Fix crash in completions following call expressions (#1417)

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

Author: Copilot

internal/fourslash/tests/completionAfterCallExpression_test.go

@@ -0,0 +1,30 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestCompletionAfterCallExpression(t *testing.T) {
+	t.Parallel()
+
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+
+	const content = `let x = someCall() /**/`
+	f := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	f.VerifyCompletions(t, "", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &defaultCommitCharacters,
+			EditRange:        ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				"satisfies",
+				"as",
+			},
+		},
+	})
+}

internal/ls/completions.go

@@ -4556,7 +4556,7 @@ type argumentInfoForCompletions struct {
 
 func getArgumentInfoForCompletions(node *ast.Node, position int, file *ast.SourceFile, typeChecker *checker.Checker) *argumentInfoForCompletions {
 	info := getImmediatelyContainingArgumentInfo(node, position, file, typeChecker)
-	if info == nil || info.isTypeParameterList || info.invocation.callInvocation == nil {
+	if info == nil || info.isTypeParameterList || info.invocation.callInvocation == nil || info.argumentIndex == nil {
 		return nil
 	}
 	return &argumentInfoForCompletions{