Type#contains should peer into RefinementTypeRef-s

Usually, `contains` should not look into class symbol infos.
For instance, we expect that:

```
scala> trait C { def foo: Int }; typeOf[C].contains(IntClass)
defined trait C
res1: Boolean = false
```

We do, however, look at the decls of a `RefinedType` in contains:

```
scala> typeOf[{ def foo: Int }].contains(IntClass)
res2: Boolean = true
```

Things get a little vague, however, when we consider a type ref
to the refinement class symbol of a refined type.

```
scala> TypeRef(NoPrefix, typeOf[{ def foo: Int }].typeSymbol, Nil)
res3: $r.intp.global.Type = AnyRef{def foo: Int}

scala> .contains(IntClass)
res4: Boolean = false
```

These show up in the first element of the base type seq of a refined
type, e.g:

```
scala> typeOf[{ def foo: Int }].typeSymbol.tpe_*
res5: $r.intp.global.Type = AnyRef{def foo: Int}

scala> typeOf[{ def foo: Int }].baseTypeSeq(0).getClass
res7: Class[_ <: $r.intp.global.Type] = class scala.reflect.internal.Types$RefinementTypeRef

scala> typeOf[{ def foo: Int }].typeSymbol.tpe_*.getClass
res6: Class[_ <: $r.intp.global.Type] = class scala.reflect.internal.Types$RefinementTypeRef
```

This commit takes the opinion that a `RefinementTypeRef` should be
transparent with respect to `contains`. This paves the way for fixing
the base type sequences of existential types over refinement types.
The implementation of `ContainsCollector` was already calling
`normalize`, which goes from `RefinementTypeRef` to `RefinedType`.
This commit maps over the result, which looks in the parents and
decls.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala

@@ -321,13 +321,20 @@ trait MethodSynthesis {
         // starts compiling (instead of failing like it's supposed to) because the typer
         // expects to be able to identify escaping locals in typedDefDef, and fails to
         // spot that brand of them. In other words it's an artifact of the implementation.
+        //
+        // JZ: ... or we could go back to uniformly using explicit result types in all cases
+        //         if we fix `dropExistential`. More details https://github.com/scala/scala-dev/issues/165
         val getterTp = derivedSym.tpe_*.finalResultType
-        val tpt = getterTp.widen match {
-          // Range position errors ensue if we don't duplicate this in some
-          // circumstances (at least: concrete vals with existential types.)
-          case _: ExistentialType => TypeTree() setOriginal (tree.tpt.duplicate setPos tree.tpt.pos.focus)
-          case _ if tree.mods.isDeferred    => TypeTree() setOriginal tree.tpt // keep type tree of original abstract field
-          case _                  => TypeTree(getterTp)
+        // Range position errors ensue if we don't duplicate this in some
+        // circumstances (at least: concrete vals with existential types.)
+        def inferredTpt = TypeTree() setOriginal (tree.tpt.duplicate setPos tree.tpt.pos.focus)
+        val tpt = getterTp match {
+          case _: ExistentialType => inferredTpt
+          case _ => getterTp.widen match {
+            case _: ExistentialType => inferredTpt
+            case _ if tree.mods.isDeferred => TypeTree() setOriginal tree.tpt // keep type tree of original abstract field
+            case _ => TypeTree(getterTp)
+          }
         }
         tpt setPos tree.tpt.pos.focus
       }

src/reflect/scala/reflect/internal/tpe/TypeMaps.scala

@@ -1012,6 +1012,9 @@ private[internal] trait TypeMaps {
           case _ =>
             tp.normalize match {
               case TypeRef(_, sym1, _) if (sym == sym1) => result = true
+              case refined: RefinedType =>
+                mapOver(tp.prefix)
+                mapOver(refined)
               case SingleType(_, sym1) if (sym == sym1) => result = true
               case _ => mapOver(tp)
             }

test/files/pos/typevar-in-prefix.scala

@@ -0,0 +1,9 @@
+trait Test1 {
+  abstract class Setting
+  def Bool: Setting
+
+  class C[T <: Setting](val s: T)
+  val setting1 = null.asInstanceOf[_1.s.type forSome { val _1: C[Setting] }]
+  // the derived accessor for this val was not using an inferred type, as was
+  // the intention of the implementation in MethodSynthesis.
+}

test/junit/scala/reflect/internal/TypesTest.scala

@@ -58,4 +58,15 @@ class TypesTest {
         Assert.fail(xs.mkString("\n"))
     }
   }
+
+  @Test
+  def testRefinementContains(): Unit = {
+    val refinement = typeOf[{def foo: Int}]
+    assert(refinement.isInstanceOf[RefinedType])
+    assert(refinement.contains(IntClass))
+    val elem0 = refinement.baseTypeSeq(0)
+    assert(elem0.isInstanceOf[RefinementTypeRef])
+    assert(elem0.contains(IntClass))
+  }
+
 }