Disallow polymorphic refinements in stuctural types.

We can't handle them with the proposed scheme. I made a note in scala#1886.

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

@@ -1039,7 +1039,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     for (refinement <- refinements1) { // TODO: get clarity whether we want to enforce these conditions
       typr.println(s"adding refinement $refinement")
       checkRefinementNonCyclic(refinement, refineCls, seen)
-    }
+      val rsym = refinement.symbol
+      if (rsym.info.isInstanceOf[PolyType] && rsym.allOverriddenSymbols.isEmpty)
+        ctx.error(i"polymorphic refinement $rsym without matching type in parent $tpt1 is no longer allowed", refinement.pos)    }
     assignType(cpy.RefinedTypeTree(tree)(tpt1, refinements1), tpt1, refinements1, refineCls)
   }
 

tests/neg/structural.scala

@@ -0,0 +1,11 @@
+object Test3 {
+  import scala.reflect.Selectable.reflectiveSelectable
+  def g(x: { type T ; def t: T ; def f(a: T): Boolean }) = x.f(x.t) // error: no ClassTag for x.T
+  g(new { type T = Int; def t = 4; def f(a:T) = true })
+  g(new { type T = Any; def t = 4; def f(a:T) = true })
+  val y: { type T = Int; def t = 4; def f(a:T) = true }
+    = new { type T = Int; def t = 4; def f(a:T) = true }
+
+  def h(x: { def f[T](a: T): Int }) = x.f[Int](4) // error: polymorphic refinement method ... no longer allowed
+
+}