Avoid hoisting local classes

The patch disables hoisting of classes local to a block into the
result type of the block.

Instead, we widen the result type of the block to one which reflects
all refinements made to the parents type of the local class.

Test cases in avoid.scala, t1569.scala.

The original t1569.scala no longer works. Why is explained in neg/t1569-failedAvoid.scala

Author: odersky

src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -49,7 +49,21 @@ trait TypeAssigner {
             case TypeAlias(ref) =>
               apply(ref)
             case info: ClassInfo =>
-              mapOver(info.instantiatedParents.reduceLeft(ctx.typeComparer.andType(_, _)))
+              val parentType = info.instantiatedParents.reduceLeft(ctx.typeComparer.andType(_, _))
+              def addRefinement(parent: Type, decl: Symbol) = {
+                val inherited = parentType.findMember(decl.name, info.cls.thisType, Private)
+                val inheritedInfo = inherited.atSignature(decl.info .signature).info
+                if (inheritedInfo.exists && decl.info <:< inheritedInfo && !(inheritedInfo <:< decl.info))
+                  typr.echo(
+                    i"add ref $parent $decl --> ",
+                    RefinedType(parent, decl.name, decl.info))
+                else
+                  parent
+              }
+              val refinableDecls = info.decls.filterNot(
+                sym => sym.is(TypeParamAccessor | Private) || sym.isConstructor)
+              val fullType = (parentType /: refinableDecls)(addRefinement)
+              mapOver(fullType)
             case _ =>
               mapOver(tp)
           }

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

@@ -415,17 +415,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   def escapingRefs(block: Block)(implicit ctx: Context): collection.Set[NamedType] = {
     var hoisted: Set[Symbol] = Set()
     lazy val locals = localSyms(block.stats).toSet
-    def isLocal(sym: Symbol): Boolean =
-      (locals contains sym) && !isHoistableClass(sym)
-    def isHoistableClass(sym: Symbol) =
-      sym.isClass && {
-        (hoisted contains sym) || {
-          hoisted += sym
-          !classLeaks(sym.asClass)
-        }
-      }
     def leakingTypes(tp: Type): collection.Set[NamedType] =
-      tp namedPartsWith (tp => isLocal(tp.symbol))
+      tp namedPartsWith (tp => locals.contains(tp.symbol))
     def typeLeaks(tp: Type): Boolean = leakingTypes(tp).nonEmpty
     def classLeaks(sym: ClassSymbol): Boolean =
       (ctx.owner is Method) || // can't hoist classes out of method bodies

test/dotc/tests.scala

@@ -99,6 +99,7 @@ class tests extends CompilerTest {
   @Test def neg_variances = compileFile(negDir, "variances", xerrors = 2)
   @Test def neg_badAuxConstr = compileFile(negDir, "badAuxConstr", xerrors = 2)
   @Test def neg_typetest = compileFile(negDir, "typetest", xerrors = 1)
+  @Test def neg_t1569_failedAvoid = compileFile(negDir, "t1569-failedAvoid", xerrors = 1)
   @Test def dotc = compileDir(dotcDir + "tools/dotc", twice)(allowDeepSubtypes)
   @Test def dotc_ast = compileDir(dotcDir + "tools/dotc/ast", twice)
   @Test def dotc_config = compileDir(dotcDir + "tools/dotc/config", twice)

tests/neg/t1569-failedAvoid.scala

@@ -0,0 +1,9 @@
+// This was t1569.scala.
+// It fails in dotty because the expected type of the anonymous function in the last line
+// is fully determined (C). So that type is taken as the type of the anonymous function.
+// 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")
+}

tests/pos/avoid.scala

@@ -0,0 +1,10 @@
+abstract class C {
+  def y: Any
+}
+
+object test {
+  val x = new C{
+    def y: String = "abc"
+  }
+  val z: String = x.y
+}

tests/pos/t1569.scala

@@ -0,0 +1,12 @@
+object Bug {
+  class C[T] { type TT = T }
+  def foo[U](x: Int)(y: C[U])(z: y.TT): Unit = {}
+  foo(3)(new C[String])("hello")
+}
+
+object Bug2 {
+  class C { type T }
+  class D extends C { type T = String }
+  def foo(x: Int)(y: C)(z: y.T): Unit = {}
+  foo(3)(new D)("hello")
+}