fix(48260): Incorrect parameter hint is highlighted when arguments contain spread syntax (#56372)

Co-authored-by: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>

src/services/completions.ts

@@ -3122,7 +3122,7 @@ function getContextualType(previousToken: Node, position: number, sourceFile: So
         case SyntaxKind.OpenBraceToken:
             return isJsxExpression(parent) && !isJsxElement(parent.parent) && !isJsxFragment(parent.parent) ? checker.getContextualTypeForJsxAttribute(parent.parent) : undefined;
         default:
-            const argInfo = SignatureHelp.getArgumentInfoForCompletions(previousToken, position, sourceFile);
+            const argInfo = SignatureHelp.getArgumentInfoForCompletions(previousToken, position, sourceFile, checker);
             return argInfo ?
                 // At `,`, treat this as the next argument after the comma.
                 checker.getContextualTypeForArgumentAtIndex(argInfo.invocation, argInfo.argumentIndex + (previousToken.kind === SyntaxKind.CommaToken ? 1 : 0)) :

src/services/signatureHelp.ts

@@ -12,6 +12,7 @@ import {
     createTextSpanFromBounds,
     createTextSpanFromNode,
     Debug,
+    ElementFlags,
     EmitHint,
     emptyArray,
     Expression,
@@ -49,6 +50,7 @@ import {
     isPropertyAccessExpression,
     isSourceFile,
     isSourceFileJS,
+    isSpreadElement,
     isTaggedTemplateExpression,
     isTemplateHead,
     isTemplateLiteralToken,
@@ -58,6 +60,7 @@ import {
     JsxTagNameExpression,
     last,
     lastOrUndefined,
+    length,
     ListFormat,
     map,
     mapToDisplayParts,
@@ -77,6 +80,7 @@ import {
     skipTrivia,
     SourceFile,
     spacePart,
+    SpreadElement,
     Symbol,
     SymbolDisplayPart,
     symbolToDisplayParts,
@@ -85,6 +89,7 @@ import {
     TemplateExpression,
     TextSpan,
     tryCast,
+    TupleTypeReference,
     Type,
     TypeChecker,
     TypeParameter,
@@ -272,25 +277,25 @@ export interface ArgumentInfoForCompletions {
     readonly argumentCount: number;
 }
 /** @internal */
-export function getArgumentInfoForCompletions(node: Node, position: number, sourceFile: SourceFile): ArgumentInfoForCompletions | undefined {
-    const info = getImmediatelyContainingArgumentInfo(node, position, sourceFile);
+export function getArgumentInfoForCompletions(node: Node, position: number, sourceFile: SourceFile, checker: TypeChecker): ArgumentInfoForCompletions | undefined {
+    const info = getImmediatelyContainingArgumentInfo(node, position, sourceFile, checker);
     return !info || info.isTypeParameterList || info.invocation.kind !== InvocationKind.Call ? undefined
         : { invocation: info.invocation.node, argumentCount: info.argumentCount, argumentIndex: info.argumentIndex };
 }
 
-function getArgumentOrParameterListInfo(node: Node, position: number, sourceFile: SourceFile): { readonly list: Node; readonly argumentIndex: number; readonly argumentCount: number; readonly argumentsSpan: TextSpan; } | undefined {
-    const info = getArgumentOrParameterListAndIndex(node, sourceFile);
+function getArgumentOrParameterListInfo(node: Node, position: number, sourceFile: SourceFile, checker: TypeChecker): { readonly list: Node; readonly argumentIndex: number; readonly argumentCount: number; readonly argumentsSpan: TextSpan; } | undefined {
+    const info = getArgumentOrParameterListAndIndex(node, sourceFile, checker);
     if (!info) return undefined;
     const { list, argumentIndex } = info;
 
