fix #9324: forbid no-arg java.lang.Enum ctor for migration mode

Author: bishabosha

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -895,9 +895,10 @@ trait Applications extends Compatibility {
           case funRef: TermRef =>
             val app = ApplyTo(tree, fun1, funRef, proto, pt)
             convertNewGenericArray(
-              widenEnumCase(
-                postProcessByNameArgs(funRef, app).computeNullable(),
-                pt))
+              guardJavaEnumCtor(
+                widenEnumCase(
+                  postProcessByNameArgs(funRef, app).computeNullable(),
+                  pt)))
           case _ =>
             handleUnexpectedFunType(tree, fun1)
         }
@@ -1113,6 +1114,12 @@ trait Applications extends Compatibility {
       tree
   }
 
+  def guardJavaEnumCtor(tree: Tree)(using Context): Tree =
+    if config.Feature.migrateTo3 && tree.symbol == defn.JavaEnumClass.primaryConstructor then
+      report.error(
+        em"""not enough arguments for constructor Enum: (name: String, ordinal: Int): ${tree.tpe}""", tree.srcPos)
+    tree
+
   /** If `tree` is a complete application of a compiler-generated `apply`
    *  or `copy` method of an enum case, widen its type to the underlying
    *  type by means of a type ascription, as long as the widened type is

tests/neg/extend-java-enum-migration.check

@@ -0,0 +1,12 @@
+-- Error: tests/neg/extend-java-enum-migration.scala:9:22 --------------------------------------------------------------
+9 |final class C extends jl.Enum[C] // error
+  |                      ^^^^^^^^^^
+  |                      not enough arguments for constructor Enum: (name: String, ordinal: Int): Enum[C]
+-- Error: tests/neg/extend-java-enum-migration.scala:11:17 -------------------------------------------------------------
+11 |object O extends jl.Enum[O.type] // error
+   |                 ^^^^^^^^^^^^^^^
+   |                 not enough arguments for constructor Enum: (name: String, ordinal: Int): Enum[O.type]
+-- Error: tests/neg/extend-java-enum-migration.scala:14:6 --------------------------------------------------------------
+14 |class Sub extends T // error
+   |      ^
+   |      not enough arguments for constructor Enum: (name: String, ordinal: Int): Enum[T]

tests/neg/extend-java-enum-migration.scala

@@ -0,0 +1,14 @@
+import java.{lang => jl}
+
+import language.`3.0-migration`
+
+// This file is different from `tests/neg/extend-java-enum.scala` as we
+// are testing that it is illegal to *not* pass arguments to jl.Enum
+// in 3.0-migration
+
+final class C extends jl.Enum[C] // error
+
+object O extends jl.Enum[O.type] // error
+
+trait T extends jl.Enum[T] // ok
+class Sub extends T // error