SI-9847 Nuance pure expr statement warning

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

@@ -2413,13 +2413,36 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           }
         }
         val stats1 = if (isPastTyper) block.stats else
-          block.stats.flatMap(stat => stat match {
+          block.stats.flatMap {
             case vd@ValDef(_, _, _, _) if vd.symbol.isLazy =>
               namer.addDerivedTrees(Typer.this, vd)
-            case _ => stat::Nil
-            })
-        val stats2 = typedStats(stats1, context.owner)
+            case stat => stat::Nil
+          }
+        val stats2 = typedStats(stats1, context.owner, warnPure = false)
         val expr1 = typed(block.expr, mode &~ (FUNmode | QUALmode), pt)
+
+        // sanity check block for unintended expr placement
+        if (!isPastTyper) {
+          val (count, result0, adapted) =
+            expr1 match {
+              case Block(expr :: Nil, Literal(Constant(()))) => (1, expr, true)
+              case Literal(Constant(()))                     => (0, EmptyTree, false)
+              case _                                         => (1, EmptyTree, false)
+            }
+          def checkPure(t: Tree, supple: Boolean): Unit =
+            if (treeInfo.isPureExprForWarningPurposes(t)) {
+              val msg = "a pure expression does nothing in statement position"
+              val parens = if (stats2.length + count > 1) "multiline expressions might require enclosing parentheses" else ""
+              val discard = if (adapted) "; a value can be silently discarded when Unit is expected" else ""
+              val text =
+                if (supple) s"${parens}${discard}"
+                else if (!parens.isEmpty) s"${msg}; ${parens}" else msg
+              context.warning(t.pos, text)
+            }
+          stats2.foreach(checkPure(_, supple = false))
+          if (result0.nonEmpty) checkPure(result0, supple = true)
+        }
+
         treeCopy.Block(block, stats2, expr1)
           .setType(if (treeInfo.isExprSafeToInline(block)) expr1.tpe else expr1.tpe.deconst)
       } finally {
@@ -2994,7 +3017,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       case _                  => log("unhandled import: "+imp+" in "+unit); imp
     }
 
-    def typedStats(stats: List[Tree], exprOwner: Symbol): List[Tree] = {
+    def typedStats(stats: List[Tree], exprOwner: Symbol, warnPure: Boolean = true): List[Tree] = {
       val inBlock = exprOwner == context.owner
       def includesTargetPos(tree: Tree) =
         tree.pos.isRange && context.unit.exists && (tree.pos includes context.unit.targetPos)
@@ -3025,9 +3048,11 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
                 ConstructorsOrderError(stat)
             }
           }
-          if (!isPastTyper && treeInfo.isPureExprForWarningPurposes(result)) context.warning(stat.pos,
-            "a pure expression does nothing in statement position; you may be omitting necessary parentheses"
-          )
+          if (warnPure && !isPastTyper && treeInfo.isPureExprForWarningPurposes(result)) {
+            val msg = "a pure expression does nothing in statement position"
+            val clause = if (stats.lengthCompare(1) > 0) "; multiline expressions may require enclosing parentheses" else ""
+            context.warning(stat.pos, s"${msg}${clause}")
+          }
           result
       }
 

test/files/neg/macro-invalidret.check

@@ -27,7 +27,7 @@ java.lang.NullPointerException
 Macros_Test_2.scala:15: error: macro implementation is missing
   foo4
   ^
-Macros_Test_2.scala:17: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+Macros_Test_2.scala:17: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
   foo6
   ^
 two warnings found

test/files/neg/scopes.check

@@ -7,7 +7,7 @@ scopes.scala:5: error: x is already defined as value x
 scopes.scala:8: error: y is already defined as value y
     val y: Float = .0f
         ^
-scopes.scala:6: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+scopes.scala:6: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
   {
   ^
 scopes.scala:11: error: x is already defined as value x

test/files/neg/stmt-expr-discard.check

@@ -1,7 +1,7 @@
-stmt-expr-discard.scala:3: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+stmt-expr-discard.scala:3: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
     + 2
     ^
-stmt-expr-discard.scala:4: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+stmt-expr-discard.scala:4: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
     - 4
       ^
 error: No warnings can be incurred under -Xfatal-warnings.

