Merge pull request #3918 from dotty-staging/fix-#1754

Fix #1754: Don't narrow GADTs to lower bounds

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -556,6 +556,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     val acc = new TreeAccumulator[List[Symbol]] {
       def apply(syms: List[Symbol], tree: Tree)(implicit ctx: Context) = tree match {
         case Bind(_, body) => apply(tree.symbol :: syms, body)
+        case Annotated(tree, id @ Ident(tpnme.BOUNDTYPE_ANNOT)) => apply(id.symbol :: syms, tree)
         case _ => foldOver(syms, tree)
       }
     }

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

@@ -143,6 +143,7 @@ object StdNames {
     val INITIALIZER_PREFIX: N         = "initial$"
     val COMPANION_MODULE_METHOD: N    = "companion$module"
     val COMPANION_CLASS_METHOD: N     = "companion$class"
+    val BOUNDTYPE_ANNOT: N            = "$boundType$"
     val QUOTE: N                      = "'"
     val TYPE_QUOTE: N                = "type_'"
     val TRAIT_SETTER_SEPARATOR: N     = str.TRAIT_SETTER_SEPARATOR