SI-6331 Avoid typing an If tree with a constant type.

The fast path in typedIf added in 8552740b avoided lubbing the if/else branch types
if they are identical, but this fails to deconst the type. This could lead to the entire
if expression being replaced by a constant.

Also introduces a new tool in partest for nicer checkfiles.

// in Test.scala
trace(if (t) -0d else 0d)

// in Test.check
trace> if (Test.this.t)
  -0.0
else
  0.0
res: Double = -0.0

1 files changed, 20 insertions, 0 deletions

diff --git a/test/files/run/t6331b.scala b/test/files/run/t6331b.scala
new file mode 100644
index 0000000000..f966abea51
--- /dev/null
+++ b/test/files/run/t6331b.scala
@@ -0,0 +1,20 @@
+import scala.tools.partest._
+import java.io._
+import scala.tools.nsc._
+import scala.tools.nsc.util.CommandLineParser
+import scala.tools.nsc.{Global, Settings, CompilerCommand}
+import scala.tools.nsc.reporters.ConsoleReporter
+
+import scala.tools.partest.trace
+import scala.util.control.Exception._
+
+
+object Test extends App {
+  def intercept = allCatch.withApply(_.getClass)
+  val t: Boolean = true
+  trace(if (t) -0d else 0d)
+  trace(if (t) 0d else -0d)
+  trace(intercept(if (???) -0d else 0d))
+  trace(intercept(if (???) 0d else 0d))
+  trace(intercept(if (???) () else ()))
+}