Fix signature help crash when nested call has trailing comma (#2315)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: DanielRosenwasser <[email protected]>

Author: Copilot

internal/checker/checker.go

@@ -9638,6 +9638,17 @@ func (c *Checker) getArgumentArityError(node *ast.Node, signatures []*Signature,
 		}
 		return diagnostic
 	default:
+		// Guard against out-of-bounds access when maxCount >= len(args).
+		// This can happen when we reach this fallback error path but the argument
+		// count actually matches the parameter count (e.g., due to trailing commas
+		// causing signature resolution to fail for other reasons).
+		if maxCount >= len(args) {
+			diagnostic := NewDiagnosticForNode(errorNode, message, parameterRange, len(args))
+			if headMessage != nil {
+				diagnostic = ast.NewDiagnosticChain(diagnostic, headMessage)
+			}
+			return diagnostic
+		}
 		sourceFile := ast.GetSourceFileOfNode(node)
 		pos := scanner.SkipTrivia(sourceFile.Text(), args[maxCount].Pos())
 		end := args[len(args)-1].End()

internal/fourslash/tests/signatureHelpNestedCallTrailingComma_test.go

@@ -0,0 +1,28 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestSignatureHelpNestedCallTrailingComma(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	// Regression test for crash when requesting signature help on a call target
+	// where the nested call has a trailing comma.
+	// Both outer and inner calls must have trailing commas, and outer must be generic.
+	const content = `declare function outer<T>(range: T): T;
+declare function inner(a: any): any;
+
+outer(inner/*1*/(undefined,),);`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	f.GoToMarker(t, "1")
+	f.VerifySignatureHelpPresent(t, &lsproto.SignatureHelpContext{
+		IsRetrigger: false,
+		TriggerKind: lsproto.SignatureHelpTriggerKindInvoked,
+	})
+}