Retain HasDefaultParams flag on export.

Fixes #14020

Author: odersky

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -531,7 +531,7 @@ object Flags {
   val RetainedModuleClassFlags: FlagSet = RetainedModuleValAndClassFlags | Enum
 
   /** Flags retained in export forwarders */
-  val RetainedExportFlags = Given | Implicit | Inline | Transparent
+  val RetainedExportFlags = Given | Implicit | Inline | Transparent | HasDefaultParams | NoDefaultParams
 
   /** Flags that apply only to classes */
   val ClassOnlyFlags = Sealed | Open | Abstract.toTypeFlags

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

@@ -1116,6 +1116,7 @@ class Namer { typer: Typer =>
                   (StableRealizable, ExprType(path.tpe.select(sym)))
                 else
                   (EmptyFlags, mbr.info.ensureMethodic)
+              sym.hasDefaultParams // ensure HasDefaultParams and NoDefaultParams flags are set
               var mbrFlags = Exported | Method | Final | maybeStable | sym.flags & RetainedExportFlags
               if sym.is(ExtensionMethod) then mbrFlags |= ExtensionMethod
               val forwarderName = checkNoConflict(alias, isPrivate = false, span)

tests/run/i14020.check

@@ -0,0 +1,3 @@
+Hello you
+Hello John
+Hello you

tests/run/i14020.scala

@@ -0,0 +1,13 @@
+class A:
+  def greeting(name: String = "you") = s"Hello $name"
+
+class B:
+  val a = A()
+  export a.*
+
+@main def Test =
+  val b = B()
+
+  println(b.a.greeting())      // works
+  println(b.greeting("John"))  // works
+  println(b.greeting())        // nope !