Add initial partial evaluation of isInstanceOf calls

1 files changed, 89 insertions, 0 deletions

diff --git a/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala b/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala
new file mode 100644
index 000000000..d4483bb18
--- /dev/null
+++ b/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala
@@ -0,0 +1,89 @@
+package dotty.tools.dotc
+package transform
+
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.Types._
+import dotty.tools.dotc.core.Decorators._
+import dotty.tools.dotc.core.Flags
+import dotty.tools.dotc.core.Symbols._
+import dotty.tools.dotc.util.Positions._
+import dotty.tools.dotc.core.TypeErasure._
+import TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+
+class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
+
+  import dotty.tools.dotc.ast.tpd._
+
+  def phaseName = "reachabilityChecker"
+
+  override def transformTypeApply(tree: TypeApply)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    val defn = ctx.definitions
+
+    def handleStaticallyKnown(tree: TypeApply, scrutinee: Type, selector: Type, inMatch: Boolean, pos: Position): Tree =
+      if (!(scrutinee <:< selector) && inMatch) {
+        ctx.error(s"this case is unreachable due to `${selector.show}` not being a subclass of `${scrutinee.show}`", pos)
+        tree
+      } else if (!(scrutinee <:< selector) && !inMatch) {
+        ctx.warning(s"this will always yield false since `${scrutinee.show}` is not a subclass of `${selector.show}` (will be optimized away)", pos)
+        rewrite(tree, to = false)
+      } else if (scrutinee <:< selector && !inMatch) {
+        ctx.warning(s"this will always yield true since `${scrutinee.show}` is a subclass of `${selector.show}` (will be optimized away)", pos)
+        rewrite(tree, to = true)
+      } else /* if (scrutinee <:< selector && inMatch) */ rewrite(tree, to = true)
+
+    def rewrite(tree: TypeApply, to: Boolean): Tree = tree
+
+    def evalTypeApply(tree: TypeApply): Tree =
+      if (tree.symbol != defn.Any_isInstanceOf) tree
+      else tree.fun match {
+        case s: Select => {
+          val scrutinee = erasure(s.qualifier.tpe.widen)
+          val selector  = erasure(tree.args.head.tpe.widen)
+
+          val scTrait = scrutinee.typeSymbol is Flags.Trait
+          val scClass =
+            scrutinee.typeSymbol.isClass &&
+            !(scrutinee.typeSymbol is Flags.Trait) &&
+            !(scrutinee.typeSymbol is Flags.Module)
+
+          val scClassNonFinal = scClass && !scrutinee.typeSymbol.is(Flags.Final)
+          val scFinalClass    = scClass && (scrutinee.typeSymbol is Flags.Final)
+
+          val selTrait = selector.typeSymbol is Flags.Trait
+          val selClass =
+            selector.typeSymbol.isClass &&
+            !(selector.typeSymbol is Flags.Trait) &&
+            !(selector.typeSymbol is Flags.Module)
+
+          val selClassNonFinal = scClass && !(selector.typeSymbol is Flags.Final)
+          val selFinalClass    = scClass && (selector.typeSymbol is Flags.Final)
+
+          // Cases ---------------------------------
+          val knownStatically = scFinalClass
+
+          val falseIfUnrelated =
+            (scClassNonFinal && selClassNonFinal) ||
+            (scClassNonFinal && selFinalClass)    ||
+            (scTrait && selFinalClass)
+
+          // Doesn't need to be calculated (since true if others are false)
+          //val happens =
+          //  (scClassNonFinal && selClassNonFinal) ||
+          //  (scTrait && selClassNonFinal)         ||
+          //  (scTrait && selTrait)
+
+          val inMatch = s.qualifier.symbol is Flags.Case
+
+          if (knownStatically)
+            handleStaticallyKnown(tree, scrutinee, selector, inMatch, tree.pos)
+          else if (falseIfUnrelated && !(selector <:< scrutinee))
+            rewrite(tree, to = false)
+          else /*if (happens)*/ tree
+        }
+
+        case _ => tree
+      }
+
+    evalTypeApply(tree)
+  }
+}