Warn about locally identifiable init order issues.

This warns about a subset of initialization order issues which
can easily be identified by inspection, such as

  val x = y
  val y = 5

The likelihood of this formulation being intentional is miniscule.

4 files changed, 68 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Constructors.scala b/src/compiler/scala/tools/nsc/transform/Constructors.scala
index a4a6c3ff31..886c790ec0 100644
--- a/src/compiler/scala/tools/nsc/transform/Constructors.scala
+++ b/src/compiler/scala/tools/nsc/transform/Constructors.scala
@@ -34,6 +34,41 @@ abstract class Constructors extends Transform with ast.TreeDSL {
       val stats = impl.body          // the transformed template body
       val localTyper = typer.atOwner(impl, clazz)
 
+      // Inspect for obvious out-of-order initialization; concrete, eager vals or vars,
+      // declared in this class, for which a reference to the member precedes its definition.
+      def checkableForInit(sym: Symbol) = (
+           (sym ne null)
+        && (sym.isVal || sym.isVar)
+        && !(sym hasFlag LAZY | DEFERRED | SYNTHETIC)
+      )
+      val uninitializedVals = mutable.Set[Symbol](
+        stats collect { case vd: ValDef if checkableForInit(vd.symbol) => vd.symbol.accessedOrSelf }: _*
+      )
+      if (uninitializedVals.nonEmpty)
+        log("Checking constructor for init order issues among: " + uninitializedVals.map(_.name).mkString(", "))
+
+      for (stat <- stats) {
+        // Checking the qualifier symbol is necessary to prevent a selection on
+        // another instance of the same class from potentially appearing to be a forward
+        // reference on the member in the current class.
+        def check(tree: Tree) = {
+          for (t <- tree) t match {
+            case t: RefTree if uninitializedVals(t.symbol.accessedOrSelf) && t.qualifier.symbol == clazz =>
+              unit.warning(t.pos, s"Reference to uninitialized ${t.symbol.accessedOrSelf}")
+            case _ =>
+          }
+        }
+        stat match {
+          case vd: ValDef      =>
+            // doing this first allows self-referential vals, which to be a conservative
+            // warner we will do because it's possible though difficult for it to be useful.
+            uninitializedVals -= vd.symbol.accessedOrSelf
+            if (!vd.symbol.isLazy)
+              check(vd.rhs)
+          case _: MemberDef    => // skip other member defs
+          case t               => check(t) // constructor body statement
+        }
+      }
       val specializedFlag: Symbol = clazz.info.decl(nme.SPECIALIZED_INSTANCE)
       val shouldGuard = (specializedFlag != NoSymbol) && !clazz.hasFlag(SPECIALIZED)
 
diff --git a/test/files/neg/constructor-init-order.check b/test/files/neg/constructor-init-order.check
new file mode 100644
index 0000000000..9ab6ac5923
--- /dev/null
+++ b/test/files/neg/constructor-init-order.check
@@ -0,0 +1,9 @@
+constructor-init-order.scala:7: warning: Reference to uninitialized value baz
+  val bar1         = baz     // warn
+                     ^
+constructor-init-order.scala:17: warning: Reference to uninitialized variable baz
+  var bar1         = baz     // warn
+                     ^
+error: No warnings can be incurred under -Xfatal-warnings.
+two warnings found
+one error found
diff --git a/test/files/neg/constructor-init-order.flags b/test/files/neg/constructor-init-order.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/neg/constructor-init-order.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
diff --git a/test/files/neg/constructor-init-order.scala b/test/files/neg/constructor-init-order.scala
new file mode 100644
index 0000000000..fe8fec87ad
--- /dev/null
+++ b/test/files/neg/constructor-init-order.scala
@@ -0,0 +1,23 @@
+trait Foo0 {
+  val quux1: String
+  val quux2 = quux1  // warning here is "future work"
+}
+
+class Foo1 extends Foo0 {
+  val bar1         = baz     // warn
+  val bar2         = lazybaz // no warn
+  val bar3         = defbaz  // no warn
+  val baz          = "oops"
+  lazy val lazybaz = "ok"
+  def defbaz       = "ok"
+  val quux1        = "oops"
+}
+
+class Foo2 {
+  var bar1         = baz     // warn
+  var bar2         = lazybaz // no warn
+  var bar3         = defbaz  // no warn
+  var baz          = "oops"
+  lazy val lazybaz = "ok"
+  def defbaz       = "ok"
+}