-    const argumentCount = getArgumentCount(list, /*ignoreTrailingComma*/ isInString(sourceFile, position, node));
+    const argumentCount = getArgumentCount(list, /*ignoreTrailingComma*/ isInString(sourceFile, position, node), checker);
     if (argumentIndex !== 0) {
         Debug.assertLessThan(argumentIndex, argumentCount);
     }
     const argumentsSpan = getApplicableSpanForArguments(list, sourceFile);
     return { list, argumentIndex, argumentCount, argumentsSpan };
 }
-function getArgumentOrParameterListAndIndex(node: Node, sourceFile: SourceFile): { readonly list: Node; readonly argumentIndex: number; } | undefined {
+function getArgumentOrParameterListAndIndex(node: Node, sourceFile: SourceFile, checker: TypeChecker): { readonly list: Node; readonly argumentIndex: number; } | undefined {
     if (node.kind === SyntaxKind.LessThanToken || node.kind === SyntaxKind.OpenParenToken) {
         // Find the list that starts right *after* the < or ( token.
         // If the user has just opened a list, consider this item 0.
@@ -304,7 +309,7 @@ function getArgumentOrParameterListAndIndex(node: Node, sourceFile: SourceFile):
         //   - On the target of the call (parent.func)
         //   - On the 'new' keyword in a 'new' expression
         const list = findContainingList(node);
-        return list && { list, argumentIndex: getArgumentIndex(list, node) };
+        return list && { list, argumentIndex: getArgumentIndex(list, node, checker) };
     }
 }
 
@@ -312,7 +317,7 @@ function getArgumentOrParameterListAndIndex(node: Node, sourceFile: SourceFile):
  * Returns relevant information for the argument list and the current argument if we are
  * in the argument of an invocation; returns undefined otherwise.
  */
-function getImmediatelyContainingArgumentInfo(node: Node, position: number, sourceFile: SourceFile): ArgumentListInfo | undefined {
+function getImmediatelyContainingArgumentInfo(node: Node, position: number, sourceFile: SourceFile, checker: TypeChecker): ArgumentListInfo | undefined {
     const { parent } = node;
     if (isCallOrNewExpression(parent)) {
         const invocation = parent;
@@ -331,7 +336,7 @@ function getImmediatelyContainingArgumentInfo(node: Node, position: number, sour
         //    Case 3:
         //          foo<T#, U#>(a#, #b#) -> The token is buried inside a list, and should give signature help
         // Find out if 'node' is an argument, a type argument, or neither
-        const info = getArgumentOrParameterListInfo(node, position, sourceFile);
+        const info = getArgumentOrParameterListInfo(node, position, sourceFile, checker);
         if (!info) return undefined;
         const { list, argumentIndex, argumentCount, argumentsSpan } = info;
         const isTypeParameterList = !!parent.typeArguments && parent.typeArguments.pos === list.pos;
@@ -397,7 +402,7 @@ function getImmediatelyContainingArgumentInfo(node: Node, position: number, sour
 }
 
 function getImmediatelyContainingArgumentOrContextualParameterInfo(node: Node, position: number, sourceFile: SourceFile, checker: TypeChecker): ArgumentListInfo | undefined {
-    return tryGetParameterInfo(node, position, sourceFile, checker) || getImmediatelyContainingArgumentInfo(node, position, sourceFile);
+    return tryGetParameterInfo(node, position, sourceFile, checker) || getImmediatelyContainingArgumentInfo(node, position, sourceFile, checker);
 }
 
 function getHighestBinary(b: BinaryExpression): BinaryExpression {
@@ -452,7 +457,7 @@ function getContextualSignatureLocationInfo(node: Node, sourceFile: SourceFile,
         case SyntaxKind.MethodDeclaration:
         case SyntaxKind.FunctionExpression:
         case SyntaxKind.ArrowFunction:
-            const info = getArgumentOrParameterListInfo(node, position, sourceFile);
+            const info = getArgumentOrParameterListInfo(node, position, sourceFile, checker);
             if (!info) return undefined;
             const { argumentIndex, argumentCount, argumentsSpan } = info;
             const contextualType = isMethodDeclaration(parent) ? checker.getContextualTypeForObjectLiteralElement(parent) : checker.getContextualType(parent as ParenthesizedExpression | FunctionExpression | ArrowFunction);
@@ -476,7 +481,7 @@ function chooseBetterSymbol(s: Symbol): Symbol {
         : s;
 }
 
