Fix property access name substitution in decorator and class field transformers by Copilot · Pull Request #3147 · microsoft/typescript-go

Copilot · 2026-03-18T17:45:58Z

Legacy decorator and class fields transformers incorrectly rename property access names when the name matches a class with an alias. For example, MyEnum.Foo inside a decorated class Foo emits as MyEnum.Foo_1, and obj.Qux inside a class expression class Qux with static fields emits as obj._a.

// @experimentalDecorators: true
@myDecorator
export class Foo {
    type = MyEnum.Foo;       // emitted as MyEnum.Foo_1 ❌
    getType() {
        return MyEnum.Foo;   // emitted as MyEnum.Foo_1 ❌
    }
}

Cause: visitIdentifier in both legacydecorators.go and classfields.go substitutes all identifiers resolving to the class, including property access names. In TypeScript, onSubstituteNode only fires for EmitHint.Expression, which excludes the .name of PropertyAccessExpression. The Go port uses a visitor instead, so it lacks that distinction.

Fix: Add explicit PropertyAccessExpression visitor functions that visit the expression child but skip visiting the Name child, preventing property access names from reaching visitIdentifier and being incorrectly substituted. This avoids relying on parent access checks, which are unreliable in transforms since previous transforms may not set parents.

Added visitPropertyAccessExpression in legacydecorators.go that visits only the expression, not the name
Added visitPropertyAccessExpressionForSubstitution in classfields.go and updated visitPropertyAccessExpression to use it for identifier names while preserving VisitEachChild for private identifier names
Handled PropertyAccessExpression with identifier names before the identifier early-out in classfields.go to prevent name substitution in non-class-field subtrees
Added comprehensive test cases across all affected transformer paths:
- legacyDecoratorsEnumAccessSameNameAsClass.ts — enum access, object property access, nested access, static member access, self-reference
- classFieldsPropertyAccessSameNameAsClass.ts — class declarations + class expression with self-reference at target es2020
- classFieldsPrivatePropertyAccessSameNameAsClass.ts — private fields with class expression at target es2020
- esDecoratorsPropertyAccessSameNameAsClass.ts — ES decorators at target es2020

Original prompt

This section details on the original issue you should resolve

<issue_title>experimentalDecorators renames enum accesses with same name as class</issue_title>
<issue_description>Given:
// @experimentalDecorators: true
export enum MyEnum {
    Foo = "FooValue",
    Bar = "BarValue",
}
 
function myDecorator(target: any) {
    return target;
}
 
@myDecorator
export class Foo {
    type: MyEnum = MyEnum.Foo;
 
    getType(): MyEnum {
        return this.type || MyEnum.Foo;
    }
}
We emit:
let Foo = Foo_1 = class Foo {
    type = MyEnum.Foo_1;
    getType() {
        return this.type || MyEnum.Foo_1;
    }
};
Incorrectly renaming Foo to Foo_1. But, obviously MyEnum.Foo is not Foo itself.</issue_description>

<agent_instructions>Create a test case FIRST! Accept the baselines, and commit that. Then fix the issue.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes experimentalDecorators renames enum accesses with same name as class #3146

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Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

…lass Skip identifier substitution when the identifier is the name part of a PropertyAccessExpression (e.g., MyEnum.Foo). In TypeScript, the substituteExpression is only invoked for EmitHint.Expression, which excludes member names in property accesses. In Go, since substitution is done via the visitor, we need to explicitly skip identifiers that are property access names. Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

jakebailey · 2026-03-18T18:08:07Z

@copilot I can imagine this bug happening all over the place, for ES decorators, class fields, etc etc. Can you write a slew of tests that could break now that we don't do this via substitution?

- Expanded legacy decorators test with more property access scenarios (object access, nested access, static member, self-reference) - Added class fields test with class expressions at target es2020 that trigger temp alias creation and demonstrate the bug - Added class fields + private fields test at target es2020 - Added ES decorators test at target es2020 - Fixed the same property access name substitution bug in classfields.go visitIdentifier (confirmed with class expression tests) Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot · 2026-03-18T18:24:04Z

@copilot I can imagine this bug happening all over the place, for ES decorators, class fields, etc etc. Can you write a slew of tests that could break now that we don't do this via sub...

Good call — confirmed the same bug exists in the class fields transformer (classfields.go). Fixed it in bd33135.

