fix(semantic): report TS18019 for abstract modifier with private identifier

[Boshen]

Summary

• Add semantic check for TypeScript error 18019: "'abstract' modifier cannot be used with a private identifier"
• This error is reported when a class member (method or property) has both the abstract modifier and a private identifier key (e.g., abstract #foo)

Fixes 2 Babel conformance tests that expected this syntax error.

Test plan

• cargo coverage -- parser passes
• Verified TypeScript reports TS18019 for these cases

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds semantic validation for TypeScript error TS18019, which prevents the use of the abstract modifier with private identifiers in class members. This addresses a gap in the semantic checker where abstract private members were previously accepted without error.

Changes:

• Added new diagnostic function abstract_cannot_be_used_with_private_identifier with TS error code 18019
• Implemented validation in check_method_definition to detect abstract methods with private identifiers
• Implemented validation in new check_property_definition function to detect abstract properties with private identifiers
• Integrated PropertyDefinition checking into the main semantic checker dispatch

Reviewed changes

Copilot reviewed 3 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/oxc_semantic/src/diagnostics.rs	Adds new diagnostic function for TS18019 error
crates/oxc_semantic/src/checker/typescript.rs	Implements checks for abstract modifier with private identifiers in both methods and properties
crates/oxc_semantic/src/checker/mod.rs	Integrates PropertyDefinition into the semantic checker dispatch
tasks/coverage/snapshots/semantic_babel.snap	Updates to show proper error reporting instead of scope mismatch
tasks/coverage/snapshots/parser_typescript.snap	Adds expected error output for TypeScript conformance test
tasks/coverage/snapshots/parser_babel.snap	Shows two previously failing tests now correctly report semantic errors

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[codspeed-hq]

Merging this PR will not alter performance

✅ 42 untouched benchmarks
⏩ 3 skipped benchmarks¹

---

_{Comparing fix/abstract-private-identifier (329a648) with main (c1c152b)}

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Dunqing]

Merge activity

• Jan 19, 1:25 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jan 19, 1:25 AM UTC: This pull request can not be added to the Graphite merge queue. Please try rebasing and resubmitting to merge when ready.
• Jan 19, 1:25 AM UTC: Graphite disabled "merge when ready" on this PR due to: a merge conflict with the target branch; resolve the conflict and try again..
• Jan 19, 1:25 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jan 19, 2:11 AM UTC: Dunqing added this pull request to the Graphite merge queue.
• Jan 19, 2:12 AM UTC: The Graphite merge queue couldn't merge this PR because it had merge conflicts.
• Jan 19, 7:22 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jan 19, 7:22 AM UTC: Boshen added this pull request to the Graphite merge queue.
• Jan 19, 7:33 AM UTC: Merged by the Graphite merge queue.