Fix: Remove definite assignment assertions from class property declaration files

[Copilot]

Fixes an issue where definite assignment assertions (!) were incorrectly preserved in TypeScript declaration files for class properties, causing compilation errors.

Problem

When generating declaration files (.d.ts), the TypeScript compiler should remove definite assignment assertions since they are not valid in ambient contexts. However, tsgo was preserving these assertions on class properties, leading to:

// Source file
export class DbObject {
    id!: string;
}

// Generated .d.ts (incorrect)
export declare class DbObject {
    id!: string;  // Error: A definite assignment assertion '!' is not permitted in this context
}

Solution

Modified the declaration transformer to detect and remove exclamation tokens (KindExclamationToken) from class property declarations when generating declaration files. The fix only applies to property declarations in classes, as property signatures in interfaces cannot have definite assignment assertions.

Files changed:

• internal/transformers/declarations/transform.go: Updated transformPropertyDeclaration to filter out definite assignment assertions

Result

The fix now correctly generates declaration files without definite assignment assertions on class properties:

// Generated .d.ts (correct)
export declare class DbObject {
    id: string;  // Clean declaration without assertion
}

This change aligns tsgo's behavior with the official TypeScript compiler and eliminates the compilation errors that were occurring in declaration files.

Testing

• Verified using existing test coverage in definiteAssignmentAssertions.ts and other submodule tests
• All existing tests pass with updated baselines showing proper removal of ! tokens from class properties
• Confirmed property signatures in interfaces remain unchanged (as they should)

Fixes #1736

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Definitely assign assertion preserved on field in declaration files</issue_title>
<issue_description>## Steps to reproduce

export class DbObject {
    id!: string;
}

Playground Link

Behavior with [email protected]

The ! is removed from declaration files

Behavior with tsgo

The ! is preserved in declaration files

export declare class DbObject {
    id!: string;
}
```</issue_description>

<agent_instructions>Add a test like:

```ts
// @declaration: true
// @strict: true
export class DbObject {
    id!: string;
}

Accept the baselines, note the output and then fix the bug in the declaration transform.</agent_instructions>

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

Fixes #1736

---

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[Copilot]

Pull Request Overview

This PR fixes a bug where definite assignment assertions (!) were being preserved in declaration files when they should be removed. The TypeScript compiler removes these assertions from .d.ts files since they are only relevant during compilation and not in the final type declarations.

• Added test cases to verify the fix for definite assignment assertions in declaration files
• Implemented logic to remove exclamation tokens from property declarations in declaration files
• Updated existing baselines to reflect the corrected behavior

Reviewed Changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

File	Description
testdata/tests/cases/compiler/definiteAssignmentAssertionInDeclaration.ts	Basic test case for definite assignment assertion removal
testdata/tests/cases/compiler/definiteAssignmentAssertionInDeclarationExtended.ts	Extended test case covering various property types and modifiers
internal/transformers/declarations/transform.go	Implementation of the fix to remove exclamation tokens from declaration files
Multiple baseline files	Updated reference outputs showing corrected behavior with exclamation tokens removed

[jakebailey]

@copilot Remove the newly added tests and baselines, leaving only the changes to the existing test baselines. The existing tests turned out to cover everything.

[Copilot]

@copilot Remove the newly added tests and baselines, leaving only the changes to the existing test baselines. The existing tests turned out to cover everything.

Removed the newly added test files and their baselines. The existing tests in definiteAssignmentAssertions.ts and other submodule tests already provide adequate coverage. Fixed in commit cd2565a.