Error message for pure expr in stat position

Author: Duhemm

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -134,7 +134,8 @@ public enum ErrorMessageID {
     TermMemberNeedsNeedsResultTypeForImplicitSearchID,
     CaseClassCannotExtendEnumID,
     ValueClassParameterMayNotBeCallByNameID,
-    NotAnExtractorID
+    NotAnExtractorID,
+    PureExpressionInStatementPositionID
     ;
 
     public int errorNumber() {

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

@@ -2131,4 +2131,12 @@ object messages {
            |For this reason, you can also define patterns through `unapplySeq` which returns `Option[Seq[T]]`.
            |This mechanism is used for instance in pattern `case List(x1, ..., xn)`""".stripMargin
   }
+
+  case class PureExpressionInStatementPosition(stat: untpd.Tree, exprOwner: Symbol)(implicit ctx: Context) extends Message(PureExpressionInStatementPositionID) {
+    val kind = "Potential Issue"
+    val msg = "a pure expression does nothing in statement position; you may be omitting necessary parentheses"
+    val explanation =
+      hl"""The pure expression `$stat` doesn't have any side effect and its result is not assigned elsewhere.
+          |It can be removed without changing the semantics of the program. This may indicate an error.""".stripMargin
+  }
 }

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

@@ -2026,7 +2026,7 @@ class Typer extends Namer
         val stat1 = typed(stat)(ctx.exprContext(stat, exprOwner))
         if (!ctx.isAfterTyper && isPureExpr(stat1) &&
             !stat1.tpe.isRef(defn.UnitClass) && !isSelfOrSuperConstrCall(stat1))
-          ctx.warning(em"a pure expression does nothing in statement position", stat.pos)
+          ctx.warning(PureExpressionInStatementPosition(stat, exprOwner), stat.pos)
         buf += stat1
         traverse(rest)
       case nil =>