Implement constraint merging

Not used yet, but we might use it as an alternative to typedArg invalidation later.

Author: odersky

src/dotty/tools/dotc/core/Constraint.scala

@@ -143,6 +143,9 @@ abstract class Constraint extends Showable {
   /** The uninstantiated typevars of this constraint */
   def uninstVars: collection.Seq[TypeVar]
 
+  /** The weakest constraint that subsumes both this constraint and `other` */
+  def & (other: Constraint)(implicit ctx: Context): Constraint
+
   /** Check that no constrained parameter contains itself as a bound */
   def checkNonCyclic()(implicit ctx: Context): Unit
 

src/dotty/tools/dotc/core/OrderingConstraint.scala

@@ -15,11 +15,13 @@ import annotation.tailrec
 
 object OrderingConstraint {
 
+  type ArrayValuedMap[T] = SimpleMap[GenericType, Array[T]]
+
   /** The type of `OrderingConstraint#boundsMap` */
-  type ParamBounds = SimpleMap[GenericType, Array[Type]]
+  type ParamBounds = ArrayValuedMap[Type]
 
   /** The type of `OrderingConstraint#lowerMap`, `OrderingConstraint#upperMap` */
-  type ParamOrdering = SimpleMap[GenericType, Array[List[PolyParam]]]
+  type ParamOrdering = ArrayValuedMap[List[PolyParam]]
 
   /** A new constraint with given maps */
   private def newConstraint(boundsMap: ParamBounds, lowerMap: ParamOrdering, upperMap: ParamOrdering)(implicit ctx: Context) : OrderingConstraint = {
@@ -495,6 +497,44 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
       }
     }
 
+  def & (other: Constraint)(implicit ctx: Context) = {
+    def merge[T](m1: ArrayValuedMap[T], m2: ArrayValuedMap[T], join: (T, T) => T): ArrayValuedMap[T] = {
+      var merged = m1
+      def mergeArrays(xs1: Array[T], xs2: Array[T]) = {
+        val xs = xs1.clone
+        for (i <- xs.indices) xs(i) = join(xs1(i), xs2(i))
+        xs
+      }
+      m2.foreachBinding { (poly, xs2) =>
+        merged = merged.updated(poly,
+            if (m1.contains(poly)) mergeArrays(m1(poly), xs2) else xs2)
+      }
+      merged
+    }
+
+    def mergeParams(ps1: List[PolyParam], ps2: List[PolyParam]) =
+      (ps1 /: ps2)((ps1, p2) => if (ps1.contains(p2)) ps1 else p2 :: ps1)
+
+    def mergeEntries(e1: Type, e2: Type): Type = e1 match {
+        case e1: TypeBounds =>
+          e2 match {
+            case e2: TypeBounds => e1 & e2
+            case _ if e1 contains e2 => e2
+            case _ => mergeError
+          }
+        case _ if e1 eq e2 => e1
+        case _ => mergeError
+    }
+
+    def mergeError = throw new AssertionError(i"cannot merge $this with $other")
+
+    val that = other.asInstanceOf[OrderingConstraint]
+    new OrderingConstraint(
+        merge(this.boundsMap, that.boundsMap, mergeEntries),
+        merge(this.lowerMap, that.lowerMap, mergeParams),
+        merge(this.upperMap, that.upperMap, mergeParams))
+  }
+
   override def checkClosed()(implicit ctx: Context): Unit = {
     def isFreePolyParam(tp: Type) = tp match {
       case PolyParam(binder: GenericType, _) => !contains(binder)

src/dotty/tools/dotc/core/TyperState.scala

@@ -128,14 +128,14 @@ extends TyperState(r) {
    */
   override def commit()(implicit ctx: Context) = {
     val targetState = ctx.typerState
-    assert(targetState eq previous)
     assert(isCommittable)
-    targetState.constraint = constraint
+    if (targetState eq previous) targetState.constraint = constraint
+    else targetState.constraint &= constraint
     constraint foreachTypeVar { tvar =>
       if (tvar.owningState eq this)
         tvar.owningState = targetState
     }
-    targetState.ephemeral = ephemeral
+    targetState.ephemeral |= ephemeral
     targetState.gc()
     reporter.flush()
     myIsCommitted = true