Fix panic in checker.GetTypeArguments for non-reference types (#4338)

[UditDewan]

Fixes #4338

Problem

The public API method Checker.GetTypeArguments forwarded straight to the internal getTypeArguments, which assumes its argument is a type reference and immediately does t.AsTypeReference(). For a non-reference type (e.g. the intrinsic string, produced by type Alias = string) that cast returns nil, so the next field access (d.resolvedTypeArguments) is a nil pointer dereference and the server panics:

panic: runtime error: invalid memory address or nil pointer dereference
  checker.(*Checker).getTypeArguments  internal/checker/checker.go:21794
  checker.(*Checker).GetTypeArguments  internal/checker/exports.go:278
  api.(*Session).handleGetTypeArguments internal/api/session.go

The native-preview API types this method as getTypeArguments(type: Type): readonly Type[], i.e. it accepts any Type, so callers can legitimately reach this path with a non-reference type.

Fix

Guard the exported GetTypeArguments so non-reference types return no type arguments instead of panicking. This matches tsc's behavior (non-references have no type arguments) and the readonly Type[] contract. The check uses ObjectFlagsReference — the same gate internal callers already apply before calling getTypeArguments — so the internal hot path is untouched.

func (c *Checker) GetTypeArguments(t *Type) []*Type {
	if t.objectFlags&ObjectFlagsReference == 0 {
		return nil
	}
	return c.getTypeArguments(t)
}

This mirrors the approach taken for the sibling crash in #4335 / PR #4345 (getBaseTypes on non-matching types returns nil).

Tests

Added regression tests to both the sync and async API suites (Checker - getTypeArguments) that call getTypeArguments on the intrinsic string type and assert it returns []. Verified that:

• Without the fix, the new test reproduces the exact nil-pointer panic from the issue.
• With the fix, both new tests and the existing getTypeArguments test pass.

gofmt and dprint check are clean.

---

Claude (Opus 4.8) helped debug this issue.

[Copilot]

Pull request overview

Fixes a server panic in the exported checker API by making Checker.GetTypeArguments safely handle non-reference types (e.g. intrinsic string) and return “no type arguments” instead of dereferencing a nil TypeReference. This aligns the native-preview API’s getTypeArguments(type: Type): readonly Type[] contract with tsc behavior.

Changes:

• Add an ObjectFlagsReference guard in Checker.GetTypeArguments to avoid nil-deref for non-reference types.
• Add sync + async native-preview regression tests asserting getTypeArguments(stringType) returns [] (issue #4338).

Reviewed changes

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

File	Description
internal/checker/exports.go	Adds an exported-API guard so non-reference Types return nil/empty instead of panicking.
_packages/native-preview/test/sync/api.test.ts	Adds sync regression test for getTypeArguments on string.
_packages/native-preview/test/async/api.test.ts	Adds async regression test for getTypeArguments on string.