Fix getChildren() dropping opening '<' token when immediately followed by '<' (#64168)

[Vaibhav Srivastava (vaibhavsrv)]

Fixes #64168

Summary

• What changed: In addSyntheticNodes (packages/typescript/src/ast/astnav.ts), when a scanned token exceeds the gap end (tokenEnd > end) and is a LessThanLessThanToken (<<), scanner.reScanLessThanToken() is invoked to split << into < (LessThanToken).
• Why it changed: Previously, when getChildren() was called on type argument lists immediately following another < (such as ReturnType<<T>(x: T) => number>), addSyntheticNodes scanned << at pos = 21 yielding tokenEnd = 23. Because 23 <= 22 evaluated to false, the < token was silently dropped, creating a gap between AST children.

Tests

• Executed:
  • ./bin/tsc -b packages/typescript
  • go test ./internal/parser ./internal/ls/...
  • AST invariant test corpus entry with type Bar = ReturnType<<T>(x: T) => number>;
• Results: Build succeeded cleanly (exit code 0) and all tests passed (exit code 0).

[Copilot]

🔵 Needs a closer look

The unrelated deferred-import feature and generated changes should be moved to a separate PR.

Pull request overview

Fixes missing < tokens from getChildren(), but also includes unrelated import.defer?.() compiler changes.

Changes:

• Re-scans << when synthesizing a single <.
• Adds AST invariant coverage.
• Adds unrelated optional-chain diagnostics and tests.

File summaries

File	Description
packages/typescript/src/ast/astnav.ts	Splits an overrunning << token.
packages/typescript/test/sync/ast.test.ts	Adds regression coverage.
tsc/internal/parser/parser.go	Flags optional deferred imports.
tsc/internal/checker/grammarchecks.go	Rejects optional import calls.
tsc/internal/diagnostics/extraDiagnosticMessages.json	Defines the new diagnostic.
tsc/internal/diagnostics/diagnostics_generated.go	Adds generated diagnostic data.
tsc/internal/diagnostics/loc_generated.go	Contains incidental import churn.
tsc/testdata/tests/cases/conformance/importDefer/importDeferOptionalChain.ts	Tests optional deferred imports.
tsc/testdata/baselines/reference/conformance/importDeferOptionalChain.errors.txt	Captures expected diagnostics.
tsc/testdata/baselines/reference/conformance/importDeferOptionalChain.js	Captures emitted JavaScript.
tsc/testdata/baselines/reference/conformance/importDeferOptionalChain.symbols	Captures symbol output.
tsc/testdata/baselines/reference/conformance/importDeferOptionalChain.types	Captures type output.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

---

💡 Add a code-review agent skill for context-aware, tailored reviews. Learn more in the docs.

[Vaibhav Srivastava (vaibhavsrv)]

Updated! Rebased the branch onto clean upstream/main to remove the deferred import commits. This PR now strictly contains only the getChildren() fix for #64168 and its AST invariant regression test (2 changed files).

[Mateusz Burzyński (Andarist)]

there is already an existing PR opened with almost exactly the same fix: #64169