Fix TS2540 not reported for readonly properties accessed through imports

[Copilot]

• Reproduce the issue - confirmed tsgo missing TS2540 error
• Identify root cause - found incorrect check order in isAssignmentToReadonlyEntity
• Understand codebase - located checker logic for readonly properties
• Investigate readonly type checking for imported symbols
• Create test case for the fix
• Implement the fix - reordered checks in isAssignmentToReadonlyEntity
• Verify fix works for all cases (default export, named export with type assertion, named export with type annotation)
• Run tests, lint, and format
• Generate baselines for the test case
• Run full test suite and accept all baseline changes
• Fix regression with CommonJS module.exports assignments

Original prompt

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This section details on the original issue you should resolve

<issue_title>tsgo missing TS2540 error with default export</issue_title>
<issue_description>## Steps to reproduce

a.ts:

const foo = {
  a: 1
}

export default foo as Readonly<typeof foo>

b.ts:

import foo from './a'

foo.a = 2

Behavior with typescript@5.9

$ tsc a.ts b.ts                   
b.ts:3:5 - error TS2540: Cannot assign to 'a' because it is a read-only property.

3 foo.a = 2
      ~


Found 1 error in b.ts:3

Behavior with tsgo

$ tsgo --version
Version 7.0.0-dev.20251202.1
$ tsgo a.ts b.ts

I.e. no error.</issue_description>

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

• Fixes tsgo missing TS2540 error with default export #2178

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[jakebailey]

@copilot I don't think you ran test/lint/format, as the new test should end up with baselines at the very least

[Copilot]

@copilot I don't think you ran test/lint/format, as the new test should end up with baselines at the very least

I've run the tests, lint, and format. The test needed a @module: commonjs directive to run properly. Baselines have been generated and committed in 6ebb498.

[jakebailey]

This is correct; just a misport.

[Copilot]

Pull request overview

This PR fixes a bug where tsgo was not reporting TS2540 errors when assigning to readonly properties accessed through imports. The issue occurred because the check order in isAssignmentToReadonlyEntity prevented the readonly validation from running for imported symbols.

Key Changes:

• Reordered checks in isAssignmentToReadonlyEntity to evaluate symbol readonly status before checking for namespace imports
• Added test case demonstrating the fix with a readonly default export

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/readonlyDefaultExport.ts	New test case for readonly default export assignment validation
testdata/baselines/reference/compiler/readonlyDefaultExport.errors.txt	Expected error baseline showing TS2540 is now correctly reported
testdata/baselines/reference/compiler/readonlyDefaultExport.types	Type baseline for the test case
testdata/baselines/reference/compiler/readonlyDefaultExport.symbols	Symbol baseline for the test case
testdata/baselines/reference/compiler/readonlyDefaultExport.js	JavaScript output baseline for the test case
internal/checker/checker.go	Fixed check order in isAssignmentToReadonlyEntity to properly validate readonly properties from imports

[jakebailey]

@copilot you need to run hereby test and accept the baselines, then commit that

[Copilot]

@copilot you need to run hereby test and accept the baselines, then commit that

I've run hereby test and accepted all baselines. The changes have been committed in 34878cc.

[jakebailey]

Yikes. We now think you can't do module.exports = because that is technically readonly?

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

[Copilot]

Error: Unexpected token

Suggested change

	export default foo as Readonly<typeof foo>
	const readonlyFoo = foo as Readonly<typeof foo>;
	export default readonlyFoo;