This outlaws explicit isInstanceOf tests on val...

This outlaws explicit isInstanceOf tests on value types, which are
nonsense in any case, as long threatened in ticket #1872. Closes #1872,
review by rytz.

3 files changed, 13 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Erasure.scala b/src/compiler/scala/tools/nsc/transform/Erasure.scala
index fa5f899da0..5de5bd9a99 100644
--- a/src/compiler/scala/tools/nsc/transform/Erasure.scala
+++ b/src/compiler/scala/tools/nsc/transform/Erasure.scala
@@ -983,9 +983,12 @@ abstract class Erasure extends AddInterfaces with typechecker.Analyzer with ast.
                   tree
             }
             // todo: also handle the case where the singleton type is buried in a compound
-          else if (fn.symbol == Any_isInstanceOf)
+          else if (fn.symbol == Any_isInstanceOf) {
             fn match {
               case TypeApply(sel @ Select(qual, name), List(targ)) =>
+                if (qual.tpe != null && isValueClass(qual.tpe.typeSymbol) && targ.tpe != null && targ.tpe <:< AnyRefClass.tpe)
+                  unit.error(sel.pos, "isInstanceOf cannot test if value types are references.")
+
                 def mkIsInstanceOf(q: () => Tree)(tp: Type): Tree =
                   Apply(
                     TypeApply(
@@ -1009,6 +1012,7 @@ abstract class Erasure extends AddInterfaces with typechecker.Analyzer with ast.
                 }
               case _ => tree
             }
+          }
           else {
             def doDynamic(fn: Tree, qual: Tree): Tree = {
               if (fn.symbol.owner.isRefinementClass && fn.symbol.allOverriddenSymbols.isEmpty)
diff --git a/test/files/neg/bug1872.check b/test/files/neg/bug1872.check
new file mode 100644
index 0000000000..9f1af33056
--- /dev/null
+++ b/test/files/neg/bug1872.check
@@ -0,0 +1,4 @@
+bug1872.scala:3: error: isInstanceOf cannot test if value types are references.
+  def f(x: Int) = x.isInstanceOf[util.Random]
+                    ^
+one error found
diff --git a/test/files/neg/bug1872.scala b/test/files/neg/bug1872.scala
new file mode 100644
index 0000000000..0ebee0b32d
--- /dev/null
+++ b/test/files/neg/bug1872.scala
@@ -0,0 +1,4 @@
+class A {
+  // a true result here would necessitate profound soul searching
+  def f(x: Int) = x.isInstanceOf[util.Random]
+}
\ No newline at end of file