Fix #2944: propagate information from GADT bounds to normal info.

This allows all other parts of pipeline to be less aware of existence
of the GADT bounds.

In particular, it makes us pickle them as normal type bounds.

Author: DarkDimius

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -45,17 +45,26 @@ import reporting.diagnostic.messages.SuperCallsNotAllowedInline
  *  (11) Minimizes `call` fields of `Inline` nodes to just point to the toplevel
  *       class from which code was inlined.
  *
+ *  (12) Converts GADT bounds into normal type bounds
+ *
  *  The reason for making this a macro transform is that some functions (in particular
  *  super and protected accessors and instantiation checks) are naturally top-down and
  *  don't lend themselves to the bottom-up approach of a mini phase. The other two functions
  *  (forwarding param accessors and synthetic methods) only apply to templates and fit
  *  mini-phase or subfunction of a macro phase equally well. But taken by themselves
  *  they do not warrant their own group of miniphases before pickling.
  */
-class PostTyper extends MacroTransform with IdentityDenotTransformer  { thisTransformer =>
+class PostTyper extends MacroTransform with SymTransformer  { thisTransformer =>
+
 
   import tpd._
 
+  def transformSym(sym: SymDenotation)(implicit ctx: Context): SymDenotation = {
+    if (sym.is(BindDefinedType) && ctx.gadt.bounds.contains(sym.symbol)) {
+      sym.copySymDenotation(info = ctx.gadt.bounds.apply(sym.symbol) & sym.info)
+    } else sym
+  }
+
   /** the following two members override abstract members in Transform */
   override def phaseName: String = "posttyper"