Fix condition where to check for pure expressions

Author: odersky

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

@@ -1708,8 +1708,10 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         traverse(stats ++ rest)
       case stat :: rest =>
         val stat1 = typed(stat)(ctx.exprContext(stat, exprOwner))
-        if ((ctx.owner.isType || rest.nonEmpty) && isPureExpr(stat1) && !ctx.isAfterTyper)
-          ctx.warning(em"a pure expression does nothing in statement position", stat1.pos)
+        if (!ctx.isAfterTyper) {
+          if (isPureExpr(stat1))
+            ctx.warning(em"a pure expression does nothing in statement position", stat.pos)
+        }
         buf += stat1
         traverse(rest)
       case nil =>

tests/neg/customArgs/pureStatement.scala

@@ -1,6 +1,20 @@
 class IOCapability
 
 object Test {
+  "" // error: pure expression does nothing in statement position
+
+  locally {
+    "" // error: pure expression does nothing in statement position
+
+    println("")
+
+    42 // error: pure expression does nothing in statement position
+
+    ((x: Int) => println("hi")) // error: pure expression does nothing in statement position
+
+    ()
+  }
+
   // Forgot to mark `ev` implicit!
   def doSideEffects(x: Int)(ev: IOCapability) = {
     println("x: " + x)