Merge pull request #192 from smarter/fix/ThisType-subtyping

Fix subtyping checks involving ThisType and PolyParam

Author: odersky

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -486,7 +486,7 @@ class TypeComparer(initctx: Context) extends DotClass {
             if (solvedConstraint && (constraint contains tp2)) isSubType(tp1, bounds(tp2).lo)
             else
               isSubTypeWhenFrozen(tp1, bounds(tp2).lo) || {
-                if (isConstrained(tp2)) addConstraint(tp2, tp1.widen, fromBelow = true)
+                if (isConstrained(tp2)) addConstraint(tp2, tp1.widenExpr, fromBelow = true)
                 else (ctx.mode is Mode.TypevarsMissContext) || secondTry(tp1, tp2)
               }
           }

test/dotc/tests.scala

@@ -56,6 +56,7 @@ class tests extends CompilerTest {
   @Test def pos_approximateUnion = compileFile(posDir, "approximateUnion", doErase)
   @Test def pos_tailcall = compileDir(posDir + "tailcall/", doErase)
   @Test def pos_nullarify = compileFile(posDir, "nullarify", "-Ycheck:nullarify" :: doErase)
+  @Test def pos_subtyping = compileFile(posDir, "subtyping", doErase)
 
   @Test def pos_all = compileFiles(posDir, twice)
   @Test def new_all = compileFiles(newDir, twice)

tests/pos/subtyping.scala

@@ -0,0 +1,6 @@
+class A {
+  def test1(): Unit = {
+    implicitly[this.type <:< this.type]
+    implicitly[this.type <:< A]
+  }
+}