Fix #7965: Deal with AndTypes in joins

Allow that the base type of a join may be a conjunction.

Author: odersky

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

@@ -212,6 +212,8 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
               tp1.derivedAppliedType(
                 mergeRefinedOrApplied(tycon1, tycon2),
                 ctx.typeComparer.lubArgs(args1, args2, tycon1.typeParams))
+            case AndType(tp21, tp22) =>
+              mergeRefinedOrApplied(tp1, tp21) & mergeRefinedOrApplied(tp1, tp22)
             case _ => fail
           }
         case tp1 @ TypeRef(pre1, _) =>
@@ -220,6 +222,8 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
               tp1.derivedSelect(pre1 | pre2)
             case _ => fail
           }
+        case AndType(tp11, tp12) =>
+          mergeRefinedOrApplied(tp11, tp2) & mergeRefinedOrApplied(tp12, tp2)
         case _ => fail
       }
     }

tests/pos/i7965.scala

@@ -0,0 +1,46 @@
+class Has[A]
+trait X
+trait Y
+trait Z
+
+abstract class Test {
+  val x: Has[X] | (Has[Y] & Has[Z])
+  def foo[T <: Has[_]](has: T): T = has
+  foo(x)
+}
+
+trait ZLayer[-RIn, +E, +ROut <: Has[_]] {
+  def >>>[E1 >: E, ROut2 <: Has[_]](that: ZLayer[ROut, E1, ROut2]): ZLayer[RIn, E1, ROut2]
+  def ++[E1 >: E, RIn2, ROut1 >: ROut <: Has[_], ROut2 <: Has[_]](that: ZLayer[RIn2, E1, ROut2]): ZLayer[RIn with RIn2, E1, ROut1 with ROut2]
+}
+object ZLayer {
+  type NoDeps[+E, +B <: Has[_]] = ZLayer[Any, E, B]
+}
+
+type ServiceA = Has[ServiceA.Service]
+object ServiceA {
+  trait Service
+  val live: ZLayer.NoDeps[Nothing, ServiceA] = ???
+}
+
+type ServiceB = Has[ServiceB.Service]
+object ServiceB {
+  trait Service
+  val live: ZLayer.NoDeps[Nothing, ServiceB] = ???
+}
+
+type ServiceC = Has[ServiceC.Service]
+object ServiceC {
+  trait Service
+  val live: ZLayer.NoDeps[Nothing, ServiceC] = ???
+}
+
+type ServiceD = Has[ServiceD.Service]
+object ServiceD {
+  trait Service
+  val live: ZLayer.NoDeps[ServiceC, ServiceD with ServiceC] = ???
+}
+
+val combined =
+    ServiceA.live >>>
+      (ServiceB.live ++ (ServiceC.live >>> ServiceD.live))