Do not constrain types under either approx

Author: abgruszecki

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

@@ -1376,7 +1376,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
    *  Test that the resulting bounds are still satisfiable.
    */
   private def narrowGADTBounds(tr: NamedType, bound: Type, approx: ApproxState, isUpper: Boolean): Boolean = {
-    val boundImprecise = if (isUpper) approx.high else approx.low
+    val boundImprecise = approx.high || approx.low
     ctx.mode.is(Mode.GADTflexible) && !frozenConstraint && !boundImprecise && {
       val tparam = tr.symbol
       gadts.println(i"narrow gadt bound of $tparam: ${tparam.info} from ${if (isUpper) "above" else "below"} to $bound ${bound.toString} ${bound.isRef(tparam)}")

tests/neg/gadt-no-approx.scala

@@ -0,0 +1,10 @@
+object `gadt-no-approx` {
+  def fo[U](u: U): U =
+    (0 : Int) match {
+      case _: u.type =>
+        val i: Int = (??? : U) // error
+        // potentially could compile
+        // val i2: Int = u
+        u
+    }
+}