Warn on selection of vals from DelayedInit subclasses.

Which are likely to yield null, if the program didn't start.

This is a common source of confusion for people new to
the language, as was seen during the Coursera course.

The test case shows that the usage pattern within Specs2
won't generate these warnings.

src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

@@ -1396,6 +1396,16 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
       }
     }
 
+    private def checkDelayedInitSelect(qual: Tree, sym: Symbol, pos: Position) = {
+      def isLikelyUninitialized = (
+           (sym.owner isSubClass DelayedInitClass)
+        && !qual.tpe.isInstanceOf[ThisType]
+        && sym.accessedOrSelf.isVal
+      )
+      if (settings.lint.value && isLikelyUninitialized)
+        unit.warning(pos, s"Selecting ${sym} from ${sym.owner}, which extends scala.DelayedInit, is likely to yield an uninitialized value")
+    }
+
     private def lessAccessible(otherSym: Symbol, memberSym: Symbol): Boolean = (
          (otherSym != NoSymbol)
       && !otherSym.isProtected
@@ -1595,6 +1605,7 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
       if(settings.Xmigration.value != NoScalaVersion)
         checkMigration(sym, tree.pos)
       checkCompileTimeOnly(sym, tree.pos)
+      checkDelayedInitSelect(qual, sym, tree.pos)
 
       if (sym eq NoSymbol) {
         unit.warning(tree.pos, "Select node has NoSymbol! " + tree + " / " + tree.tpe)

test/files/neg/delayed-init-ref.check

@@ -0,0 +1,10 @@
+delayed-init-ref.scala:17: error: Selecting value vall from object O, which extends scala.DelayedInit, is likely to yield an uninitialized value
+  println(O.vall)     // warn
+            ^
+delayed-init-ref.scala:19: error: Selecting value vall from object O, which extends scala.DelayedInit, is likely to yield an uninitialized value
+  println(vall)       // warn
+          ^
+delayed-init-ref.scala:40: error: Selecting value foo from trait UserContext, which extends scala.DelayedInit, is likely to yield an uninitialized value
+    println({locally(()); this}.foo)  // warn (spurious, but we can't discriminate)
+                                ^
+three errors found

test/files/neg/delayed-init-ref.flags

@@ -0,0 +1 @@
+-Xlint -Xfatal-warnings

test/files/neg/delayed-init-ref.scala

@@ -0,0 +1,42 @@
+trait T {
+  val traitVal = ""
+}
+
+object O extends App with T {
+  val vall = ""
+  lazy val lazyy = ""
+  def deff = ""
+
+  println(vall)       // no warn
+  new {
+    println(vall)     // no warn
+  }
+}
+
+object Client {
+  println(O.vall)     // warn
+  import O.vall
+  println(vall)       // warn
+
+  println(O.lazyy)    // no warn
+  println(O.deff)     // no warn
+  println(O.traitVal) // no warn
+}
+
+// Delayed init usage pattern from Specs2
+// See: https://groups.google.com/d/msg/scala-sips/wP6dL8nIAQs/ogjoPE-MSVAJ
+trait Before extends DelayedInit {
+  def before()
+  override def delayedInit(x: => Unit): Unit = { before; x }
+}
+object Spec {
+  trait UserContext extends Before {
+    def before() = ()
+    val foo = "foo"
+  }
+  new UserContext {
+    println(foo)                      // no warn
+    println(this.foo)                 // no warn
+    println({locally(()); this}.foo)  // warn (spurious, but we can't discriminate)
+  }
+}