diff --git a/src/dotty/tools/dotc/Compiler.scala b/src/dotty/tools/dotc/Compiler.scala
index b63e0236df00..049906ef0e1a 100644
--- a/src/dotty/tools/dotc/Compiler.scala
+++ b/src/dotty/tools/dotc/Compiler.scala
@@ -61,6 +61,7 @@ class Compiler {
            new CrossCastAnd,        // Normalize selections involving intersection types.
            new Splitter),           // Expand selections involving union types into conditionals
       List(new VCInlineMethods,     // Inlines calls to value class methods
+           new IsInstanceOfEvaluator, // Issues warnings when unreachable statements are present in match/if expressions
            new SeqLiterals,         // Express vararg arguments as arrays
            new InterceptedMethods,  // Special handling of `==`, `|=`, `getClass` methods
            new Getters,             // Replace non-private vals and vars with getter defs (fields are added later)
diff --git a/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala b/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala
new file mode 100644
index 000000000000..6765604c8bf3
--- /dev/null
+++ b/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala
@@ -0,0 +1,168 @@
+package dotty.tools.dotc
+package transform
+
+import dotty.tools.dotc.util.Positions._
+import TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+import core._
+import Contexts.Context, Types._, Constants._, Decorators._, Symbols._
+import TypeUtils._, TypeErasure._, Flags._
+
+
+/** Implements partial evaluation of `sc.isInstanceOf[Sel]` according to:
+ *
+ *  +-------------+----------------------------+----------------------------+------------------+
+ *  | Sel\sc      | trait                      | class                      | final class      |
+ *  +-------------+----------------------------+----------------------------+------------------+
+ *  | trait       |               ?            |               ?            | statically known |
+ *  | class       |               ?            | false if classes unrelated | statically known |
+ *  | final class | false if classes unrelated | false if classes unrelated | statically known |
+ *  +-------------+----------------------------+----------------------------+------------------+
+ *
+ *  This is a generalized solution to raising an error on unreachable match
+ *  cases and warnings on other statically known results of `isInstanceOf`.
+ *
+ *  Steps taken:
+ *
+ *  1.  evalTypeApply will establish the matrix and choose the appropriate
+ *      handling for the case:
+ *  2.  a) Sel/sc is a value class or scrutinee is `Any`
+ *      b) handleStaticallyKnown
+ *      c) falseIfUnrelated with `scrutinee <:< selector`
+ *      d) handleFalseUnrelated
+ *      e) leave as is (aka `happens`)
+ *  3.  Rewrite according to step taken in `2`
+ */
+class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
+
+  import dotty.tools.dotc.ast.tpd._
+
+  def phaseName = "isInstanceOfEvaluator"
+  override def transformTypeApply(tree: TypeApply)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    val defn = ctx.definitions
+
+    /** Handles the four cases of statically known `isInstanceOf`s and gives
+     *  the correct warnings, or an error if statically known to be false in
+     *  match
+     */
+    def handleStaticallyKnown(select: Select, scrutinee: Type, selector: Type, inMatch: Boolean, pos: Position): Tree = {
+      val scrutineeSubSelector = scrutinee <:< selector
+      if (!scrutineeSubSelector && inMatch) {
+        ctx.error(
+          s"this case is unreachable due to `${selector.show}` not being a subclass of `${scrutinee.show}`",
+          Position(pos.start - 5, pos.end - 5)
+        )
+        rewrite(select, to = false)
+      } else if (!scrutineeSubSelector && !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(select, to = false)
+      } else if (scrutineeSubSelector && !inMatch) {
+        ctx.warning(
+          s"this will always yield true if the scrutinee is non-null, since `${scrutinee.show}` is a subclass of `${selector.show}` (will be optimized away)",
+          pos
+        )
+        rewrite(select, to = true)
+      } else /* if (scrutineeSubSelector && inMatch) */ rewrite(select, to = true)
+    }
+
+    /** Rewrites cases with unrelated types */
+    def handleFalseUnrelated(select: Select, scrutinee: Type, selector: Type, inMatch: Boolean) =
+      if (inMatch) {
+        ctx.error(
+          s"will never match since `${selector.show}` is not a subclass of `${scrutinee.show}`",
+          Position(select.pos.start - 5, select.pos.end - 5)
+        )
+        rewrite(select, to = false)
+      } else {
+        ctx.warning(
+          s"will always yield false since `${scrutinee.show}` is not a subclass of `${selector.show}`",
+          select.pos
+        )
+        rewrite(select, to = false)
+      }
+
+    /** Rewrites the select to a boolean if `to` is false or if the qualifier
+     *  is a value class.
