Exclude self and super constructor calls from purity warnings

Don't issue a "pure expression does nothing in statement position" for self and
super constructor calls. They are conceptually unit-returning in this position anyway.

Author: odersky

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

@@ -1917,7 +1917,8 @@ class Typer extends Namer
         traverse(stats ++ rest)
       case stat :: rest =>
         val stat1 = typed(stat)(ctx.exprContext(stat, exprOwner))
-        if (!ctx.isAfterTyper && isPureExpr(stat1) && !stat1.tpe.isRef(defn.UnitClass))
+        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)
         buf += stat1
         traverse(rest)

tests/neg-custom-args/fatal-warnings/i2333.scala

@@ -1,4 +1,5 @@
 @deprecated("bla", "2.11.0") class Foo {
+  println("")
   def this(x: Int) = this()
 }