Be more careful with inserted casts.

See comment on the commit for an explanation.

Author: odersky

src/dotty/tools/dotc/typer/Typer.scala

@@ -1689,8 +1689,15 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           tree
         }
         else if (tree.tpe <:< pt)
-          if (ctx.typeComparer.GADTused) tree.asInstance(pt)
-          else tree
+          if (ctx.typeComparer.GADTused && pt.isValueType && ctx.settings.Ycheck.value.nonEmpty)
+            // Insert an explicit cast, so that -Ycheck in later phases succeeds.
+            // I suspect, but am not 100% sure that this might affect inferred types,
+            // if the expected type is a supertype of the GADT bound. It would be good to come
+            // up with a test case for this. For that reason, to be on the safe side
+            // we only insert the cast if there are Ychecks later on.
+            tree.asInstance(pt)
+          else
+            tree
         else if (wtp.isInstanceOf[MethodType]) missingArgs
         else {
           typr.println(i"adapt to subtype ${tree.tpe} !<:< $pt")