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

The fast path in typedIf added in 8552740 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

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -4067,7 +4067,7 @@ trait Typers extends Modes with Adaptations with Tags {
             if ( opt.virtPatmat && !isPastTyper
               && thenp1.tpe.annotations.isEmpty && elsep1.tpe.annotations.isEmpty // annotated types need to be lubbed regardless (at least, continations break if you by pass them like this)
               && thenTp =:= elseTp
-               ) (thenp1.tpe, false) // use unpacked type
+               ) (thenp1.tpe.deconst, false) // use unpacked type. Important to deconst, as is done in ptOrLub, otherwise `if (???) 0 else 0` evaluates to 0 (SI-6331)
             // TODO: skolemize (lub of packed types) when that no longer crashes on files/pos/t4070b.scala
             else ptOrLub(thenp1.tpe :: elsep1.tpe :: Nil, pt)
 

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
+    }
+  }
 }

test/files/run/t6331b.check

@@ -0,0 +1,30 @@
+trace> if (Test.this.t)
+  -0.0
+else
+  0.0
+res: Double = -0.0
+
+trace> if (Test.this.t)
+  0.0
+else
+  -0.0
+res: Double = 0.0
+
+trace> Test.this.intercept.apply[Any](if (scala.this.Predef.???)
+  -0.0
+else
+  0.0)
+res: Any = class scala.NotImplementedError
+
+trace> Test.this.intercept.apply[Any](if (scala.this.Predef.???)
+  0.0
+else
+  0.0)
+res: Any = class scala.NotImplementedError
+
+trace> Test.this.intercept.apply[Any](if (scala.this.Predef.???)
+  ()
+else
+  ())
+res: Any = class scala.NotImplementedError
+

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 ()))
+}