext), causing all dated targets (ES2022–ES2025) to require tslib even though native private fields are supported since ES2022 and no helper is ever emitted.

Root Cause

In checker.go, the private field helper requirement was gated by:

if c.languageVersion < LanguageFeatureMinimumTarget.PrivateNamesAndClassStaticBlocks ||
    c.languageVersion < LanguageFeatureMinimumTarget.ClassAndClassElementDecorators ||
    !c.compilerOptions.GetUseDefineForClassFields() {
    // ...checkExternalEmitHelpers
}

LanguageFeatureMinimumTarget.ClassAndClassElementDecorators is pinned to ScriptTarget.ESNext because TC39 decorators have never been assigned to a dated ECMAScript edition. This made the condition unconditionally true for every dated target, regardless of whether the file uses decorators.

Fix

Removed the decorator check from private field helper requirements in four locations:

1. checkPropertyAccessExpressionOrQualifiedName (lines 11361-11363) - private field get/set
2. checkInExpression (lines 13179-13181) - private field in checks
3. setNodeLinksForPrivateIdentifierScope (lines 10621-10623)
4. getFirstTransformableStaticClassElement (line 10150)

Now private fields only require helpers when:

• Target is before ES2022 (PrivateNamesAndClassStaticBlocks)
• useDefineForClassFields is false (legacy class field semantics)

Testing

• All existing tests pass
• Verified fix works for ES2022, ES2023, ES2024, ES2025 targets
• Verified ESNext still works
• Verified decorators still correctly require tslib

Fixes #63728

[Copilot]

Pull request overview

Updates private-field helper detection to avoid unnecessary tslib requirements for native private fields.

Changes:

• Removes broad decorator-target gating from four checker paths.
• Documents intended ES2022 and class-field behavior.

Suppressed comments (3)

tsc/internal/checker/checker.go:11371

• Standard class decorators can still force static private members to be transformed at ES2022+. In esDecorators-classDeclaration-fields-staticPrivate(target=es2022).js, accesses to the class-decorated D.#field1 emit __classPrivateFieldGet and __classPrivateFieldSet (lines 58–59). With this condition, importHelpers no longer validates or resolves those helpers, so compilation can succeed and then emit imports that are missing or incompatible. Retain a decorator-specific branch limited to static private declarations in a standard-decorated class.
  tsc/internal/checker/checker.go:13190
• The same decorator-specific lowering applies to brand checks on static private members of a class with a standard class decorator. Such a #x in value expression is rewritten to __classPrivateFieldIn even at ES2022+, but this condition no longer registers the imported helper. Please preserve the helper check when the resolved private declaration is static and its containing class is transformed by standard decorators.
  tsc/internal/checker/checker.go:11371
• useDefineForClassFields: false does not lower native private names at ES2022+; it only relocates field initializers. The existing ESNext baseline privateNameWhenNotUseDefineForClassFieldsInEsNext(target=esnext).js preserves every private access (for example lines 84–110), so no private helper is emitted. Keeping this branch means the reported TS2354 false positive still occurs for users who explicitly disable useDefineForClassFields. Base the check on actual private-name lowering (plus the targeted standard-decorator/static-private case), and update the parallel in and collision conditions as well.

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[Arsh Verma (ArshVermaGit)]

not approvable yet, as the Copilot review flags a high-severity regression where decorated classes may still require the decorator transform/helper.