Better types for class type parameters

If we see a class type parameter that has a wildcard argument, we now intersect
the original info of the class type parameter and the argument. Previously we
replaced the class type parameter info with the argument info, but that might lose
information.

Fixes #15940

Author: odersky

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

@@ -1120,7 +1120,7 @@ object Denotations {
       then this
       else if symbol.isAllOf(ClassTypeParam) then
         val arg = symbol.typeRef.argForParam(pre, widenAbstract = true)
-        if arg.exists then derivedSingleDenotation(symbol, arg.bounds, pre)
+        if arg.exists then derivedSingleDenotation(symbol, symbol.info.bounds & arg.bounds, pre)
         else derived(symbol.info)
       else derived(symbol.info)
     }

tests/pos/i15940.scala

@@ -0,0 +1,19 @@
+object Ops:
+  implicit class EitherSeqOps[E, T](private val seq: Seq[Either[E, T]]) extends AnyVal:
+    def sequence: Either[::[E], Seq[T]] = ???
+
+trait BuildException
+case class CompositeBuildException(ex: ::[BuildException]) extends BuildException
+
+trait ActionableDiagnostic
+trait ActionableHandler[A <: ActionableDiagnostic]:
+  def exec: Either[BuildException, Seq[A]]
+
+import Ops._
+
+val test: Either[BuildException, Seq[ActionableDiagnostic]] =
+  // Can be replaced with Seq[Either[BuildException, Seq[ _ <: ActionableDiagnostic]]] , but current version matches better type of missing implicit
+  Seq.empty[ActionableHandler[_]].map(_.exec)
+    .sequence
+    .left.map(_.head)
+    .map(_.flatten) // error