Add explanation doc

Author: EugeneFlesselle

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

@@ -4644,7 +4644,11 @@ object Types extends TypeUtils {
           case tycon: HKTypeLambda => defn.AnyType
           case tycon: TypeRef if tycon.symbol.isClass => tycon
           case tycon: TypeProxy =>
-            if validSuper != Nowhere && args.exists(_.isProvisional) then validSuper = Nowhere
+            if validSuper != Nowhere && args.exists(_.isProvisional) then
+              // applyIfParameterized may perform eta-reduction leading to different
+              // variance annotations depending on the instantiation of type params
+              // see tests/pos/typeclass-encoding3b.scala:348 for an example
+              validSuper = Nowhere
             tycon.superType.applyIfParameterized(args)
           case _ => defn.AnyType
       cachedSuper

tests/pos/typeclass-encoding3b.scala

@@ -345,5 +345,12 @@ object functors {
   }
 
   MonadFlatten.flattened(List(List(1, 2, 3), List(4, 5))) // ok, synthesizes (using ListMonad)
-  MonadFlatten.flattened(List(List(1, 2, 3), List(4, 5)))(using ListMonad)
+  MonadFlatten.flattened(List(List(1, 2, 3), List(4, 5)))(using ListMonad) // was an error
+  /*
+  When checking `ListMonad <:< functors.Monad.Impl[T]`
+  we eventually get to the comparison `[X] =>> T[X] <:< [+X] =>> List[X]`
+  because the `This` type member of `ListMonad` has a covariance annotation.
+  This fails the variance conformance checks despite the fact that T has been instantiated to List,
+  since it has been substituted into the refinement (and cached) before its instantiation.
+  */
 }