Error message for pure expr in stat position

Author: Duhemm

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

@@ -135,7 +135,8 @@ public enum ErrorMessageID {
     CaseClassCannotExtendEnumID,
     ValueClassParameterMayNotBeCallByNameID,
     NotAnExtractorID,
-    MemberWithSameNameAsStaticID
+    MemberWithSameNameAsStaticID,
+    PureExpressionInStatementPositionID
     ;
 
     public int errorNumber() {

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

@@ -2139,4 +2139,14 @@ object messages {
     override def kind: String = "Syntax"
     override def explanation: String = ""
   }
+
+  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

@@ -2013,7 +2013,7 @@ 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 =>