1 files changed, 48 insertions, 0 deletions
diff --git a/src/dotty/tools/dotc/core/TypeOps.scala b/src/dotty/tools/dotc/core/TypeOps.scala
index 272e93dd3..6070b4f49 100644
--- a/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/src/dotty/tools/dotc/core/TypeOps.scala
@@ -113,6 +113,54 @@ trait TypeOps { this: Context =>
     }
   }
 
+  type VarianceMap = SimpleMap[TypeVar, Integer]
+
+  /** Add occurrences of type variables in type `tp` to variance map `vmap` */
+  final def addVariances(vmap: VarianceMap, tp: Type, variance: Int, include: TypeVar => Boolean, accu: VariancesAccumulator): VarianceMap = tp match {
+    case tp: TypeRef =>
+      if (tp.symbol.isStatic) vmap
+      else {
+        val prefix = tp.prefix
+        val tp1 = tp.lookupRefined(prefix, tp.name)
+        addVariances(vmap, if (tp1.exists) tp1 else prefix, variance, include, accu)
+      }
+    case tp: TermRef =>
+      if (tp.symbol.isStatic) vmap
+      else addVariances(vmap, tp.prefix, variance, include, accu)
+
+    case _: ThisType
+      | _: BoundType
+      | NoPrefix => vmap
+
+    case tp: RefinedType =>
+      addVariances(
+        addVariances(vmap, tp.parent, variance, include, accu),
+        tp.refinedInfo, variance, include, accu)
+
+    case bounds @ TypeBounds(lo, hi) if lo eq hi =>
+      addVariances(vmap, lo, variance * bounds.variance, include, accu)
+
+    case tp: TypeVar if !tp.isInstantiated && (typerState.constraint contains tp) && include(tp) =>
+      val v = vmap(tp)
+      if (v == null) vmap.updated(tp, variance)
+      else if (v == variance) vmap
+      else vmap.updated(tp, 0)
+
+    case _ =>
+      (if (accu != null) accu else new VariancesAccumulator(include)).runOver(vmap, tp, variance)
+  }
+
+  class VariancesAccumulator(include: TypeVar => Boolean) extends TypeAccumulator[VarianceMap] {
+    def apply(vmap: VarianceMap, tp: Type): VarianceMap = addVariances(vmap, tp, variance, include, this)
+    def runOver(vmap: VarianceMap, tp: Type, variance: Int): VarianceMap = {
+      val saved = this.variance
+      this.variance = variance
+      val result = foldOver(vmap, tp)
+      this.variance = saved
+      result
+    }
+  }
+
   private def enterArgBinding(formal: Symbol, info: Type, cls: ClassSymbol, decls: Scope) = {
     val lazyInfo = new LazyType { // needed so we do not force `formal`.
       def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {