Change rules for exports to make them more useful for enums

Two changes:

 - The forwarder alising a value definition gets a singleton type referring
   to the alias
 - Synthetic members are not exported by a wildcard export

This means we can now safely write
```
  enum MatchDegree {
    case NoMatch, ParamMatch, FullMatch
  }
  export MatchDegree._
```
and export just the three enum values with the most precise types. (precision
is necessary for exhaustiveness checks, among others).

Author: odersky

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

@@ -976,12 +976,11 @@ class Namer { typer: Typer =>
                   fwdInfo(path.tpe.select(mbr.symbol), mbr.info),
                   coord = span)
               else {
-                val maybeStable = if (mbr.symbol.isStableMember) StableRealizable else EmptyFlags
-                ctx.newSymbol(
-                  cls, alias,
-                  Exported | Method | Final | maybeStable | mbr.symbol.flags & RetainedExportFlags,
-                  mbr.info.ensureMethodic,
-                  coord = span)
+                val (maybeStable, mbrInfo) =
+                  if (mbr.symbol.isStableMember) (StableRealizable, ExprType(path.tpe.select(mbr.symbol)))
+                  else (EmptyFlags, mbr.info.ensureMethodic)
+                val mbrFlags = Exported | Method | Final | maybeStable | mbr.symbol.flags & RetainedExportFlags
+                ctx.newSymbol(cls, alias, mbrFlags, mbrInfo, coord = span)
               }
             val forwarderDef =
               if (forwarder.isType) tpd.TypeDef(forwarder.asType)
@@ -1010,7 +1009,8 @@ class Namer { typer: Typer =>
         }
 
         def addForwardersExcept(seen: List[TermName], span: Span): Unit =
-          for (mbr <- path.tpe.allMembers) {
+          for (mbr <- path.tpe.membersBasedOnFlags(
+              required = EmptyFlags, excluded = PrivateOrSynthetic)) {
             val alias = mbr.name.toTermName
             if (!seen.contains(alias)) addForwarder(alias, mbr, span)
           }

docs/docs/reference/enums/desugarEnums.md

@@ -157,7 +157,7 @@ map into case classes or vals.
 
 Non-generic enums `E` that define one or more singleton cases
 are called _enumerations_. Companion objects of enumerations define
-the following additional members.
+the following additional synthetic members.
 
    - A method `valueOf(name: String): E`. It returns the singleton case value whose
      `toString` representation is `name`.

docs/docs/reference/other-new-features/export.md

@@ -54,7 +54,7 @@ of one of the following forms:
  - An _omitting selector_ `x => _` prevents `x` from being aliased by a subsequent
    wildcard selector.
  - A _wildcard selector_ creates aliases for all eligible members of `path` except for
-   those members that are named by a previous simple, renaming, or omitting selector.
+   synthetic members generated by the compiler and those members that are named by a previous simple, renaming, or omitting selector.
 
 A member is _eligible_ if all of the following holds:
 
@@ -75,7 +75,7 @@ Export aliases are always `final`. Aliases of delegates are again defines as del
    not marked `override`.
  - However, export aliases can implement deferred members of base classes.
 
-Export aliases for value definitions are marked by the compiler as "stable". This means
+Export aliases for value definitions are marked by the compiler as "stable" and their result types are the singleton types of the aliased definitions. This means
 that they can be used as parts of stable identifier paths, even though they are technically methods. For instance, the following is OK:
 ```scala
   class C { type T }