Include companion objects of abstract types in implicit scope

Author: odersky

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

@@ -478,12 +478,14 @@ trait ImplicitRunInfo { self: Run =>
    *
    *  - If `tp` is a class reference, S contains a reference to the companion object of the class,
    *    if it exists, as well as the implicit scopes of all of `tp`'s parent class references.
-   *  - If `tp` is an opaque type alias `p.A` of type `tp'`, S contains a reference to an object `A` defined in the
-   *    same scope as the opaque type, if it exists, as well as the implicit scope of `tp'`.
-   *  - If `tp` is a reference `p.T` to a class or opaque type alias, S also contains all object references
-   *    on the prefix path `p`. Under Scala-2 mode, package objects of package references on `p` also
-   *    count towards the implicit scope.
-   *  - If `tp` is an alias of `tp'`, S contains the implicit scope of `tp'`.
+   *  - If `tp` is an opaque type alias or abstract type `p.A`, S contains a reference to an
+   *    object `A` defined in the same scope as type `A`, if it exists, as well as
+   *    the implicit scope of `tp`s upper bound.
+   *  - If `tp` is a reference `p.T` to a class, an opaque type alias, or an abstract type,
+   *    S also contains all object references on the prefix path `p`.
+   *    Under Scala-2 mode, package objects of package references on `p` also count
+   *    towards the implicit scope.
+   *  - If `tp` is a (non-opaque) alias of `tp'`, S contains the implicit scope of `tp'`.
    *  - If `tp` is some other type, its implicit scope is the union of the implicit scopes of
    *    its parts (parts defined as in the spec).
    *
@@ -501,7 +503,8 @@ trait ImplicitRunInfo { self: Run =>
     /** Is `sym` an anchor type for which givens may exist? Anchor types are classes,
      *  opaque type aliases, and abstract types, but not type parameters
      */
-    def isAnchor(sym: Symbol) = sym.isClass && !sym.is(Package) || sym.isOpaqueAlias
+    def isAnchor(sym: Symbol) =
+      sym.isClass && !sym.is(Package) || sym.is(DeferredType)
 
     def anchors(tp: Type): List[Type] = tp match {
       case tp: NamedType if isAnchor(tp.symbol) => tp :: Nil