Warn about surprising shadowing.

It's hidden behind -Xlint and pretty specific, but makes me feel better
anyway. References SI-4762, no review.

3 files changed, 59 insertions, 0 deletions

diff --git a/test/files/neg/t4762.check b/test/files/neg/t4762.check
new file mode 100644
index 0000000000..5e67f2022a
--- /dev/null
+++ b/test/files/neg/t4762.check
@@ -0,0 +1,7 @@
+t4762.scala:15: error: private[this] value x in class B shadows mutable x inherited from class A.  Changes to x will not be visible within class B - you may want to give them distinct names.
+    /* (99,99) */  (this.x, this.y),
+                         ^
+t4762.scala:48: error: private[this] value x in class Derived shadows mutable x inherited from class Base.  Changes to x will not be visible within class Derived - you may want to give them distinct names.
+  class Derived( x : Int ) extends Base( x ) { override def toString = x.toString }
+                                                                       ^
+two errors found
diff --git a/test/files/neg/t4762.flags b/test/files/neg/t4762.flags
new file mode 100644
index 0000000000..e93641e931
--- /dev/null
+++ b/test/files/neg/t4762.flags
@@ -0,0 +1 @@
+-Xlint -Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/t4762.scala b/test/files/neg/t4762.scala
new file mode 100644
index 0000000000..8757f4a103
--- /dev/null
+++ b/test/files/neg/t4762.scala
@@ -0,0 +1,51 @@
+// https://issues.scala-lang.org/browse/SI-4762
+
+// In A, x and y are -1.
+class A(var x: Int) {
+  val y: Int = -1
+}
+
+// In B, x and y are 99 and private[this], implicitly so
+// in the case of x.
+class B(x: Int) extends A(-1) {
+  private[this] def y: Int = 99
+
+  // Three distinct results.
+  def f = List(
+    /* (99,99) */  (this.x, this.y),
+    /* (-1,99) */  ((this: B).x, (this: B).y),
+    /* (-1,-1) */  ((this: A).x, (this: A).y)
+  )
+
+  // The 99s tell us we are reading the private[this]
+  // data of a different instance.
+  def g(b: B) = List(
+    /* (-1,99) */  (b.x, b.y),
+    /* (-1,99) */  ((b: B).x, (b: B).y),
+    /* (-1,-1) */  ((b: A).x, (b: A).y)
+  )
+}
+
+object Test {
+  def f(x: A)  = /* -2 */  x.x + x.y
+  def g1(x: B) = /* -2 */  (x: A).x + (x: A).y
+  def g2(x: B) = (x: B).x + (x: B).y
+  // java.lang.IllegalAccessError: tried to access method B.y()I from class Test$
+
+  def main(args: Array[String]): Unit = {
+    val b = new B(99)
+    b.f foreach println
+    b.g(new B(99)) foreach println
+
+    println(f(b))
+    println(g1(b))
+    println(g2(b))
+  }
+}
+
+class bug4762 {
+  class Base( var x : Int ) { def increment() { x = x + 1 } }
+  class Derived( x : Int ) extends Base( x ) { override def toString = x.toString }
+
+  val derived = new Derived( 1 )
+}