Streamline isConcrete a bit more

Author: odersky

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

@@ -34,24 +34,10 @@ object MatchTypes:
         true
       case tp1: TermRef =>
         !tp1.symbol.is(Param) && isConcrete(tp1.underlying)
-      case tp1: TermParamRef =>
+      case _: (ParamRef | MatchType) =>
         false
-      case tp1: SingletonType =>
+      case tp1: TypeProxy =>
         isConcrete(tp1.underlying)
-      case tp1: ExprType =>
-        isConcrete(tp1.underlying)
-      case tp1: AnnotatedType =>
-        isConcrete(tp1.parent)
-      case tp1: RefinedOrRecType =>
-        isConcrete(tp1.underlying)
-      case tp1: AndOrType =>
-        isConcrete(tp1.tp1) && isConcrete(tp1.tp2)
-      case tp1: TypeVar =>
-        isConcrete(tp1.underlying)
-      case tp1: LazyRef =>
-        isConcrete(tp1.ref)
-      case tp1: FlexibleType =>
-        isConcrete(tp1.hi)
       case _ =>
         false
   end isConcrete

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

@@ -76,6 +76,7 @@ object Types extends TypeUtils {
    *        |              +- HKTypeLambda
    *        |              +- MatchType
    *        |              +- FlexibleType
+   *        |              +- LazyRef
    *        |
    *        +- GroundType -+- AndType
    *                       +- OrType