add safety for unknown type

Author: bishabosha

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

@@ -873,11 +873,22 @@ object TypeOps:
   end healPrefix
 
   /** lift the prefix of an applied type, without normalisation or forcing */
-  def appliedTypePrefix(tpe: Type): Type =
+  def appliedTypePrefix(tpe: Type)(using Context): Option[Type] =
     tpe match
-      case AppliedType(tycon, _) => appliedTypePrefix(tycon)
-      case tpe: NamedType        => tpe.prefix
-      case tpe: ThisType         => tpe.tref.prefix
-      case _                     => NoPrefix
+      case tpe: NamedType        => Some(tpe.prefix)
+      case tpe: ThisType         => appliedTypePrefix(tpe.tref)
+      case tpe: AppliedType      => appliedTypePrefix(tpe.tycon)
+      case tpe: AnnotatedType    => appliedTypePrefix(tpe.parent) // TODO: test
+      case tpe: AndOrType        => Some(NoPrefix) // TODO: test
+      case _ =>
+        // ignore HKTypeLambda/TypeBounds
+        // what do do about
+        // - MatchType        -- will this occur or will it be reduced?
+        // - TypeParamRef     -- likely collapse to bounds?
+        // - TypeVar/ExprType -- when can this occur
+        // - RecThis          -- what is the prefix of this
+        // - RefinedOrRecType -- do we need to refine the children?
+        // - AnnotatedType    -- do we need to annotate the children?
+        None
 
 end TypeOps

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

@@ -236,7 +236,9 @@ 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 = TypeOps.appliedTypePrefix(mirroredType)
+    val pre = TypeOps.appliedTypePrefix(mirroredType) match
+      case Some(pre) => pre
+      case _         => return EmptyTree
     mirroredType match
       case AndType(tp1, tp2) =>
         productMirror(tp1, formal, span).orElse(productMirror(tp2, formal, span))
@@ -285,7 +287,9 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
   end productMirror
 
   private def sumMirror(mirroredType: Type, formal: Type, span: Span)(using Context): Tree =
-    val pre = TypeOps.appliedTypePrefix(mirroredType)
+    val pre = TypeOps.appliedTypePrefix(mirroredType) match
+      case Some(pre) => pre
+      case _         => return EmptyTree
     val cls = mirroredType.classSymbol
     val useCompanion = cls.useCompanionAsMirror
     val declScope = if useCompanion then cls.linkedClass else ctx.owner