fix(33286): Code folding fail for arrow function when param list has no parentheses

src/services/outliningElementsCollector.ts

@@ -276,9 +276,7 @@ namespace ts.OutliningElementsCollector {
     }
 
     function functionSpan(node: FunctionLike, body: Block, sourceFile: SourceFile): OutliningSpan | undefined {
-        const openToken = isNodeArrayMultiLine(node.parameters, sourceFile)
-            ? findChildOfKind(node, SyntaxKind.OpenParenToken, sourceFile)
-            : findChildOfKind(body, SyntaxKind.OpenBraceToken, sourceFile);
+        const openToken = tryGetFunctionOpenToken(node, body, sourceFile);
         const closeToken = findChildOfKind(body, SyntaxKind.CloseBraceToken, sourceFile);
         return openToken && closeToken && spanBetweenTokens(openToken, closeToken, node, sourceFile, /*autoCollapse*/ node.kind !== SyntaxKind.ArrowFunction);
     }
@@ -291,4 +289,14 @@ namespace ts.OutliningElementsCollector {
     function createOutliningSpan(textSpan: TextSpan, kind: OutliningSpanKind, hintSpan: TextSpan = textSpan, autoCollapse = false, bannerText = "..."): OutliningSpan {
         return { textSpan, kind, hintSpan, bannerText, autoCollapse };
     }
+
+    function tryGetFunctionOpenToken(node: FunctionLike, body: Block, sourceFile: SourceFile): Node | undefined {
+        if (isNodeArrayMultiLine(node.parameters, sourceFile)) {
+            const openParenToken = findChildOfKind(node, SyntaxKind.OpenParenToken, sourceFile);
+            if (openParenToken) {
+                return openParenToken;
+            }
+        }
+        return findChildOfKind(body, SyntaxKind.OpenBraceToken, sourceFile);
+    }
 }

tests/cases/fourslash/outliningSpansForFunction.ts

@@ -1,14 +1,84 @@
 /// <reference path="fourslash.ts"/>
 
-////function f(x: number, y: number)[| {
-////    return x + y;
+////[|(
+////    a: number,
+////    b: number
+////) => {
+////    return a + b;
+////}|]
+/////
+////(a: number, b: number) => [|{
+////    return a + b;
+////}|]
+////
+////const f1 = function[| (
+////    a: number
+////    b: number
+////) {
+////    return a + b;
+////}|]
+////
+////const f2 = function (a: number, b: number)[| {
+////    return a + b;
 ////}|]
 ////
-////function g[|(
-////    x: number,
-////    y: number,
-////): number {
-////    return x + y;
+////function f3[| (
+////    a: number
+////    b: number
+////) {
+////    return a + b;
 ////}|]
+////
+////function f4(a: number, b: number)[| {
+////    return a + b;
+////}|]
+////
+////class Foo[| {
+////    constructor[|(
+////        a: number,
+////        b: number
+////    ) {
+////        this.a = a;
+////        this.b = b;
+////    }|]
+////
+////    m1[|(
+////        a: number,
+////        b: number
+////    ) {
+////        return a + b;
+////    }|]
+////
+////    m1(a: number, b: number)[| {
+////        return a + b;
+////    }|]
+////}|]
+////
+////declare function foo(props: any): void;
+////foo(
+////    a =>[| {
+////
+////    }|]
+////)
+////
+////foo(
+////    (a) =>[| {
+////
+////    }|]
+////)
+////
+////foo(
+////    (a, b, c) =>[| {
+////
+////    }|]
+////)
+////
+////foo([|
+////    (a,
+////     b,
+////     c) => {
+////
+////    }|]
+////)
 
 verify.outliningSpansInCurrentFile(test.ranges());