Implement -Wvalue-discard warning

Warn when an expression `e` with non-Unit type is adapted by embedding
it into a block `{ e; () }` because the expected type is Unit.

Author: cb372

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -156,6 +156,7 @@ private sealed trait WarningSettings:
   self: SettingGroup =>
   val Whelp: Setting[Boolean] = BooleanSetting("-W", "Print a synopsis of warning options.")
   val XfatalWarnings: Setting[Boolean] = BooleanSetting("-Werror", "Fail the compilation if there are any warnings.", aliases = List("-Xfatal-warnings"))
+  val YwarnValueDiscard: Setting[Boolean] = BooleanSetting("-Wvalue-discard", "Warn when non-Unit expression results are unused.", aliases = List("-Ywarn-value-discard"))
 
   val Wunused: Setting[List[String]] = MultiChoiceSetting(
     name = "-Wunused",

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -184,6 +184,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case ImplicitSearchTooLargeID // errorNumber: 168
   case TargetNameOnTopLevelClassID // errorNumber: 169
   case NotClassTypeID // errorNumber 170
+  case ValueDiscardingID // errorNumber 171
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2535,3 +2535,9 @@ import cc.CaptureSet.IdentityCaptRefMap
   extends TypeMsg(NotClassTypeID), ShowMatchTrace(tp):
     def msg = ex"$tp is not a class type"
     def explain = ""
+
+  class ValueDiscarding(tp: Type)(using Context)
+  extends Message(ValueDiscardingID):
+    def kind = MessageKind.PotentialIssue
+    def msg = em"discarded non-Unit value of type $tp"
+    def explain = ""

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3905,6 +3905,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         // so will take the code path that decides on inlining
         val tree1 = adapt(tree, WildcardType, locked)
         checkStatementPurity(tree1)(tree, ctx.owner)
+        if (!ctx.isAfterTyper && !tree.isInstanceOf[Inlined] && ctx.settings.YwarnValueDiscard.value) {
+          report.warning(ValueDiscarding(tree.tpe), tree.srcPos)
+        }
         return tpd.Block(tree1 :: Nil, Literal(Constant(())))
       }
 

tests/neg/warn-value-discard.scala

@@ -0,0 +1,18 @@
+// scalac: -Wvalue-discard -Werror
+
+import scala.util.{Either, Right, Left}
+
+case class Failed(msg: String)
+
+def doSomething(): Either[Failed, Unit] =
+  Right(())
+
+def log(): Either[Failed, Unit] =
+  Left(Failed("whoops you should have flatMapped me"))
+
+def singleExpr(): Either[Failed, Unit] =
+  doSomething().map(_ => log()) // error
+
+def block(): Either[Failed, Unit] = {
+  doSomething().map(_ => log()) // error
+}