[backport] Fix SI-6637 (misoptimization in erasure)

commit f9ef5300ab561628e53c654df9000c75f488d74a
Author: Jan Niehusmann <jan@gondor.com>
Date:   Fri Nov 9 15:05:58 2012 +0100

    Fix SI-6637 (misoptimization in erasure)

    Move the optimization one level deeper so the expression
    being tested with isInstanceOf is always evaluated.
    (cherry picked from commit b540aae)

src/compiler/scala/tools/nsc/transform/Erasure.scala

@@ -1057,17 +1057,17 @@ abstract class Erasure extends AddInterfaces
                     Apply(Select(qual, cmpOp), List(gen.mkAttributedQualifier(targ.tpe)))
                   }
                 case RefinedType(parents, decls) if (parents.length >= 2) =>
-                  // Optimization: don't generate isInstanceOf tests if the static type
-                  // conforms, because it always succeeds.  (Or at least it had better.)
-                  // At this writing the pattern matcher generates some instance tests
-                  // involving intersections where at least one parent is statically known true.
-                  // That needs fixing, but filtering the parents here adds an additional
-                  // level of robustness (in addition to the short term fix.)
-                  val parentTests = parents filterNot (qual.tpe <:< _)
-
-                  if (parentTests.isEmpty) Literal(Constant(true))
-                  else gen.evalOnce(qual, currentOwner, unit) { q =>
-                    atPos(tree.pos) {
+                  gen.evalOnce(qual, currentOwner, unit) { q =>
+                    // Optimization: don't generate isInstanceOf tests if the static type
+                    // conforms, because it always succeeds.  (Or at least it had better.)
+                    // At this writing the pattern matcher generates some instance tests
+                    // involving intersections where at least one parent is statically known true.
+                    // That needs fixing, but filtering the parents here adds an additional
+                    // level of robustness (in addition to the short term fix.)
+                    val parentTests = parents filterNot (qual.tpe <:< _)
+
+                    if (parentTests.isEmpty) Literal(Constant(true))
+                    else atPos(tree.pos) {
                       parentTests map mkIsInstanceOf(q) reduceRight gen.mkAnd
                     }
                   }

test/files/run/t6637.check

@@ -0,0 +1 @@
+ok

test/files/run/t6637.scala

@@ -0,0 +1,8 @@
+
+object Test extends App {
+  try {
+    class A ; class B ; List().head.isInstanceOf[A with B]
+  } catch {
+    case _ :java.util.NoSuchElementException => println("ok")
+  }
+}