Fix debug failure from fixUnusedIdentifier

src/services/codefixes/fixUnusedIdentifier.ts

@@ -146,25 +146,9 @@ namespace ts.codefix {
                     break;
                 }
 
-                if (isArrowFunction(oldFunction) && oldFunction.parameters.length === 1) {
-                    // Lambdas with exactly one parameter are special because, after removal, there
-                    // must be an empty parameter list (i.e. `()`) and this won't necessarily be the
-                    // case if the parameter is simply removed (e.g. in `x => 1`).
-                    const newFunction = updateArrowFunction(
-                        oldFunction,
-                        oldFunction.modifiers,
-                        oldFunction.typeParameters,
-                        /*parameters*/ undefined,
-                        oldFunction.type,
-                        oldFunction.equalsGreaterThanToken,
-                        oldFunction.body);
-
-                    // Drop leading and trailing trivia of the new function because we're only going
-                    // to replace the span (vs the full span) of the old function - the old leading
-                    // and trailing trivia will remain.
-                    suppressLeadingAndTrailingTrivia(newFunction);
-
-                    changes.replaceNode(sourceFile, oldFunction, newFunction);
+                if (isArrowFunction(oldFunction) && oldFunction.parameters.length === 1 && !findChildOfKind(oldFunction, SyntaxKind.OpenParenToken, sourceFile)) {
+                    // `x => {}` becomes `() => {}`
+                    changes.replaceNodeWithText(sourceFile, oldFunction.parameters[0], "()");
                 }
                 else {
                     changes.deleteNodeInList(sourceFile, parent);

src/services/textChanges.ts

@@ -315,6 +315,10 @@ namespace ts.textChanges {
             return this.replaceRangeWithNodes(sourceFile, getAdjustedRange(sourceFile, oldNode, oldNode, options), newNodes, options);
         }
 
+        public replaceNodeWithText(sourceFile: SourceFile, oldNode: Node, text: string, options: ChangeNodeOptions = useNonAdjustedPositions) {
+            return this.replaceRangeWithText(sourceFile, getAdjustedRange(sourceFile, oldNode, oldNode, options), text);
+        }
+
         public replaceNodeRangeWithNodes(sourceFile: SourceFile, startNode: Node, endNode: Node, newNodes: ReadonlyArray<Node>, options: ReplaceWithMultipleNodesOptions & ConfigurableStartEnd = useNonAdjustedPositions) {
             return this.replaceRangeWithNodes(sourceFile, getAdjustedRange(sourceFile, startNode, endNode, options), newNodes, options);
         }

tests/cases/fourslash/codeFixUnusedIdentifier_formatterDebugFailure.ts

@@ -0,0 +1,15 @@
+/// <reference path='fourslash.ts' />
+
+// Regression test for https://github.com/Microsoft/TypeScript/issues/23942
+
+// @allowJs: true
+// @target: esnext
+
+// @Filename: /a.js
+////(o){"
+
+verify.codeFix({
+    description: "Remove declaration for: 'o'",
+    index: 0,
+    newFileContent: '(){"',
+});

tests/cases/fourslash/unusedParameterInLambda1.ts

@@ -4,9 +4,8 @@
 // @noUnusedParameters: true
 ////[|/*~a*/(/*~b*/x/*~c*/:/*~d*/number/*~e*/)/*~f*/ => /*~g*/{/*~h*/}/*~i*/|]
 
-// In a perfect world, /*~f*/ and /*~h*/ would probably be retained.
 verify.codeFix({
     description: "Remove declaration for: 'x'",
     index: 0,
-    newRangeContent: "/*~a*/() => /*~g*/ { }/*~i*/",
+    newRangeContent: "/*~a*/(/*~e*/)/*~f*/ => /*~g*/{/*~h*/}/*~i*/",
 });

tests/cases/fourslash/unusedParameterInLambda2.ts

@@ -4,9 +4,8 @@
 // @noUnusedParameters: true
 ////[|/*~a*/x/*~b*/ /*~c*/=>/*~d*/ {/*~e*/}/*~f*/|]
 
-// In a perfect world, /*~c*/ and /*~e*/ would probably be retained.
 verify.codeFix({
     description: "Remove declaration for: 'x'",
     index: 0,
-    newRangeContent: "/*~a*/() => /*~d*/ { }/*~f*/",
+    newRangeContent: "/*~a*/()/*~b*/ /*~c*/=>/*~d*/ {/*~e*/}/*~f*/",
 });