Added tests across all three transformer paths:

Legacy decorators: expanded test with enum access, object property access, nested access, static member access, and self-reference scenarios
Class fields (classFieldsPropertyAccessSameNameAsClass): includes a class expression with self-reference + static fields at target es2020, which creates a temp alias and demonstrated obj2.Qux being incorrectly emitted as obj2._a
Class fields + private fields (classFieldsPrivatePropertyAccessSameNameAsClass): same pattern with private fields at target es2020
ES decorators (esDecoratorsPropertyAccessSameNameAsClass): at target es2020 to exercise both ES decorator and class fields transformers

internal/transformers/estransforms/classfields.go

Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

internal/transformers/utilities.go

…cess in transformers and checker Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot

Pull request overview

Fixes an identifier-substitution bug in the Go port’s legacy decorators + class fields transformers where property-access names (e.g. MyEnum.Foo’s Foo) could be incorrectly substituted when they resolve to a class alias, aligning behavior with TypeScript’s emit-time substitution rules.

Changes:

Skip class-alias substitution when an identifier is the .name of a PropertyAccessExpression in legacy decorators and class fields transformers.
Refactor checker helpers to reuse existing AST access-expression predicates.
Add new compiler regression tests and accepted baselines covering legacy decorators, ES decorators, and class fields/private fields scenarios.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/legacyDecoratorsEnumAccessSameNameAsClass.ts	New legacy-decorators regression covering enum/member/property access name substitution behavior.
testdata/tests/cases/compiler/esDecoratorsPropertyAccessSameNameAsClass.ts	New ES-decorators regression covering property access name substitution behavior.
testdata/tests/cases/compiler/classFieldsPropertyAccessSameNameAsClass.ts	New class-fields regression including class expression + static field lowering path.
testdata/tests/cases/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.ts	New class-fields regression for private fields + aliasing behavior.
testdata/baselines/reference/compiler/legacyDecoratorsEnumAccessSameNameAsClass.types	Accepted baseline output for the new legacy-decorators test (types).
testdata/baselines/reference/compiler/legacyDecoratorsEnumAccessSameNameAsClass.symbols	Accepted baseline output for the new legacy-decorators test (symbols).
testdata/baselines/reference/compiler/legacyDecoratorsEnumAccessSameNameAsClass.js	Accepted baseline output for the new legacy-decorators test (JS emit).
testdata/baselines/reference/compiler/esDecoratorsPropertyAccessSameNameAsClass.types	Accepted baseline output for the new ES-decorators test (types).
testdata/baselines/reference/compiler/esDecoratorsPropertyAccessSameNameAsClass.symbols	Accepted baseline output for the new ES-decorators test (symbols).
testdata/baselines/reference/compiler/esDecoratorsPropertyAccessSameNameAsClass.js	Accepted baseline output for the new ES-decorators test (JS emit).
testdata/baselines/reference/compiler/classFieldsPropertyAccessSameNameAsClass.types	Accepted baseline output for the new class-fields test (types).
testdata/baselines/reference/compiler/classFieldsPropertyAccessSameNameAsClass.symbols	Accepted baseline output for the new class-fields test (symbols).
testdata/baselines/reference/compiler/classFieldsPropertyAccessSameNameAsClass.js	Accepted baseline output for the new class-fields test (JS emit).
testdata/baselines/reference/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.types	Accepted baseline output for the new private-fields test (types).
testdata/baselines/reference/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.symbols	Accepted baseline output for the new private-fields test (symbols).
testdata/baselines/reference/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.js	Accepted baseline output for the new private-fields test (JS emit).
internal/transformers/tstransforms/legacydecorators.go	Skip substitution for property-access names while resolving class-alias references.
internal/transformers/estransforms/classfields.go	Skip substitution for property-access names in the class-fields alias substitution path.
internal/checker/utilities.go	Refactor access-expression helper to reuse AST predicates.
internal/checker/checker.go	Refactor index-signature lookup to use `ast.IsRightSideOfPropertyAccess`.

