Only instantiate toplevel typevars

Author: odersky

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

@@ -81,6 +81,19 @@ object Inferencing {
     if (depVars.nonEmpty) instantiateSelected(tp, depVars.toList)
   }
 
+  /** If `tp` is type variable with a lower bound in the current constraint,
+   *  instantiate it from below.
+   */
+  def couldInstantiateTypeVar(tp: Type)(using Context): Boolean = tp.dealias match
+    case tvar: TypeVar
+    if !tvar.isInstantiated
+       && ctx.typerState.constraint.contains(tvar)
+       && tvar.hasLowerBound =>
+      tvar.instantiate(fromBelow = true)
+      true
+    case _ =>
+      false
+
   /** The accumulator which forces type variables using the policy encoded in `force`
    *  and returns whether the type is fully defined. The direction in which
    *  a type variable is instantiated is determined as follows:

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

@@ -12,7 +12,7 @@ import config.Printers.typr
 import ast.Trees._
 import NameOps._
 import ProtoTypes._
-import Inferencing.isFullyDefined
+import Inferencing.couldInstantiateTypeVar
 import collection.mutable
 import reporting._
 import Checking.{checkNoPrivateLeaks, checkNoWildcard}
@@ -160,7 +160,7 @@ trait TypeAssigner {
       TryDynamicCallType
     else if (qualType.isErroneous || name.toTermName == nme.ERROR)
       UnspecifiedErrorType
-    else if !isFullyDefined(qualType, ForceDegree.none) && isFullyDefined(qualType, ForceDegree.failBottom) then
+    else if couldInstantiateTypeVar(qualType) then
       // try again with more defined qualifier type
       selectionType(tree, qual1)
     else if (name == nme.CONSTRUCTOR)