Fixed #264 - failure to typecheck GADTs

The previous scheme derived the right bounds, but then failed to use them
because a TypeRef already has a set info (its bounds). Changing the bounds in
the symbol by a side effect does not affect that. This is good! But it showed
that the previous scheme was too fragile because it used a sneaky side effect
when updating the symbol info which failed to propgate into the cached
info in TypeRef.

We now keep GADT computed bounds separate form the symbol info
in a map `gadt` in the current context.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/dotty/tools/dotc/core/Contexts.scala b/src/dotty/tools/dotc/core/Contexts.scala
index 805b7660c..2477fd4a4 100644
--- a/src/dotty/tools/dotc/core/Contexts.scala
+++ b/src/dotty/tools/dotc/core/Contexts.scala
@@ -146,6 +146,11 @@ object Contexts {
     protected def diagnostics_=(diagnostics: Option[StringBuilder]) = _diagnostics = diagnostics
     def diagnostics: Option[StringBuilder] = _diagnostics
 
+    /** The current bounds in force for type parameters appearing in a GADT */
+    private var _gadt: GADTMap = _
+    protected def gadt_=(gadt: GADTMap) = _gadt = gadt
+    def gadt: GADTMap = _gadt
+
     /** A map in which more contextual properties can be stored */
     private var _moreProperties: Map[String, Any] = _
     protected def moreProperties_=(moreProperties: Map[String, Any]) = _moreProperties = moreProperties
@@ -418,6 +423,8 @@ object Contexts {
     def setSetting[T](setting: Setting[T], value: T): this.type =
       setSettings(setting.updateIn(sstate, value))
 
+    def setFreshGADTBounds: this.type = { this.gadt = new GADTMap(gadt.bounds); this }
+
     def setDebug = setSetting(base.settings.debug, true)
   }
 
@@ -439,6 +446,7 @@ object Contexts {
     moreProperties = Map.empty
     typeComparer = new TypeComparer(this)
     searchHistory = new SearchHistory(0, Map())
+    gadt = new GADTMap(SimpleMap.Empty)
   }
 
   object NoContext extends Context {
@@ -593,6 +601,8 @@ object Contexts {
     implicit val ctx: Context = initctx
   }
 
+  class GADTMap(var bounds: SimpleMap[Symbol, TypeBounds])
+
   /** Initial size of superId table */
   private final val InitialSuperIdsSize = 4096