Fix scala#1828: add warning when patternmatching on generics

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

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