Don't reformat body of arrow function when deleting its parameter

Author: Andy Hanson

src/services/textChanges.ts

@@ -329,6 +329,10 @@ namespace ts.textChanges {
             this.insertText(sourceFile, token.getStart(sourceFile), text);
         }
 
+        public replaceNodeWithText(sourceFile: SourceFile, oldNode: Node, text: string, options: ChangeNodeOptions = useNonAdjustedPositions): void {
+            return this.replaceRangeWithText(sourceFile, getAdjustedRange(sourceFile, oldNode, oldNode, options), text);
+        }
+
         public replaceRangeWithText(sourceFile: SourceFile, range: TextRange, text: string) {
             this.changes.push({ kind: ChangeKind.Text, sourceFile, range, text });
         }
@@ -1010,25 +1014,9 @@ namespace ts.textChanges {
             switch (node.kind) {
                 case SyntaxKind.Parameter: {
                     const oldFunction = node.parent;
-                    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!, // TODO: GH#18217
-                            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 {
                         deleteNodeInList(changes, deletedNodesInLists, sourceFile, node);

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*/",
 });