fix(printer): don't wrap broken when-guards in parentheses

src/printers/expressions.ts

@@ -705,15 +705,12 @@ export default {
   },
 
   guard(path, print) {
-    return [
-      "when ",
-      group([
-        ifBreak("("),
-        indent([softline, path.call(print, "namedChildren", 0)]),
-        softline,
-        ifBreak(")")
-      ])
-    ];
+    // Break after `when` without wrapping the guard expression in parentheses
+    // (issue #1001). A space is used when the group fits on one line.
+    return group([
+      "when",
+      indent([line, path.call(print, "namedChildren", 0)])
+    ]);
   }
 } satisfies Partial<NamedNodePrinters>;
 

test/unit-test/switch-when-guard-parens/.prettierrc.json

@@ -0,0 +1,3 @@
+{
+  "printWidth": 40
+}

test/unit-test/switch-when-guard-parens/_input.java

@@ -0,0 +1,8 @@
+class T {
+  boolean f(Object x) {
+    return switch (x) {
+      case A a when a.equals(a.aaaa) -> true;
+      default -> false;
+    };
+  }
+}

test/unit-test/switch-when-guard-parens/_output.java

@@ -0,0 +1,9 @@
+class T {
+  boolean f(Object x) {
+    return switch (x) {
+      case A a when
+        a.equals(a.aaaa) -> true;
+      default -> false;
+    };
+  }
+}

test/unit-test/unnamed-variables-and-patterns/_output.java

@@ -120,10 +120,9 @@ int wrappingMultipleSwitchPatterns() {
       case
         MyRecord(LongTypeName longVariableName, LongTypeName longVariableName),
         MyRecord(LongTypeName longVariableName, LongTypeName longVariableName)
-      when (
+      when
         this.longVariableName > longVariableName &&
-        this.longVariableName > longVariableName
-      ) -> longMethodName(
+        this.longVariableName > longVariableName -> longMethodName(
         longVariableName,
         longVariableName,
         longVariableName,