-function getArgumentIndex(argumentsList: Node, node: Node) {
+function getArgumentIndex(argumentsList: Node, node: Node, checker: TypeChecker) {
     // The list we got back can include commas.  In the presence of errors it may
     // also just have nodes without commas.  For example "Foo(a b c)" will have 3
     // args without commas. We want to find what index we're at.  So we count
@@ -488,20 +493,39 @@ function getArgumentIndex(argumentsList: Node, node: Node) {
     // on.  In that case, even if we're after the trailing comma, we'll still see
     // that trailing comma in the list, and we'll have generated the appropriate
     // arg index.
+    const args = argumentsList.getChildren();
     let argumentIndex = 0;
-    for (const child of argumentsList.getChildren()) {
+    for (let pos = 0; pos < length(args); pos++) {
+        const child = args[pos];
         if (child === node) {
             break;
         }
-        if (child.kind !== SyntaxKind.CommaToken) {
-            argumentIndex++;
+        if (isSpreadElement(child)) {
+            argumentIndex = argumentIndex + getSpreadElementCount(child, checker) + (pos > 0 ? pos : 0);
+        }
+        else {
+            if (child.kind !== SyntaxKind.CommaToken) {
+                argumentIndex++;
+            }
         }
     }
-
     return argumentIndex;
 }
 
-function getArgumentCount(argumentsList: Node, ignoreTrailingComma: boolean) {
+function getSpreadElementCount(node: SpreadElement, checker: TypeChecker) {
+    const spreadType = checker.getTypeAtLocation(node.expression);
+    if (checker.isTupleType(spreadType)) {
+        const { elementFlags, fixedLength } = (spreadType as TupleTypeReference).target;
+        if (fixedLength === 0) {
+            return 0;
+        }
+        const firstOptionalIndex = findIndex(elementFlags, f => !(f & ElementFlags.Required));
+        return firstOptionalIndex < 0 ? fixedLength : firstOptionalIndex;
+    }
+    return 0;
+}
+
+function getArgumentCount(argumentsList: Node, ignoreTrailingComma: boolean, checker: TypeChecker) {
     // The argument count for a list is normally the number of non-comma children it has.
     // For example, if you have "Foo(a,b)" then there will be three children of the arg
     // list 'a' '<comma>' 'b'.  So, in this case the arg count will be 2.  However, there
@@ -515,7 +539,14 @@ function getArgumentCount(argumentsList: Node, ignoreTrailingComma: boolean) {
     // arg count of 3.
     const listChildren = argumentsList.getChildren();
 
-    let argumentCount = countWhere(listChildren, arg => arg.kind !== SyntaxKind.CommaToken);
+    let argumentCount = 0;
+    for (const child of listChildren) {
+        if (isSpreadElement(child)) {
+            argumentCount = argumentCount + getSpreadElementCount(child, checker);
+        }
+    }
+
+    argumentCount = argumentCount + countWhere(listChildren, arg => arg.kind !== SyntaxKind.CommaToken);
     if (!ignoreTrailingComma && listChildren.length > 0 && last(listChildren).kind === SyntaxKind.CommaToken) {
         argumentCount++;
     }

src/services/stringCompletions.ts

@@ -407,7 +407,7 @@ function getStringLiteralCompletionEntries(sourceFile: SourceFile, node: StringL
         case SyntaxKind.NewExpression:
         case SyntaxKind.JsxAttribute:
             if (!isRequireCallArgument(node) && !isImportCall(parent)) {
-                const argumentInfo = SignatureHelp.getArgumentInfoForCompletions(parent.kind === SyntaxKind.JsxAttribute ? parent.parent : node, position, sourceFile);
+                const argumentInfo = SignatureHelp.getArgumentInfoForCompletions(parent.kind === SyntaxKind.JsxAttribute ? parent.parent : node, position, sourceFile, typeChecker);
                 // Get string literal completions from specialized signatures of the target
                 // i.e. declare function f(a: 'A');
                 // f("/*completion position*/")