test/files/neg/t1181.check

@@ -1,10 +1,10 @@
-t1181.scala:8: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
-     case (Nil, Nil) => map
-                        ^
 t1181.scala:9: error: type mismatch;
  found   : scala.collection.immutable.Map[Symbol,Symbol]
  required: Symbol
      _ => buildMap(map.updated(keyList.head, valueList.head), keyList.tail, valueList.tail)
                   ^
+t1181.scala:8: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
+     case (Nil, Nil) => map
+                        ^
 one warning found
 one error found

test/files/neg/t9847.check

@@ -0,0 +1,45 @@
+t9847.scala:4: warning: discarded non-Unit value
+  def f(): Unit = 42
+                  ^
+t9847.scala:4: warning: a pure expression does nothing in statement position
+  def f(): Unit = 42
+                  ^
+t9847.scala:5: warning: discarded non-Unit value
+  def g = (42: Unit)
+           ^
+t9847.scala:5: warning: a pure expression does nothing in statement position
+  def g = (42: Unit)
+           ^
+t9847.scala:7: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
+    1
+    ^
+t9847.scala:12: warning: discarded non-Unit value
+  + 1
+  ^
+t9847.scala:12: warning: a pure expression does nothing in statement position
+  + 1
+  ^
+t9847.scala:11: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
+    1
+    ^
+t9847.scala:12: warning: multiline expressions might require enclosing parentheses; a value can be silently discarded when Unit is expected
+  + 1
+  ^
+t9847.scala:16: warning: discarded non-Unit value
+    x + 1
+      ^
+t9847.scala:19: warning: discarded non-Unit value
+  def j(): Unit = x + 1
+                    ^
+t9847.scala:21: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
+  class C { 42 }
+            ^
+t9847.scala:22: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
+  class D { 42 ; 17 }
+            ^
+t9847.scala:22: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
+  class D { 42 ; 17 }
+                 ^
+error: No warnings can be incurred under -Xfatal-warnings.
+14 warnings found
+one error found

test/files/neg/t9847.flags

@@ -0,0 +1 @@
+-Xfatal-warnings -Ywarn-value-discard

test/files/neg/t9847.scala

@@ -0,0 +1,23 @@
+
+trait T {
+
+  def f(): Unit = 42
+  def g = (42: Unit)
+  def h = {
+    1
+  + 1
+  }
+  def hh(): Unit = {
+    1
+  + 1
+  }
+  def i(): Unit = {
+    val x = 1
+    x + 1
+  }
+  def x = 42
+  def j(): Unit = x + 1
+
+  class C { 42 }
+  class D { 42 ; 17 }
+}

test/files/neg/unit-returns-value.check

@@ -1,13 +1,13 @@
-unit-returns-value.scala:4: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+unit-returns-value.scala:4: warning: a pure expression does nothing in statement position
     if (b) return 5
                   ^
 unit-returns-value.scala:4: warning: enclosing method f has result type Unit: return value discarded
     if (b) return 5
            ^
-unit-returns-value.scala:22: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+unit-returns-value.scala:22: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     i1 // warn
     ^
-unit-returns-value.scala:23: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+unit-returns-value.scala:23: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     i2 // warn
     ^
 error: No warnings can be incurred under -Xfatal-warnings.

test/files/run/contrib674.check

@@ -1,3 +1,6 @@
-contrib674.scala:15: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+contrib674.scala:15: warning: a pure expression does nothing in statement position
+    1
+    ^
+contrib674.scala:15: warning: multiline expressions might require enclosing parentheses; a value can be silently discarded when Unit is expected
     1
     ^

test/files/run/contrib674.scala

@@ -1,7 +1,7 @@
 // causes VerifyError with scala-2.5.1
 
 object Test extends App {
-  def bad() {
+  def bad(): Unit = {
     try {
       1
     } catch {

test/files/run/delay-bad.check

@@ -1,7 +1,7 @@
-delay-bad.scala:53: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+delay-bad.scala:53: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
     f(new C { 5 })
               ^
-delay-bad.scala:73: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+delay-bad.scala:73: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
     f(new { val x = 5 } with E() { 5 })
                                    ^
 warning: there was one deprecation warning (since 2.11.0); re-run with -deprecation for details

test/files/run/delay-good.check

@@ -1,7 +1,7 @@
-delay-good.scala:53: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+delay-good.scala:53: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
     f(new C { 5 })
               ^
-delay-good.scala:73: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+delay-good.scala:73: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
     f(new { val x = 5 } with E() { 5 })
                                    ^
 

