experiment with supertypes

Author: bishabosha

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

@@ -251,6 +251,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
   private def companionPath(mirroredType: Type, cls: Symbol, pre: Type, span: Span)(using Context) =
     val clsType =
       // ok to assume healPrefix will succeed because `cls` and `pre` derive from the same type.
+      // TODO: will fail, e.g. for value alias to case object - we still need to support `scala.package.None.type`
       TypeOps.healPrefix(mirroredType.mirrorCompanionRef, pre).toOption.get
     val ref = pathFor(clsType)
     assert(ref.symbol.is(Module) && (cls.is(ModuleClass) || (ref.symbol.companionClass == cls)))
@@ -330,6 +331,10 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       mirrorRef.cast(mirrorType)
     end makeProductMirror
 
+    if pre == NoPrefix then
+      report.error(i"$mirroredType is not a valid prod type, because it does not have a prefix")
+    else
+      report.warning(i"prod mirror with $mirroredType: pre $pre")
     mirroredType match
       case AndType(tp1, tp2) =>
         productMirror(tp1, formal, span).orElse(productMirror(tp2, formal, span))
@@ -359,6 +364,10 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
     val pre = TypeOps.extractPrefix(mirroredType) match
       case pre if pre.exists => pre
       case _                 => return EmptyTree
+    // if pre == NoPrefix then
+    //   report.error(i"$mirroredType is not a valid sum type, because it does not have a prefix")
+    // else
+    //   report.warning(i"sum mirror with $mirroredType: pre $pre")
     val cls = mirroredType.classSymbol
     val useCompanion = cls.useCompanionAsSumMirror
 

tests/neg/anon-mirror-gen-super-a.scala

@@ -0,0 +1,14 @@
+import scala.deriving.Mirror
+
+@main def Test = {
+
+  class Foo {
+    final val foo: Qux.type = Qux
+    case object Qux
+  }
+
+  class Bar extends Foo {
+    summon[Mirror.Of[Bar.super.foo.type]] //
+  }
+
+}

tests/neg/anon-mirror-gen-this-b.scala

@@ -8,9 +8,9 @@ import scala.deriving.Mirror
 
   object Foo {
 
-    sealed trait Item { thisItem =>
+    sealed trait Item {
 
-      val mItem = summon[Mirror.Of[thisItem.type]] // error: Bar.A.type is not a subtype of thisItem.type
+      val mItem = summon[Mirror.Of[Item.this.type]] // error: Bar.A.type is not a subtype of thisItem.type
 
     }
   }