Don't copy implicit flags from getter to setter

Author: odersky

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

@@ -158,7 +158,7 @@ object desugar {
         vparamss = (setterParam :: Nil) :: Nil,
         tpt      = TypeTree(defn.UnitType),
         rhs      = setterRhs
-      ).withMods((mods | Accessor) &~ CaseAccessor)
+      ).withMods((mods | Accessor) &~ (CaseAccessor | Implicit | Given))
       Thicket(vdef, setter)
     }
     else vdef

tests/pos-special/fatal-warnings/i6290.scala

@@ -0,0 +1,15 @@
+class TC { type T }
+
+class C given (TC { type T = Int })
+
+def f1 given (x: TC) = ???
+def f2 given (@unchecked x: TC) = ???
+inline def f3 given (inline x: TC) = ???
+
+class C1 given (x: TC)
+class C2 given (@unchecked x: TC)
+class C3 given (val x: TC)
+class C4 given (var x: TC)
+class C5 given (private val x: TC)
+class C6 given (private[this] val x: TC)
+