test/files/run/exceptions-2.check

@@ -1,4 +1,4 @@
-exceptions-2.scala:267: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+exceptions-2.scala:267: warning: a pure expression does nothing in statement position
     try { 1 } catch { case e: java.io.IOException => () }
           ^
 nested1: 

test/files/run/lazy-locals.check

@@ -1,7 +1,7 @@
-lazy-locals.scala:153: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+lazy-locals.scala:153: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
   {
   ^
-lazy-locals.scala:159: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+lazy-locals.scala:159: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
     {
     ^
 forced lazy val q

test/files/run/macro-duplicate.check

@@ -1,3 +1,3 @@
-Test_2.scala:5: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+Test_2.scala:5: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
   Macros.foo
          ^

test/files/run/misc.check

@@ -1,25 +1,25 @@
-misc.scala:46: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:46: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     42;
     ^
-misc.scala:47: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:47: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     42l;
     ^
-misc.scala:48: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:48: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     23.5f;
     ^
-misc.scala:49: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:49: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     23.5;
     ^
-misc.scala:50: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:50: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     "Hello";
     ^
-misc.scala:51: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:51: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     32 + 45;
        ^
-misc.scala:62: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:62: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     x;
     ^
-misc.scala:74: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+misc.scala:74: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     1 < 2;
       ^
 ### Hello

test/files/run/names-defaults.check

@@ -1,4 +1,7 @@
-names-defaults.scala:269: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+names-defaults.scala:269: warning: a pure expression does nothing in statement position
+    spawn(b = { val ttt = 1; ttt }, a = 0)
+                             ^
+names-defaults.scala:269: warning: multiline expressions might require enclosing parentheses; a value can be silently discarded when Unit is expected
     spawn(b = { val ttt = 1; ttt }, a = 0)
                              ^
 warning: there were four deprecation warnings; re-run with -deprecation for details

test/files/run/patmatnew.check

@@ -1,10 +1,16 @@
-patmatnew.scala:351: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+patmatnew.scala:351: warning: a pure expression does nothing in statement position
         case 1 => "OK"
                   ^
-patmatnew.scala:352: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+patmatnew.scala:352: warning: a pure expression does nothing in statement position
         case 2 => assert(false); "KO"
                                  ^
-patmatnew.scala:353: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+patmatnew.scala:352: warning: multiline expressions might require enclosing parentheses; a value can be silently discarded when Unit is expected
+        case 2 => assert(false); "KO"
+                                 ^
+patmatnew.scala:353: warning: a pure expression does nothing in statement position
+        case 3 => assert(false); "KO"
+                                 ^
+patmatnew.scala:353: warning: multiline expressions might require enclosing parentheses; a value can be silently discarded when Unit is expected
         case 3 => assert(false); "KO"
                                  ^
 patmatnew.scala:670: warning: This catches all Throwables. If this is really intended, use `case e : Throwable` to clear this warning.

test/files/run/reify_lazyunit.check

@@ -1,4 +1,4 @@
-reify_lazyunit.scala:6: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+reify_lazyunit.scala:6: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
     lazy val x = { 0; println("12")}
                    ^
 12

test/files/run/repl-bare-expr.check

@@ -1,12 +1,12 @@
 
 scala> 2 ; 3
-<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+<console>:11: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
        2 ;;
        ^
 res0: Int = 3
 
 scala> { 2 ; 3 }
-<console>:12: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+<console>:12: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
        { 2 ; 3 }
          ^
 res1: Int = 3
@@ -15,16 +15,16 @@ scala> 5 ; 10 ; case object Cow ; 20 ; class Moo { override def toString = "Mooo
   1 +
   2 +
   3 } ; bippy+88+11
-<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+<console>:11: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
        5 ; 10 ; case object Cow ; 20 ; class Moo { override def toString = "Moooooo" } ; 30 ; def bippy = {
        ^
-<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+<console>:11: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
        5 ; 10 ; case object Cow ; 20 ; class Moo { override def toString = "Moooooo" } ; 30 ; def bippy = {
            ^
-<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+<console>:11: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
        5 ; 10 ; case object Cow ; 20 ; class Moo { override def toString = "Moooooo" } ; 30 ; def bippy = {
                                   ^
-<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
+<console>:11: warning: a pure expression does nothing in statement position; multiline expressions may require enclosing parentheses
        5 ; 10 ; case object Cow ; 20 ; class Moo { override def toString = "Moooooo" } ; 30 ; def bippy = {
                                                                                          ^
 defined object Cow