Optionally check variance of Lambda classes.

Make sure that Lambda Arg refinements have the same variance as the Lambda classes
they instantiate. Controlled by a Config parameter.

Author: odersky

src/dotty/tools/dotc/config/Config.scala

@@ -27,7 +27,7 @@ object Config {
   /** Show subtype traces for all deep subtype recursions */
   final val traceDeepSubTypeRecursions = false
 
-  final val verboseExplainSubtype = true
+  final val verboseExplainSubtype = false
 
   /** When set, use new signature-based matching.
    *  Advantantage of doing so: It's supposed to be faster
@@ -43,4 +43,9 @@ object Config {
    *  for large constraints.
    */
   final val trackConstrDeps = true
+
+  /** Check that variances of lambda arguments match the
+   *  variance of the underlying lambda class.
+   */
+  final val checkLambdaVariance = false
 }

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

@@ -1420,6 +1420,19 @@ object Types {
 
     override def underlying(implicit ctx: Context) = parent
 
+    private def checkInst(implicit ctx: Context): this.type = {
+      if (Config.checkLambdaVariance)
+        refinedInfo match {
+          case refinedInfo: TypeBounds if refinedInfo.variance != 0 && refinedName.isLambdaArgName =>
+            val cls = parent.LambdaClass(forcing = false)
+            if (cls.exists)
+              assert(refinedInfo.variance == cls.typeParams.apply(refinedName.lambdaArgIndex).variance,
+                  s"variance mismatch for $this, $cls, ${cls.typeParams}, ${cls.typeParams.apply(refinedName.lambdaArgIndex).variance}, ${refinedInfo.variance}")
+          case _ =>
+        }
+      this
+    }
+
     /** Derived refined type, with a twist: A refinement with a higher-kinded type param placeholder
      *  is transformed to a refinement of the original type parameter if that one exists.
      */
@@ -1476,10 +1489,10 @@ object Types {
       else make(RefinedType(parent, names.head, infoFns.head), names.tail, infoFns.tail)
 
     def apply(parent: Type, name: Name, infoFn: RefinedType => Type)(implicit ctx: Context): RefinedType =
-      ctx.base.uniqueRefinedTypes.enterIfNew(new CachedRefinedType(parent, name, infoFn))
+      ctx.base.uniqueRefinedTypes.enterIfNew(new CachedRefinedType(parent, name, infoFn)).checkInst
 
     def apply(parent: Type, name: Name, info: Type)(implicit ctx: Context): RefinedType = {
-      ctx.base.uniqueRefinedTypes.enterIfNew(parent, name, info)
+      ctx.base.uniqueRefinedTypes.enterIfNew(parent, name, info).checkInst
     }
   }