Support Instance Main Methods in google-java-format

core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java

@@ -453,8 +453,15 @@ protected void dropEmptyDeclarations() {
     }
   }
 
+  // Replace with Flags.IMPLICIT_CLASS once JDK 25 is the minimum supported version
+  private static final int IMPLICIT_CLASS = 1 << 19;
+
   @Override
   public Void visitClass(ClassTree tree, Void unused) {
+    if ((TreeInfo.flags((JCTree) tree) & IMPLICIT_CLASS) == IMPLICIT_CLASS) {
+      visitImplicitClass(tree);
+      return null;
+    }
     switch (tree.getKind()) {
       case ANNOTATION_TYPE -> visitAnnotationType(tree);
       case CLASS, INTERFACE -> visitClassDeclaration(tree);
@@ -465,6 +472,12 @@ public Void visitClass(ClassTree tree, Void unused) {
     return null;
   }
 
+  private void visitImplicitClass(ClassTree node) {
+    builder.open(minusTwo);
+    addBodyDeclarations(node.getMembers(), BracesOrNot.NO, FirstDeclarationsOrNot.YES);
+    builder.close();
+  }
+
   public void visitAnnotationType(ClassTree node) {
     sync(node);
     builder.open(ZERO);

core/src/test/java/com/google/googlejavaformat/java/FormatterIntegrationTest.java

@@ -59,7 +59,7 @@ public class FormatterIntegrationTest {
               "I981",
               "I1020",
               "I1037")
-          .putAll(25, "ModuleImport")
+          .putAll(25, "ModuleImport", "InstanceMain")
           .build();
 
   @Parameters(name = "{index}: {0}")

core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.input

@@ -0,0 +1,5 @@
+String greeting = "Hello, World!";
+
+void main() {
+    System.out.println(greeting);
+}

core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.output

@@ -0,0 +1,5 @@
+String greeting = "Hello, World!";
+
+void main() {
+  System.out.println(greeting);
+}