Re-enabled checkbounds tests

Now that F-bunded types are treated more robustly, we can check bounds for
non-emptyness during Typer.

This unvealed one wrong test (wonder how that passed scalac?), which got
moved to neg.

Author: odersky

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

@@ -735,7 +735,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   def typedAppliedTypeTree(tree: untpd.AppliedTypeTree)(implicit ctx: Context): AppliedTypeTree = track("typedAppliedTypeTree") {
     val tpt1 = typed(tree.tpt)
     val args1 = tree.args mapconserve (typed(_))
-    // todo in later phase: check arguments conform to parameter bounds
+    // check that arguments conform to bounds is done in phase FirstTransform
     assignType(cpy.AppliedTypeTree(tree, tpt1, args1), tpt1, args1)
   }
 
@@ -748,9 +748,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     val TypeBoundsTree(lo, hi) = desugar.typeBoundsTree(tree)
     val lo1 = typed(lo)
     val hi1 = typed(hi)
-    // need to do in later phase, as this might cause a cyclic reference error. See pos/t0039.scala
-    //  if (!(lo1.tpe <:< hi1.tpe))
-    //    ctx.error(d"lower bound ${lo1.tpe} does not conform to upper bound ${hi1.tpe}", tree.pos)
+    if (!(lo1.tpe <:< hi1.tpe))
+      ctx.error(d"lower bound ${lo1.tpe} does not conform to upper bound ${hi1.tpe}", tree.pos)
     assignType(cpy.TypeBoundsTree(tree, lo1, hi1), lo1, hi1)
   }
 

test/dotc/tests.scala

@@ -84,6 +84,7 @@ class tests extends CompilerTest {
   @Test def neg_tailcall = compileFile(negDir, "tailcall/tailrec", xerrors = 7)
   @Test def neg_tailcall2 = compileFile(negDir, "tailcall/tailrec-2", xerrors = 2)
   @Test def neg_tailcall3 = compileFile(negDir, "tailcall/tailrec-3", xerrors = 2)
+  @Test def nef_t1279a = compileFile(negDir, "t1279a", xerrors = 1)
   @Test def neg_t1843 = compileFile(negDir, "t1843", xerrors = 1)
   @Test def neg_t1843_variances = compileFile(negDir, "t1843-variances", xerrors = 1)
   @Test def neg_t2994 = compileFile(negDir, "t2994", xerrors = 2)

tests/pos/t1279a.scala renamed to tests/neg/t1279a.scala

@@ -1,10 +1,10 @@
 // covariant linked list
 abstract class M {
-  self =>
+  self: M =>
 
   type T
   final type selfType = M {type T <: self.T}
-  type actualSelfType >: self.type <: selfType
+  type actualSelfType >: self.type <: selfType // this no longer compiles because self.type is not a subtype of selfType
 
   def next: selfType