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, 27 insertions, 0 deletions

diff --git a/src/partest/scala/tools/partest/package.scala b/src/partest/scala/tools/partest/package.scala
index 08934ef143..49d3ed301c 100644
--- a/src/partest/scala/tools/partest/package.scala
+++ b/src/partest/scala/tools/partest/package.scala
@@ -73,4 +73,31 @@ package object partest {
 
   def isPartestDebug: Boolean =
     propOrEmpty("partest.debug") == "true"
+
+
+  import language.experimental.macros
+
+  /**
+   * `trace("".isEmpty)` will return `true` and as a side effect print the following to standard out.
+   * {{{
+   *   trace> "".isEmpty
+   *   res: Boolean = true
+   *
+   * }}}
+   *
+   * An alternative to [[scala.tools.partest.ReplTest]] that avoids the inconvenience of embedding
+   * test code in a string.
+   */
+  def trace[A](a: A) = macro traceImpl[A]
+
+  import scala.reflect.macros.Context
+  def traceImpl[A: c.AbsTypeTag](c: Context)(a: c.Expr[A]): c.Expr[A] = {
+    import c.universe._
+    val exprCode = c.literal(show(a.tree))
+    val exprType = c.literal(show(a.actualType))
+    reify {
+      println(s"trace> ${exprCode.splice}\nres: ${exprType.splice} = ${a.splice}\n")
+      a.splice
+    }
+  }
 }