Fix #1569: Improve avoidance algorithm

The essential change is that we do not throw away more
precise info of the avoided type if the expected type
is fully defined.

Author: odersky

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

@@ -642,12 +642,18 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     }
     val leaks = escapingRefs(tree, localSyms)
     if (leaks.isEmpty) tree
-    else if (isFullyDefined(pt, ForceDegree.none)) ascribeType(tree, pt)
     else if (!forcedDefined) {
       fullyDefinedType(tree.tpe, "block", tree.pos)
-      val tree1 = ascribeType(tree, avoid(tree.tpe, localSyms))
-      ensureNoLocalRefs(tree1, pt, localSyms, forcedDefined = true)
-    } else
+      val avoidingType = avoid(tree.tpe, localSyms)
+      if (isFullyDefined(pt, ForceDegree.none) && !(avoidingType <:< pt))
+        ascribeType(tree, pt)
+      else {
+        val tree1 = ascribeType(tree, avoidingType)
+        ensureNoLocalRefs(tree1, pt, localSyms, forcedDefined = true)
+      }
+    } else if (isFullyDefined(pt, ForceDegree.none))
+      ascribeType(tree, pt)
+    else
       errorTree(tree,
           em"local definition of ${leaks.head.name} escapes as part of expression's type ${tree.tpe}"/*; full type: ${result.tpe.toString}"*/)
   }

tests/neg/t1569-failedAvoid.scala

@@ -0,0 +1,6 @@
+// See pos/t1569a.scala for related examples that work.
+object Bug {
+  class C { type T }
+  def foo(x: Int)(y: C)(z: y.T): Unit = {}
+  foo(3)(new C { type T = String })("hello")
+}