Allow signature help argument count to be equal to argument index (#58203)

src/services/signatureHelp.ts

@@ -287,9 +287,6 @@ function getArgumentOrParameterListInfo(node: Node, position: number, sourceFile
     const { list, argumentIndex } = info;
 
     const argumentCount = getArgumentCount(checker, list);
-    if (argumentIndex !== 0) {
-        Debug.assertLessThan(argumentIndex, argumentCount);
-    }
     const argumentsSpan = getApplicableSpanForArguments(list, sourceFile);
     return { list, argumentIndex, argumentCount, argumentsSpan };
 }
@@ -660,9 +657,6 @@ function createSignatureHelpItems(
     const callTargetDisplayParts = callTargetSymbol ? symbolToDisplayParts(typeChecker, callTargetSymbol, useFullPrefix ? sourceFile : undefined, /*meaning*/ undefined) : emptyArray;
     const items = map(candidates, candidateSignature => getSignatureHelpItem(candidateSignature, callTargetDisplayParts, isTypeParameterList, typeChecker, enclosingDeclaration, sourceFile));
 
-    if (argumentIndex !== 0) {
-        Debug.assertLessThan(argumentIndex, argumentCount);
-    }
     let selectedItemIndex = 0;
     let itemsSeen = 0;
     for (let i = 0; i < items.length; i++) {

tests/cases/fourslash/completionsBeforeRestArg1.ts

@@ -0,0 +1,12 @@
+/// <reference path="fourslash.ts" />
+
+// @target: esnext
+// @lib: esnext
+
+//// const layers = Object.assign({}, /*1*/...[]);
+
+verify.completions({
+    marker: "1",
+    includes: completion.globals,
+    isNewIdentifierLocation: true,
+});

tests/cases/fourslash/signatureHelpRestArgs3.ts

@@ -0,0 +1,8 @@
+/// <reference path="fourslash.ts" />
+
+// @target: esnext
+// @lib: esnext
+
+//// const layers = Object.assign({}, /*1*/...[]);
+
+verify.baselineSignatureHelp();