fix retyper in checking Select in patterns

Author: liufengyun

compiler/src/dotty/tools/dotc/typer/ReTyper.scala

@@ -35,7 +35,8 @@ class ReTyper extends Typer {
 
   override def typedSelect(tree: untpd.Select, pt: Type)(implicit ctx: Context): Tree = {
     assert(tree.hasType, tree)
-    val qual1 = typed(tree.qualifier, AnySelectionProto)
+    // a qualifier cannot be a pattern
+    val qual1 = typed(tree.qualifier, AnySelectionProto)(ctx.retractMode(Mode.Pattern))
     untpd.cpy.Select(tree)(qual1, tree.name).withType(tree.typeOpt)
   }
 

tests/run/i3812.scala

@@ -9,7 +9,6 @@ object Test {
     x match { case { "42".toInt }   => () } // ok
     x match { case { Y.toInt }      => () } // ok
     x match { case { Y }.toInt      => () } // ok
-
-    // x match { case Y.toInt          => () } // compiles but rejected by Ycheck
+    x match { case Y.toInt          => () } // ok
   }
 }