tests/baselines/reference/signatureHelpRestArgs.baseline

@@ -0,0 +1,709 @@
+// === SignatureHelp ===
+=== /tests/cases/fourslash/signatureHelpRestArgs.ts ===
+// function fn(a: number, b: number, c: number) {}
+// const a = [1, 2] as const;
+// const b = [1] as const;
+// 
+// fn(...a, );
+//          ^
+// | ----------------------------------------------------------------------
+// | fn(a: number, b: number, **c: number**): void
+// | ----------------------------------------------------------------------
+// fn(, ...a);
+//    ^
+// | ----------------------------------------------------------------------
+// | fn(**a: number**, b: number, c: number): void
+// | ----------------------------------------------------------------------
+// 
+// fn(...b, );
+//          ^
+// | ----------------------------------------------------------------------
+// | fn(a: number, **b: number**, c: number): void
+// | ----------------------------------------------------------------------
+// fn(, ...b, );
+//    ^
+// | ----------------------------------------------------------------------
+// | fn(**a: number**, b: number, c: number): void
+// | ----------------------------------------------------------------------
+//            ^
+// | ----------------------------------------------------------------------
+// | fn(a: number, b: number, **c: number**): void
+// | ----------------------------------------------------------------------
+
+[
+  {
+    "marker": {
+      "fileName": "/tests/cases/fourslash/signatureHelpRestArgs.ts",
+      "position": 109,
+      "name": "1"
+    },
+    "item": {
+      "items": [
+        {
+          "isVariadic": false,
+          "prefixDisplayParts": [
+            {
+              "text": "fn",
+              "kind": "functionName"
+            },
+            {
+              "text": "(",
+              "kind": "punctuation"
+            }
+          ],
+          "suffixDisplayParts": [
+            {
+              "text": ")",
+              "kind": "punctuation"
+            },
+            {
+              "text": ":",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            },
+            {
+              "text": "void",
+              "kind": "keyword"
+            }
+          ],
+          "separatorDisplayParts": [
+            {
+              "text": ",",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            }
+          ],
+          "parameters": [
+            {
+              "name": "a",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "a",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "b",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "b",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "c",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "c",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            }
+          ],
+          "documentation": [],
+          "tags": []
+        }
+      ],
+      "applicableSpan": {
+        "start": 103,
+        "length": 6
+      },
+      "selectedItemIndex": 0,
+      "argumentIndex": 2,
+      "argumentCount": 4
+    }
+  },
+  {
+    "marker": {
+      "fileName": "/tests/cases/fourslash/signatureHelpRestArgs.ts",
+      "position": 115,
+      "name": "2"
+    },
+    "item": {
+      "items": [
+        {
+          "isVariadic": false,
+          "prefixDisplayParts": [
+            {
+              "text": "fn",
+              "kind": "functionName"
+            },
+            {
+              "text": "(",
+              "kind": "punctuation"
+            }
+          ],
+          "suffixDisplayParts": [
+            {
+              "text": ")",
+              "kind": "punctuation"
+            },
+            {
+              "text": ":",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            },
+            {
+              "text": "void",
+              "kind": "keyword"
+            }
+          ],
+          "separatorDisplayParts": [
+            {
+              "text": ",",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            }
+          ],
+          "parameters": [
+            {
+              "name": "a",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "a",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "b",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "b",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "c",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "c",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            }
+          ],
+          "documentation": [],
+          "tags": []
+        }
+      ],
+      "applicableSpan": {
+        "start": 115,
+        "length": 6
+      },
+      "selectedItemIndex": 0,
+      "argumentIndex": 0,
+      "argumentCount": 3
+    }
+  },
+  {
+    "marker": {
+      "fileName": "/tests/cases/fourslash/signatureHelpRestArgs.ts",
+      "position": 134,
+      "name": "3"
+    },
+    "item": {
+      "items": [
+        {
+          "isVariadic": false,
+          "prefixDisplayParts": [
+            {
+              "text": "fn",
+              "kind": "functionName"
+            },
+            {
+              "text": "(",
+              "kind": "punctuation"
+            }
+          ],
+          "suffixDisplayParts": [
+            {
+              "text": ")",
+              "kind": "punctuation"
+            },
