[Harsh Vardhan Chauhan (1-DARK)]

Fixes #64980

Problem

With experimentalDecorators and define semantics (useDefineForClassFields, the default at target ES2022+), a decorated abstract property was emitted as a class field. Every instance then got an own data: undefined that shadowed whatever the decorator installed on the prototype. tsc 6.0.3 erased the field.

abstract class Base {
  @dec abstract data: string;
}

Before:

class Base { data; }
// plus the __decorate call

After:

class Base { }
// plus the __decorate call

Cause

For decorated declare/abstract properties under experimentalDecorators, typeeraser.go keeps the node so LegacyDecoratorsTransformer can emit __decorate. It rebuilds the node with VisitModifiers, which elides abstract. visitPropertyDeclaration in legacydecorators.go then finds neither NodeFlagsAmbient nor ModifierFlagsAbstract and emits a field. declare is unaffected because the node keeps NodeFlagsAmbient.

Fix

When preserving a decorated abstract property, append a declare modifier so the decorator transform erases the field. This matches Strada, which appends ModifierFlags.Ambient to the preserved node.

Tests

Added legacyDecoratorAbstractPropertyDefine (target ES2022, experimentalDecorators, useDefineForClassFields). The baseline shows no field on Base and the __decorate call intact. Also checked manually that the repro prints false from prototype.

[Copilot]

🟡 Changes recommended

A decorated abstract computed property with a unique-symbol key can emit an undeclared temporary because its rebuilt node lacks NodeFlagsAmbient, causing a ReferenceError during class evaluation.

1 open issue

• Mark rebuilt abstract computed properties ambient · New

What changed in this PR

Addresses #64980 by erasing decorated abstract fields under define semantics while preserving legacy decorator calls, restoring TypeScript 6 behavior.

Changes:

• Adds a synthetic declare modifier to abstract properties preserved for the legacy decorator transform.
• Adds an ES2022 regression test and baseline showing field erasure with decoration intact.

File	Description
tsc/​testdata/​tests/​cases/​compiler/​legacyDecoratorAbstractPropertyDefine.ts	Adds regression coverage for decorated abstract fields.
tsc/​testdata/​baselines/​reference/​compiler/​legacyDecoratorAbstractPropertyDefine.js	Records erased fields and the retained __decorate call.
tsc/​internal/​transformers/​tstransforms/​typeeraser.go	Marks preserved abstract properties with a synthetic declare modifier.

_{Review effort: Balanced}