3 files changed, 9 insertions, 4 deletions
diff --git a/src/dotty/tools/dotc/typer/Checking.scala b/src/dotty/tools/dotc/typer/Checking.scala
index 25e6a7aa7..e3024b9a6 100644
--- a/src/dotty/tools/dotc/typer/Checking.scala
+++ b/src/dotty/tools/dotc/typer/Checking.scala
@@ -45,13 +45,16 @@ trait Checking extends NoChecking {
   }
 
   /** Check that type arguments `args` conform to corresponding bounds in `poly` */
-  override def checkBounds(args: List[tpd.Tree], poly: PolyType, pos: Position)(implicit ctx: Context): Unit =
+  override def checkBounds(args: List[tpd.Tree], poly: PolyType, pos: Position)(implicit ctx: Context): Unit = {
+    val argTypes = args.tpes
+    def substituted(tp: Type) = tp.substParams(poly, argTypes)
     for ((arg, bounds) <- args zip poly.paramBounds) {
       def notConforms(which: String, bound: Type) =
         ctx.error(i"Type argument ${arg.tpe} does not conform to $which bound $bound", arg.pos)
-      if (!(arg.tpe <:< bounds.hi)) notConforms("upper", bounds.hi)
+      if (!(arg.tpe <:< substituted(bounds.hi))) notConforms("upper", bounds.hi)
       if (!(bounds.lo <:< arg.tpe)) notConforms("lower", bounds.lo)
     }
+  }
 
   /** Check that type `tp` is stable.
    *  @return The type itself
diff --git a/src/dotty/tools/dotc/typer/Namer.scala b/src/dotty/tools/dotc/typer/Namer.scala
index c24021936..9c8522b83 100644
--- a/src/dotty/tools/dotc/typer/Namer.scala
+++ b/src/dotty/tools/dotc/typer/Namer.scala
@@ -614,6 +614,7 @@ class Namer { typer: Typer =>
 
   def typeDefSig(tdef: TypeDef, sym: Symbol)(implicit ctx: Context): Type = {
     completeParams(tdef.tparams)
+    sym.info = TypeBounds.empty // avoid cyclic reference errors for F-bounds
     val tparamSyms = tdef.tparams map symbolOfTree
     val rhsType = typedAheadType(tdef.rhs).tpe
 
diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 6a1d279df..e195c2807 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -699,8 +699,9 @@ 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)
-    if (!(lo1.tpe <:< hi1.tpe))
-      ctx.error(i"lower bound ${lo1.tpe} does not conform to upper bound ${hi1.tpe}", tree.pos)
+    if (false) // 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(i"lower bound ${lo1.tpe} does not conform to upper bound ${hi1.tpe}", tree.pos)
     assignType(cpy.TypeBoundsTree(tree, lo1, hi1), lo1, hi1)
   }