Emit "pure expression does nothing in statement position" warning

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

@@ -1707,7 +1707,10 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       case Thicket(stats) :: rest =>
         traverse(stats ++ rest)
       case stat :: rest =>
-        buf += typed(stat)(ctx.exprContext(stat, exprOwner))
+        val stat1 = typed(stat)(ctx.exprContext(stat, exprOwner))
+        if (!ctx.isAfterTyper && isPureExpr(stat1))
+          ctx.warning(em"a pure expression does nothing in statement position", stat.pos)
+        buf += stat1
         traverse(rest)
       case nil =>
         buf.toList

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -163,6 +163,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("../tests/neg/customArgs/noimports2.scala", defaultOptions.and("-Yno-imports")) +
     compileFile("../tests/neg/customArgs/overloadsOnAbstractTypes.scala", allowDoubleBindings) +
     compileFile("../tests/neg/customArgs/xfatalWarnings.scala", defaultOptions.and("-Xfatal-warnings")) +
+    compileFile("../tests/neg/customArgs/pureStatement.scala", defaultOptions.and("-Xfatal-warnings")) +
     compileFile("../tests/neg/customArgs/phantom-overload.scala", allowDoubleBindings) +
     compileFile("../tests/neg/tailcall/t1672b.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/t3275.scala", defaultOptions) +

tests/neg/customArgs/pureStatement.scala

@@ -0,0 +1,28 @@
+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)
+  }
+
+  implicit val cap: IOCapability = new IOCapability
+
+  2 // error: pure expression does nothing in statement position
+
+  doSideEffects(1) // error: pure expression does nothing in statement position
+}