Fix signature help crash with trailing comma and update test with minimal repro

Co-authored-by: DanielRosenwasser <972891+DanielRosenwasser@users.noreply.github.com>

Author: Copilot

internal/checker/checker.go

@@ -9648,9 +9648,10 @@ func (c *Checker) getArgumentArityError(node *ast.Node, signatures []*Signature,
 		}
 		return diagnostic
 	default:
-		// Handle edge case where maxCount >= len(args)
-		// This can happen when signature resolution fails for reasons other than argument count,
-		// such as when trailing commas create OmittedExpressions that affect type inference.
+		// 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 {

internal/fourslash/tests/signatureHelpNestedCallTrailingComma_test.go

@@ -4,7 +4,6 @@ import (
 	"testing"
 
 	"github.com/microsoft/typescript-go/internal/fourslash"
-	. "github.com/microsoft/typescript-go/internal/fourslash/tests/util"
 	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
 	"github.com/microsoft/typescript-go/internal/testutil"
 )
@@ -16,23 +15,14 @@ func TestSignatureHelpNestedCallTrailingComma(t *testing.T) {
 	// 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, b: any, c: any, d: any, e: any): any;
+declare function inner(a: any): any;
 
-outer(
-    inner(/*1*/
-        undefined,
-        undefined,
-        undefined,
-        undefined,
-        undefined,
-    ),
-);`
+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,
-		TriggerCharacter: PtrTo("("),
-		TriggerKind:      lsproto.SignatureHelpTriggerKindTriggerCharacter,
+		IsRetrigger: false,
+		TriggerKind: lsproto.SignatureHelpTriggerKindInvoked,
 	})
 }