From 758db16649ef569122354f44f10c85303e6f268a Mon Sep 17 00:00:00 2001
From: Felix Mulder <felix.mulder@gmail.com>
Date: Mon, 19 Dec 2016 15:22:32 +0100
Subject: [PATCH] Fix #1828: add warning when patternmatching on generics

---
 .../dotc/transform/IsInstanceOfEvaluator.scala    |  7 +++++--
 tests/repl/errmsgs.check                          | 15 +++++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala b/compiler/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala
index 8bc4a2aa91ea..35de182676cc 100644
--- a/compiler/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala
+++ b/compiler/src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala
@@ -147,8 +147,11 @@ class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
 
           val inMatch = s.qualifier.symbol is Case
 
-          if (valueClassesOrAny) tree
-          else if (knownStatically)
+          if (valueClassesOrAny) {
+            if (selector eq defn.ObjectType)
+              ctx.warning(i"abstract type pattern is unchecked since it is eliminated by erasure", tree.pos)
+            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
diff --git a/tests/repl/errmsgs.check b/tests/repl/errmsgs.check
index f0ccdf53f675..0199bb369ff4 100644
--- a/tests/repl/errmsgs.check
+++ b/tests/repl/errmsgs.check
@@ -78,4 +78,19 @@ scala> class Foo() { def bar: Int = 1 }; val foo = new Foo(); foo.barr
 4 |class Foo() { def bar: Int = 1 }; val foo = new Foo(); foo.barr
   |                                                       ^^^^^^^^
   |        value `barr` is not a member of Foo(foo) - did you mean `foo.bar`?
+scala> def unsafeCast[S](a: Any) = a match { case s: S => s; case _ => ??? }
+-- Warning: <console> ----------------------------------------------------------
+4 |def unsafeCast[S](a: Any) = a match { case s: S => s; case _ => ??? }
+  |                                                ^
+  |      abstract type pattern is unchecked since it is eliminated by erasure
+def unsafeCast[S](a: Any): [S] => (a: Any)S
+scala> unsafeCast[String](1)
+java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
+	at .<init>(<console>:6)
+	at .<clinit>(<console>)
+	at RequestResult$.<init>(<console>:3)
+	at RequestResult$.<clinit>(<console>)
+	at RequestResult$result(<console>)
+	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
+	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.jav...
 scala> :quit