SI-7763 Avoid dropping casts in erasure

466b7d29f avoided quadratic complexity in Erasure's treatment
of chained `asInstanceOf` calls. It did so by using the typechecked
qualifier, rather than discarding it.

However, that also dropped the cast altogether! In many cases this
was masked by later inclusion of a cast to the expected type
by `adaptToType`:

	  at scala.tools.nsc.transform.Erasure$Eraser.cast(Erasure.scala:636)
	  at scala.tools.nsc.transform.Erasure$Eraser.scala$tools$nsc$transform$Erasure$Eraser$$adaptToType(Erasure.scala:665)
	  at scala.tools.nsc.transform.Erasure$Eraser.adapt(Erasure.scala:766)
	  at scala.tools.nsc.typechecker.Typers$Typer.runTyper$1(Typers.scala:5352)

This commit re-wraps the typechecked `qual` in its original
`<qual>.asInstanceOf[T]` to preserve semantics while avoiding
the big-O blowup.

The test includes the compiler option `-Ynooptimize` because dead code
elimination *also* thinks that this cast is superfluous. I'll follow up
on that problem seprately.

2 files changed, 21 insertions, 0 deletions

diff --git a/test/files/run/t7763.flags b/test/files/run/t7763.flags
new file mode 100644
index 0000000000..7f4215d1bc
--- /dev/null
+++ b/test/files/run/t7763.flags
@@ -0,0 +1 @@
+-Ynooptimize
\ No newline at end of file
diff --git a/test/files/run/t7763.scala b/test/files/run/t7763.scala
new file mode 100644
index 0000000000..638077e64a
--- /dev/null
+++ b/test/files/run/t7763.scala
@@ -0,0 +1,20 @@
+object Test {
+  class A; class B
+  def main(args: Array[String]) {
+    def noExpectedType() {
+      a().asInstanceOf[B] // cast elided!
+    }
+    def withExpectedType(): B = {
+      a().asInstanceOf[B]
+    }
+    def test(a: => Any) = try {
+      a
+      sys.error("no CCE!")
+    } catch {case _: ClassCastException => }
+
+    test(noExpectedType())
+    test(withExpectedType())
+  }
+
+  def a(): Object = new A
+}