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jakebailey · 2026-03-18T19:07:23Z

internal/checker/checker.go

+	if ast.IsIdentifier(node) && ast.IsRightSideOfPropertyAccess(node) {
 		keyType := c.getLiteralTypeFromPropertyName(node)


@copilot apply changes based on this feedback

Added node.Parent != nil guard before ast.IsRightSideOfPropertyAccess(node) in getIndexSignaturesAtLocation to prevent a potential nil dereference. See 1d02f5b.

testdata/tests/cases/compiler/legacyDecoratorsEnumAccessSameNameAsClass.ts

testdata/tests/cases/compiler/esDecoratorsPropertyAccessSameNameAsClass.ts

testdata/tests/cases/compiler/classFieldsPropertyAccessSameNameAsClass.ts

testdata/tests/cases/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.ts

…sRightSideOfPropertyAccess Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot

Pull request overview

Fixes incorrect class-alias substitution where identifiers used as the name in property access expressions (e.g. MyEnum.Foo) were being treated as standalone references and renamed during decorator/class-field transforms in the TypeScript-Go port.

Changes:

Add new compiler regression tests (legacy decorators, ES decorators, and class fields/private fields) covering property access scenarios that must not be substituted.
Update legacy decorators and class fields transformers to skip substitution when an identifier is the right side of a property access.
Reuse ast.IsRightSideOfPropertyAccess in checker utilities and add a nil-safety guard in getIndexSignaturesAtLocation.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/legacyDecoratorsEnumAccessSameNameAsClass.ts	New legacy-decorators regression test reproducing enum-member access rename issue.
testdata/tests/cases/compiler/esDecoratorsPropertyAccessSameNameAsClass.ts	New ES-decorators regression test for property/enum member access vs class aliasing.
testdata/tests/cases/compiler/classFieldsPropertyAccessSameNameAsClass.ts	New class-fields regression test for aliasing and property access names.
testdata/tests/cases/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.ts	New private-fields regression test ensuring property access names aren’t substituted.
testdata/baselines/reference/compiler/legacyDecoratorsEnumAccessSameNameAsClass.types	Baseline updates for the new legacy-decorators test.
testdata/baselines/reference/compiler/legacyDecoratorsEnumAccessSameNameAsClass.symbols	Symbol baseline updates for the new legacy-decorators test.
testdata/baselines/reference/compiler/legacyDecoratorsEnumAccessSameNameAsClass.js	Emit baseline showing corrected output for legacy decorators transform.
testdata/baselines/reference/compiler/esDecoratorsPropertyAccessSameNameAsClass.types	Baseline updates for the new ES-decorators test.
testdata/baselines/reference/compiler/esDecoratorsPropertyAccessSameNameAsClass.symbols	Symbol baseline updates for the new ES-decorators test.
testdata/baselines/reference/compiler/esDecoratorsPropertyAccessSameNameAsClass.js	Emit baseline for ES decorators ensuring property names remain stable.
testdata/baselines/reference/compiler/classFieldsPropertyAccessSameNameAsClass.types	Baseline updates for the class-fields regression test.
testdata/baselines/reference/compiler/classFieldsPropertyAccessSameNameAsClass.symbols	Symbol baseline updates for the class-fields regression test.
testdata/baselines/reference/compiler/classFieldsPropertyAccessSameNameAsClass.js	Emit baseline confirming property access names aren’t renamed in class-fields transform.
testdata/baselines/reference/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.types	Baseline updates for the private-fields regression test.
testdata/baselines/reference/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.symbols	Symbol baseline updates for the private-fields regression test.
testdata/baselines/reference/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.js	Emit baseline confirming property access names aren’t renamed with private fields.
internal/transformers/tstransforms/legacydecorators.go	Skip substitution for identifiers that are property-access names in legacy decorators transformer.
internal/transformers/estransforms/classfields.go	Skip substitution for identifiers that are property-access names in class fields transformer.
internal/checker/utilities.go	Refactor to reuse shared AST helper for “right side of access expression” checks.
internal/checker/checker.go	Add nil-guard before calling `ast.IsRightSideOfPropertyAccess` in index-signature lookup.

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testdata/tests/cases/compiler/classFieldsPrivatePropertyAccessSameNameAsClass.ts

internal/transformers/tstransforms/legacydecorators.go

testdata/tests/cases/compiler/legacyDecoratorsEnumAccessSameNameAsClass.ts

testdata/tests/cases/compiler/esDecoratorsPropertyAccessSameNameAsClass.ts

testdata/tests/cases/compiler/classFieldsPropertyAccessSameNameAsClass.ts

weswigham · 2026-03-19T23:22:14Z

internal/transformers/estransforms/classfields.go

 func (tx *classFieldsTransformer) visitIdentifier(node *ast.Identifier) *ast.Node {
-	declaration := tx.resolver.GetReferencedValueDeclaration(tx.EmitContext().MostOriginal(node.AsNode()))
+	original := tx.EmitContext().MostOriginal(node.AsNode())
+	if original.Parent != nil && ast.IsRightSideOfPropertyAccess(original) {


Parent accesses in transforms are a nonstarter - it's rng if this gets hit or not, as a previous transform running will block this logic by parents not getting set. Fortunately, disabling substitutions on the Names of property accesses is really easy - just add an explicit PropertyAccessExpression visitor func and don't visit the Name.