+            {
+              "text": ":",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            },
+            {
+              "text": "void",
+              "kind": "keyword"
+            }
+          ],
+          "separatorDisplayParts": [
+            {
+              "text": ",",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            }
+          ],
+          "parameters": [
+            {
+              "name": "a",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "a",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "b",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "b",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "c",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "c",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            }
+          ],
+          "documentation": [],
+          "tags": []
+        }
+      ],
+      "applicableSpan": {
+        "start": 128,
+        "length": 6
+      },
+      "selectedItemIndex": 0,
+      "argumentIndex": 1,
+      "argumentCount": 3
+    }
+  },
+  {
+    "marker": {
+      "fileName": "/tests/cases/fourslash/signatureHelpRestArgs.ts",
+      "position": 140,
+      "name": "4"
+    },
+    "item": {
+      "items": [
+        {
+          "isVariadic": false,
+          "prefixDisplayParts": [
+            {
+              "text": "fn",
+              "kind": "functionName"
+            },
+            {
+              "text": "(",
+              "kind": "punctuation"
+            }
+          ],
+          "suffixDisplayParts": [
+            {
+              "text": ")",
+              "kind": "punctuation"
+            },
+            {
+              "text": ":",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            },
+            {
+              "text": "void",
+              "kind": "keyword"
+            }
+          ],
+          "separatorDisplayParts": [
+            {
+              "text": ",",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            }
+          ],
+          "parameters": [
+            {
+              "name": "a",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "a",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "b",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "b",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "c",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "c",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            }
+          ],
+          "documentation": [],
+          "tags": []
+        }
+      ],
+      "applicableSpan": {
+        "start": 140,
+        "length": 8
+      },
+      "selectedItemIndex": 0,
+      "argumentIndex": 0,
+      "argumentCount": 3
+    }
+  },
+  {
+    "marker": {
+      "fileName": "/tests/cases/fourslash/signatureHelpRestArgs.ts",
+      "position": 148,
+      "name": "5"
+    },
+    "item": {
+      "items": [
+        {
+          "isVariadic": false,
+          "prefixDisplayParts": [
+            {
+              "text": "fn",
+              "kind": "functionName"
+            },
+            {
+              "text": "(",
+              "kind": "punctuation"
+            }
+          ],
+          "suffixDisplayParts": [
+            {
+              "text": ")",
+              "kind": "punctuation"
+            },
+            {
+              "text": ":",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            },
+            {
+              "text": "void",
+              "kind": "keyword"
+            }
+          ],
+          "separatorDisplayParts": [
+            {
+              "text": ",",
+              "kind": "punctuation"
+            },
+            {
+              "text": " ",
+              "kind": "space"
+            }
+          ],
+          "parameters": [
+            {
+              "name": "a",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "a",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "b",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "b",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            },
+            {
+              "name": "c",
+              "documentation": [],
+              "displayParts": [
+                {
+                  "text": "c",
+                  "kind": "parameterName"
+                },
+                {
+                  "text": ":",
+                  "kind": "punctuation"
+                },
+                {
+                  "text": " ",
+                  "kind": "space"
+                },
+                {
+                  "text": "number",
+                  "kind": "keyword"
+                }
+              ],
+              "isOptional": false,
+              "isRest": false
+            }
+          ],
+          "documentation": [],
+          "tags": []
+        }
+      ],
+      "applicableSpan": {
+        "start": 140,
+        "length": 8
+      },
+      "selectedItemIndex": 0,
+      "argumentIndex": 2,
+      "argumentCount": 3
+    }
+  }
+]

tests/cases/fourslash/signatureHelpRestArgs.ts

@@ -0,0 +1,13 @@
+/// <reference path="fourslash.ts" />
+
+////function fn(a: number, b: number, c: number) {}
+////const a = [1, 2] as const;
+////const b = [1] as const;
+////
+////fn(...a, /*1*/);
+////fn(/*2*/, ...a);
+////
+////fn(...b, /*3*/);
+////fn(/*4*/, ...b, /*5*/);
+
+verify.baselineSignatureHelp();