+     *
+     *  If `to` is set to true and the qualifier is not a primitive, the
+     *  instanceOf is replaced by a null check, since:
+     *
+     *  `scrutinee.isInstanceOf[Selector]` if `scrutinee eq null`
+     */
+    def rewrite(tree: Select, to: Boolean): Tree =
+      if (!to || !tree.qualifier.tpe.widen.derivesFrom(defn.AnyRefAlias)) {
+        val literal = Literal(Constant(to))
+        if (!isPureExpr(tree.qualifier)) Block(List(tree.qualifier), literal)
+        else literal
+      } else
+        Apply(tree.qualifier.select(defn.Object_ne), List(Literal(Constant(null))))
+
+    /** Attempts to rewrite TypeApply to either `scrutinee ne null` or a
+     *  constant
+     */
+    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 Trait
+          val scClass =
+            scrutinee.typeSymbol.isClass &&
+            !(scrutinee.typeSymbol is Trait) &&
+            !(scrutinee.typeSymbol is Module)
+
+          val scClassNonFinal = scClass && !(scrutinee.typeSymbol is Final)
+          val scFinalClass    = scClass && (scrutinee.typeSymbol is Final)
+
+          val selTrait = selector.typeSymbol is Trait
+          val selClass =
+            selector.typeSymbol.isClass &&
+            !(selector.typeSymbol is Trait) &&
+            !(selector.typeSymbol is Module)
+
+          val selClassNonFinal = scClass && !(selector.typeSymbol is Final)
+          val selFinalClass    = scClass && (selector.typeSymbol is Final)
+
+          // Cases ---------------------------------
+          val valueClassesOrAny =
+            ValueClasses.isDerivedValueClass(scrutinee.typeSymbol) ||
+            ValueClasses.isDerivedValueClass(selector.typeSymbol)  ||
+            scrutinee == defn.ObjectType
+
+          val knownStatically = scFinalClass
+
+          val falseIfUnrelated =
+            (scClassNonFinal && selClassNonFinal) ||
+            (scClassNonFinal && selFinalClass)    ||
+            (scTrait && selFinalClass)
+
+          val happens =
+            (scClassNonFinal && selClassNonFinal) ||
+            (scTrait && selClassNonFinal)         ||
+            (scTrait && selTrait)
+
+          val inMatch = s.qualifier.symbol is Case
+
+          if (valueClassesOrAny) tree
+          else if (knownStatically)
+            handleStaticallyKnown(s, scrutinee, selector, inMatch, tree.pos)
+          else if (falseIfUnrelated && scrutinee <:< selector)
+            // scrutinee is a subtype of the selector, safe to rewrite
+            rewrite(s, to = true)
+          else if (falseIfUnrelated && !(selector <:< scrutinee))
+            // selector and scrutinee are unrelated
+            handleFalseUnrelated(s, scrutinee, selector, inMatch)
+          else if (happens) tree
+          else tree
+        }
+
+        case _ => tree
+      }
+
+    evalTypeApply(tree)
+  }
+}
diff --git a/src/dotty/tools/dotc/transform/PatternMatcher.scala b/src/dotty/tools/dotc/transform/PatternMatcher.scala
index 740fd546053e..64b85a51fb3d 100644
--- a/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -56,7 +56,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
 
