Allow implementing deferred members by export aliases

So far, this gave a "member already exists" error.
Also, fix info of type export aliases.

Author: odersky

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

@@ -949,7 +949,8 @@ class Namer { typer: Typer =>
           if (sym.is(ImplicitOrImplied) != exp.impliedOnly) s"is ${if (exp.impliedOnly) "not " else ""}implied"
           else if (!sym.isAccessibleFrom(path.tpe)) "is not accessible"
           else if (sym.isConstructor || sym.is(ModuleClass) || sym.is(Bridge)) "_"
-          else if (cls.derivesFrom(sym.owner)) i"is already a member of $cls"
+          else if (cls.derivesFrom(sym.owner) &&
+                   (sym.owner == cls || !sym.is(Deferred))) i"is already a member of $cls"
           else ""
         }
 
@@ -965,7 +966,7 @@ class Namer { typer: Typer =>
               case _: ClassInfo =>
                 HKTypeLambda.fromParams(info.typeParams, ref)
               case _: TypeBounds =>
-                ref
+                TypeAlias(ref)
               case info: HKTypeLambda =>
                 info.derivedLambdaType(info.paramNames, info.paramInfos,
                   fwdInfo(ref.appliedTo(info.paramRefs), info.resultType))

tests/pos/export-proxies.scala

@@ -0,0 +1,15 @@
+trait Session{
+  def call1(): Unit
+  def call2(): Unit
+  def call3(): Unit
+  type T
+}
+
+class SessionProxy(val session:Session) extends Session {
+  export session.{call2, call3, T}
+
+  def call1(): Unit = {
+    println("call1")
+    session.call1()
+  }
+}