Java parser: add support for unnamed classes (JEP-445)

Fixes #18584

compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala

@@ -826,6 +826,26 @@ object JavaParsers {
       addCompanionObject(statics, cls)
     }
 
+    def unnamedClassDecl(priorTypes: List[Tree], firstMemberMods: Modifiers, start: Offset): List[Tree] = {
+      val name = source.name.replaceAll("\\.java$", "").nn.toTypeName
+      val (statics, body) = typeBodyDecls(CLASS, name, parentTParams = Nil, firstMemberMods = Some(firstMemberMods))
+
+      val priorStatics = priorTypes.map {
+        case t: (TypeDef | ModuleDef) => t.withMods(t.mods.withFlags(t.mods.flags | Flags.JavaStatic))
+        case other => throw new Error(s"$other is neither TypeDef nor ModuleDef")
+      }
+
+      val cls = atSpan(start, 0) {
+        TypeDef(name, makeTemplate(
+          parents = List(javaLangObject()),
+          stats = body,
+          tparams = Nil, 
+          needsDummyConstr = true)
+        ).withMods(Modifiers(Flags.Private | Flags.Final))
+      }
+      addCompanionObject(priorStatics ::: statics, cls)
+    }
+
     def recordDecl(start: Offset, mods: Modifiers): List[Tree] =
       accept(RECORD)
       val nameOffset = in.offset
@@ -899,13 +919,13 @@ object JavaParsers {
       defs
     }
 
-    def typeBodyDecls(parentToken: Int, parentName: Name, parentTParams: List[TypeDef]): (List[Tree], List[Tree]) = {
+    def typeBodyDecls(parentToken: Int, parentName: Name, parentTParams: List[TypeDef], firstMemberMods: Option[Modifiers] = None): (List[Tree], List[Tree]) = {
       val inInterface = definesInterface(parentToken)
       val statics = new ListBuffer[Tree]
       val members = new ListBuffer[Tree]
       while (in.token != RBRACE && in.token != EOF) {
         val start = in.offset
-        var mods = modifiers(inInterface)
+        var mods = (if (statics.isEmpty && members.isEmpty) firstMemberMods else None).getOrElse(modifiers(inInterface))
         if (in.token == LBRACE) {
           skipAhead() // skip init block, we just assume we have seen only static
           accept(RBRACE)
@@ -1067,16 +1087,35 @@ object JavaParsers {
       val buf = new ListBuffer[Tree]
       while (in.token == IMPORT)
         buf ++= importDecl()
+
+      val afterImports = in.offset
+      val typesBuf = new ListBuffer[Tree]
+
       while (in.token != EOF && in.token != RBRACE) {
         while (in.token == SEMI) in.nextToken()
         if (in.token != EOF) {
           val start = in.offset
           val mods = modifiers(inInterface = false)
           adaptRecordIdentifier() // needed for typeDecl
-          buf ++= typeDecl(start, mods)
+
+          in.token match {
+            case ENUM | INTERFACE | AT | CLASS | RECORD => typesBuf ++= typeDecl(start, mods)
+            case _ =>
+              if (thisPackageName == tpnme.EMPTY_PACKAGE) {
+                // upon encountering non-types directly at a compilation unit level in an unnamed package,
+                // the entire compilation unit is treated as a JEP-445 unnamed class
+                val cls = unnamedClassDecl(priorTypes = typesBuf.toList, firstMemberMods = mods, start = afterImports)
+                typesBuf.clear()
+                typesBuf ++= cls
+              } else {
+                in.nextToken()
+                syntaxError(em"illegal start of type declaration", skipIt = true)
+                List(errorTypeTree)
+              }
+          }
         }
       }
-      val unit = atSpan(start) { PackageDef(pkg, buf.toList) }
+      val unit = atSpan(start) { PackageDef(pkg, (buf ++ typesBuf).toList) }
       accept(EOF)
       unit match
         case PackageDef(Ident(nme.EMPTY_PACKAGE), Nil) => EmptyTree

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -50,6 +50,9 @@ class CompilationTests {
     if scala.util.Properties.isJavaAtLeast("16") then
       tests ::= compileFilesInDir("tests/pos-java16+", defaultOptions.and("-Ysafe-init"))
 
+    if scala.util.Properties.isJavaAtLeast("21") then
+      tests ::= compileFilesInDir("tests/pos-java21+", defaultOptions.withJavacOnlyOptions("--enable-preview", "--release", "21"))  
+
     aggregateTests(tests*).checkCompile()
   }
 

compiler/test/dotty/tools/dotc/parsing/JavaJep445ParserTest.scala

@@ -0,0 +1,71 @@
+package dotty.tools.dotc.parsing
+
+import dotty.tools.DottyTest
+import dotty.tools.dotc.ast.Trees.{Ident, PackageDef, TypeDef}
+import dotty.tools.dotc.ast.untpd
+import dotty.tools.dotc.ast.untpd.ModuleDef
+import dotty.tools.dotc.core.Contexts.{Context, ContextBase}
+import dotty.tools.dotc.core.StdNames.tpnme
+import dotty.tools.dotc.printing.{PlainPrinter, Printer}
+import dotty.tools.dotc.util.SourceFile
+import dotty.tools.io.PlainFile
+import org.junit.Assert.{assertTrue, fail}
+import org.junit.Test
+import JavaParsers.JavaParser
+
+class JavaJep445ParserTest extends DottyTest {
+
+  @Test def `the parser produces same trees for a class and an equivalent unnamed class`
+      : Unit = {
+
+    val unnamed = JavaParser(
+      SourceFile.virtual(
+        "MyUnnamed.java",
+        s"""
+          |import some.pkg.*;
+          |
+          |@interface InnerAnnotation {}
+          |
+          |interface InnerInterface {}
+          |
+          |@Magic
+          |public volatile double d;
+          |
+          |void main() {}
+          |
+          |interface SecondInnerInterface {}
+          |
+          |""".stripMargin
+      )
+    ).parse()
+
+    val named = JavaParser(
+      SourceFile.virtual(
+        "SomeFile.java",
+        s"""
+          |import some.pkg.*;
+          |
+          |private final class MyUnnamed {
+          |
+          |  @interface InnerAnnotation {}
+          |  
+          |  interface InnerInterface {}
+          |  
+          |  @Magic
+          |  public volatile double d;
+          |  
+          |  void main() {}
+          |  
+          |  interface SecondInnerInterface {}
+          |
+          |}
+          |""".stripMargin
+      )
+    ).parse()
+
+    assertTrue(
+      "expected same trees for named and unnamed classes",
+      unnamed.sameTree(named)
+    )
+  }
+}

tests/pos-java21+/jep445/B.scala

@@ -0,0 +1 @@
+class B

tests/pos-java21+/jep445/UnnamedMainOnly.java

@@ -0,0 +1 @@
+void main() {}

tests/pos-java21+/jep445/UnnamedStartsWithAnnotatedField.java

@@ -0,0 +1,5 @@
+
+@MyAnnotation
+int myInt = 10;
+
+void main() {}

tests/pos-java21+/jep445/UnnamedStartsWithField.java

@@ -0,0 +1,9 @@
+private volatile int myInt = 10;
+
+String hello() {
+    return "hello";
+}
+
+interface Inner {}
+
+void main() {}

tests/pos-java21+/jep445/UnnamedStartsWithType.java

@@ -0,0 +1,4 @@
+
+class InnerOfUnnamed {}
+
+void main() {}