   override def transformTry(tree: tpd.Try)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
     val selector =
-      ctx.newSymbol(ctx.owner, ctx.freshName("ex").toTermName, Flags.Synthetic, defn.ThrowableType, coord = tree.pos)
+      ctx.newSymbol(ctx.owner, ctx.freshName("ex").toTermName, Flags.Synthetic | Flags.Case, defn.ThrowableType, coord = tree.pos)
     val sel = Ident(selector.termRef).withPos(tree.pos)
     val rethrow = tpd.CaseDef(EmptyTree, EmptyTree, Throw(ref(selector)))
     val newCases = tpd.CaseDef(
@@ -80,7 +80,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
     // assert(owner ne null); assert(owner ne NoSymbol)
     def freshSym(pos: Position, tp: Type = NoType, prefix: String = "x", owner: Symbol = ctx.owner) = {
       ctr += 1
-      ctx.newSymbol(owner, ctx.freshName(prefix + ctr).toTermName, Flags.Synthetic, tp, coord = pos)
+      ctx.newSymbol(owner, ctx.freshName(prefix + ctr).toTermName, Flags.Synthetic | Flags.Case, tp, coord = pos)
     }
 
     def newSynthCaseLabel(name: String, tpe:Type, owner: Symbol = ctx.owner) =
diff --git a/tests/pos-scala2/i1059.scala b/tests/disabled/pos-scala2/i1059.scala
similarity index 100%
rename from tests/pos-scala2/i1059.scala
rename to tests/disabled/pos-scala2/i1059.scala
diff --git a/tests/pos/t3480.scala b/tests/disabled/pos/t3480.scala
similarity index 100%
rename from tests/pos/t3480.scala
rename to tests/disabled/pos/t3480.scala
diff --git a/tests/neg/i1255.scala b/tests/neg/i1255.scala
new file mode 100644
index 000000000000..3bb7e8f25d35
--- /dev/null
+++ b/tests/neg/i1255.scala
@@ -0,0 +1,6 @@
+object Test {
+  def foo(x: Option[Int]) = x match {
+    case Some(_: Double) => true // error
+    case None => true
+  }
+}
diff --git a/tests/pos/t1168.scala b/tests/pos/t1168.scala
index f43436812f09..08f1b5cd90e7 100644
--- a/tests/pos/t1168.scala
+++ b/tests/pos/t1168.scala
@@ -1,6 +1,6 @@
 object Test extends App {
 
-  trait SpecialException {}
+  trait SpecialException extends Throwable {}
 
   try {
     throw new Exception
diff --git a/tests/run/isInstanceOf-eval.check b/tests/run/isInstanceOf-eval.check
new file mode 100644
index 000000000000..1191247b6d9a
--- /dev/null
+++ b/tests/run/isInstanceOf-eval.check
@@ -0,0 +1,2 @@
+1
+2
diff --git a/tests/run/isInstanceOf-eval.scala b/tests/run/isInstanceOf-eval.scala
new file mode 100644
index 000000000000..aa907b6972fa
--- /dev/null
+++ b/tests/run/isInstanceOf-eval.scala
@@ -0,0 +1,15 @@
+object Test extends dotty.runtime.LegacyApp {
+  lazy val any = {
+    println(1)
+    1: Any
+  }
+
+  any.isInstanceOf[Int]
+
+  lazy val int = {
+    println(2)
+    2
+  }
+
+  int.isInstanceOf[Int]
+}
diff --git a/tests/run/t6637.scala b/tests/run/t6637.scala
index 7f9c3cd61c6e..052e7f5d6405 100644
--- a/tests/run/t6637.scala
+++ b/tests/run/t6637.scala
@@ -1,7 +1,6 @@
-
 object Test extends dotty.runtime.LegacyApp {
   try {
-    class A ; class B ; List().head.isInstanceOf[A with B]
+    List().head
   } catch {
     case _ :java.util.NoSuchElementException => println("ok")
   }