This is Original, no?

We can change it, but I assumed this was somewhat fair game.

In any case, @copilot revert all changes and try doing explicit visiting of PropertyAccessExpressions

Reverted all parent-access checks and checker changes. Now using explicit PropertyAccessExpression visitor functions:

legacydecorators.go: Added KindPropertyAccessExpression case → visitPropertyAccessExpression that visits only the expression, not the name

classfields.go: Handle PropertyAccessExpression with identifier names before the identifier early-out; updated visitPropertyAccessExpression fallthrough to use visitPropertyAccessExpressionForSubstitution for identifier names while preserving VisitEachChild for private identifiers

See a15fecc.

In this case, moving the bail check to after the .Original does make the parent check not crash, it's true, but does just make it not do the right thing - it doesn't respect the current, transformed shape of the AST.

Wasn't satisfied with what it did, so made an explicit visitor for this in classfields.

The decorators pass doesn't need a special separate visitor, I think.

…sitor to skip name substitution Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Initial plan

d60a794

Copilot AI assigned Copilot and jakebailey Mar 18, 2026

Copilot started work on behalf of jakebailey March 18, 2026 17:46 View session

Copilot AI and others added 2 commits March 18, 2026 17:57

Add test case for experimentalDecorators enum access rename bug

01f26ce

Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix enum access renaming issue in experimental decorators~~ Fix experimentalDecorators renaming enum accesses with same name as class Mar 18, 2026

Copilot AI requested a review from jakebailey March 18, 2026 18:00

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Copilot started work on behalf of jakebailey March 18, 2026 18:08 View session

Copilot AI changed the title ~~Fix experimentalDecorators renaming enum accesses with same name as class~~ Fix property access name substitution in decorator and class field transformers Mar 18, 2026

Copilot finished work on behalf of jakebailey March 18, 2026 18:25

jakebailey reviewed Mar 18, 2026

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internal/transformers/estransforms/classfields.go Outdated Show resolved Hide resolved

Copilot started work on behalf of jakebailey March 18, 2026 18:31 View session

Extract IsPropertyAccessName helper to deduplicate property access check

07f8c8f

Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot AI requested a review from jakebailey March 18, 2026 18:40

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internal/transformers/utilities.go Outdated Show resolved Hide resolved

Copilot started work on behalf of jakebailey March 18, 2026 18:47 View session

Remove IsPropertyAccessName helper; reuse ast.IsRightSideOfPropertyAc…

4beb0a4

…cess in transformers and checker Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot AI requested a review from jakebailey March 18, 2026 19:00

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Copilot started work on behalf of jakebailey March 18, 2026 19:07 View session

Add node.Parent != nil guard in getIndexSignaturesAtLocation before I…

1d02f5b

…sRightSideOfPropertyAccess Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot finished work on behalf of jakebailey March 18, 2026 19:15

jakebailey requested a review from Copilot March 18, 2026 19:21

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jakebailey requested a review from weswigham March 19, 2026 21:13

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Copilot started work on behalf of jakebailey March 19, 2026 23:43 View session

Revert parent-access checks; use explicit PropertyAccessExpression vi…

a15fecc

…sitor to skip name substitution Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot finished work on behalf of jakebailey March 19, 2026 23:58

jakebailey added 2 commits March 19, 2026 19:02

Use a substitution visitor

1561b81

Say Strada, not TypeScript

c37f1e2

jakebailey requested a review from weswigham March 20, 2026 02:06

jakebailey added 2 commits March 19, 2026 19:16

Further testing

db10c65

Drop the names now no longer used

ddd0e20

		if ast.IsIdentifier(node) && ast.IsRightSideOfPropertyAccess(node) {
		keyType := c.getLiteralTypeFromPropertyName(node)

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Copilot AI commented Mar 18, 2026 •

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