use do not force symbols

Author: bishabosha

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -872,4 +872,12 @@ object TypeOps:
 
   end healPrefix
 
+  /** lift the prefix of an applied type, without normalisation or forcing */
+  def appliedTypePrefix(tpe: Type): Type =
+    tpe match
+      case AppliedType(tycon, _) => appliedTypePrefix(tycon)
+      case tpe: NamedType        => tpe.prefix
+      case tpe: ThisType         => tpe.tref.prefix
+      case _                     => NoPrefix
+
 end TypeOps

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

@@ -236,7 +236,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
     monoMap(mirroredType.resultType)
 
   private def productMirror(mirroredType: Type, formal: Type, span: Span)(using Context): Tree =
-    val pre = mirroredType.normalizedPrefix
+    val pre = TypeOps.appliedTypePrefix(mirroredType)
     mirroredType match
       case AndType(tp1, tp2) =>
         productMirror(tp1, formal, span).orElse(productMirror(tp2, formal, span))
@@ -285,7 +285,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
   end productMirror
 
   private def sumMirror(mirroredType: Type, formal: Type, span: Span)(using Context): Tree =
-    val pre = mirroredType.normalizedPrefix
+    val pre = TypeOps.appliedTypePrefix(mirroredType)
     val cls = mirroredType.classSymbol
     val useCompanion = cls.useCompanionAsMirror
     val declScope = if useCompanion then cls.linkedClass else ctx.owner

tests/pos/anon-mirror-gen-this-a.scala

@@ -0,0 +1,18 @@
+import scala.deriving.Mirror
+
+class Outer { self =>
+
+  object Inner {
+    sealed trait Item { thisItem => // both children and parent share a common sub-prefix
+
+      val mItem = summon[Mirror.Of[thisItem.type]]
+      type derivedA = Tuple.Head[mItem.MirroredElemTypes]
+      val mA = summon[Mirror.Of[derivedA]]
+
+    }
+
+    case object A extends self.Inner.Item
+    case object B extends self.Inner.Item
+  }
+
+}

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

@@ -0,0 +1,20 @@
+import scala.deriving.Mirror
+
+def local = {
+
+  object Bar {
+    case object A extends Foo.Item
+  }
+
+  object Foo {
+
+    sealed trait Item { thisItem => // both children and parent share a common sub-prefix
+
+      val mItem = summon[Mirror.Of[thisItem.type]]
+      type derivedA = Tuple.Head[mItem.MirroredElemTypes]
+      val mA = summon[Mirror.Of[derivedA]]
+
+    }
+  }
+
+}