Fix #5827: Don't copy protected flag to apply method of case class

Author: odersky

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

@@ -629,9 +629,12 @@ object desugar {
           (creatorExpr /: widenDefs)((rhs, meth) => Apply(Ident(meth.name), rhs :: Nil))
         val applyMeths =
           if (mods is Abstract) Nil
-          else
-            DefDef(nme.apply, derivedTparams, derivedVparamss, applyResultTpt, widenedCreatorExpr)
-              .withFlags(Synthetic | constr1.mods.flags & (DefaultParameterized | copiedAccessFlags)) :: widenDefs
+          else {
+            val copiedFlagsMask = DefaultParameterized | (copiedAccessFlags & Private)
+            val app = DefDef(nme.apply, derivedTparams, derivedVparamss, applyResultTpt, widenedCreatorExpr)
+              .withFlags(Synthetic | constr1.mods.flags & copiedFlagsMask)
+            app :: widenDefs
+          }
         val unapplyMeth = {
           val unapplyParam = makeSyntheticParameter(tpt = classTypeRef)
           val unapplyRHS = if (arity == 0) Literal(Constant(true)) else Ident(unapplyParam.name)

tests/pos/i5827.scala

@@ -0,0 +1 @@
+case class Foo protected(a: Int)