Don't compute least upper bounds for expressions in statement positio…

…ns inside blocks.

This may save huge amount of time (Fixes SI-5862) for complicated lubs. I had to remove the a check in adapt for
the part that transforms <expr> into { <expr>; () } when the expected type is Unit. The reason is in the
code.

As a side effect, we get more warnings for pure expressions in statement positions (see the change in the test file).

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

@@ -86,6 +86,17 @@ trait Modes {
    */
   final val TYPEPATmode   = 0x10000
 
+  /** STATmode is set when typing statements inside a block.
+   *
+   *  This is useful only for skipping lub computations in 
+   *  such positions, when the expected type is known to be Unit. This
+   *  saves time in pathological cases where lubs can take many seconds 
+   *  but in the end is discarded.
+   *
+   *  TODO: Remove when lubs become types in their own right. 
+   */
+  final val STATmode      = 0x20000
+
   final private val StickyModes   = EXPRmode | PATTERNmode | TYPEmode | ALTmode
 
   final def onlyStickyModes(mode: Int) =
@@ -128,7 +139,8 @@ trait Modes {
     (1 << 13) -> "ALTmode",
     (1 << 14) -> "HKmode",
     (1 << 15) -> "BYVALmode",
-    (1 << 16) -> "TYPEPATmode"
+    (1 << 16) -> "TYPEPATmode",
+    (1 << 17) -> "STATmode"
   )
   def modeString(mode: Int): String =
     if (mode == 0) "NOmode"

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

@@ -1127,9 +1127,11 @@ trait Typers extends Modes with Adaptations with Taggings {
               if (inExprModeButNot(mode, FUNmode)) {
                 pt.normalize match {
                   case TypeRef(_, sym, _) =>
-                    // note: was if (pt.typeSymbol == UnitClass) but this leads to a potentially
-                    // infinite expansion if pt is constant type ()
-                    if (sym == UnitClass && tree.tpe <:< AnyClass.tpe) { // (12)
+                    // note: there was an additional '&& tree.tpe <:< AnyClass.tpe', claiming that
+                    // it would save us from a potentially infinite expansion if pt is constant type ()
+                    // didn't seem to do any good, and the test would miss the fact that Object.tpe <:< AnyClass.tpe 
+                    // after erasure (triggered by try-catches added by cleanup for structural types)
+                    if (sym == UnitClass) { // (12)
                       if (settings.warnValueDiscard.value)
                         context.unit.warning(tree.pos, "discarded non-Unit value")
                       return typed(atPos(tree.pos)(Block(List(tree), Literal(Constant()))), mode, pt)
@@ -2566,7 +2568,7 @@ trait Typers extends Modes with Adaptations with Taggings {
                 } else newTyper(context.make(stat, exprOwner))
                 // XXX this creates a spurious dead code warning if an exception is thrown
                 // in a constructor, even if it is the only thing in the constructor.
-                val result = checkDead(localTyper.typed(stat, EXPRmode | BYVALmode, WildcardType))
+                val result = checkDead(localTyper.typed(stat, EXPRmode | BYVALmode | STATmode, WildcardType))
 
                 if (treeInfo.isSelfOrSuperConstrCall(result)) {
                   context.inConstructorSuffix = true
@@ -3861,6 +3863,7 @@ trait Typers extends Modes with Adaptations with Taggings {
               && thenTp =:= elseTp
                ) (thenp1.tpe, false) // use unpacked type
             // TODO: skolemize (lub of packed types) when that no longer crashes on files/pos/t4070b.scala
+            else if ((mode & STATmode) != 0) (definitions.UnitClass.tpe, true)
             else ptOrLub(List(thenp1.tpe, elsep1.tpe), pt)
 
           if (needAdapt) { //isNumericValueType(owntype)) {
@@ -4744,7 +4747,7 @@ trait Typers extends Modes with Adaptations with Taggings {
           var catches1 = typedCases(catches, ThrowableClass.tpe, pt)
           val finalizer1 = if (finalizer.isEmpty) finalizer
                            else typed(finalizer, UnitClass.tpe)
-          val (owntype, needAdapt) = ptOrLub(block1.tpe :: (catches1 map (_.tpe)), pt)
+          val (owntype, needAdapt) = if ((mode & STATmode) != 0) (definitions.UnitClass.tpe, true) else ptOrLub(block1.tpe :: (catches1 map (_.tpe)), pt)
           if (needAdapt) {
             block1 = adapt(block1, mode, owntype)
             catches1 = catches1 map (adaptCase(_, mode, owntype))

test/files/neg/checksensible.check

@@ -77,8 +77,8 @@ checksensible.scala:63: error: comparing a fresh object using `!=' will always y
   new Exception() != new Exception()
                   ^
 checksensible.scala:66: error: comparing values of types Int and Null using `==' will always yield false
-  if (foo.length == null) "plante" else "plante pas"
-                 ^
+  val dummy = if (foo.length == null) "plante" else "plante pas"
+                             ^
 checksensible.scala:71: error: comparing values of types Bip and Bop using `==' will always yield false
   (x1 == x2)
       ^

test/files/neg/checksensible.scala

@@ -63,7 +63,7 @@ class EqEqRefTest {
   new Exception() != new Exception()
 
   val foo: Array[String] = Array("1","2","3")
-  if (foo.length == null) "plante" else "plante pas"
+  val dummy = if (foo.length == null) "plante" else "plante pas"
 
   // final classes with default equals
   val x1 = new Bip