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

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(ref: SymDenotation)(implicit ctx: Context): SymDenotation = {
+    if (ref.is(BindDefinedType) && ctx.gadt.bounds.contains(ref.symbol)) {
+      ref.copySymDenotation(info = ctx.gadt.bounds.apply(ref.symbol) & ref.info)
+    } else ref
+  }
+
   /** the following two members override abstract members in Transform */
   override def phaseName: String = "posttyper"
 

tests/pos/i2944.scala

@@ -0,0 +1,10 @@
+trait Map2[K] {
+  def get(k: K): K = k
+  def foo: K = {
+    this match {
+      case that: Map2[b] => that.get(3.asInstanceOf[b])
+      //case that: Map2[c] => that.get(4.asInstanceOf[K])
+      case _ => get(5.asInstanceOf[K])
+    }
+  }
+}