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.

4 files changed, 64 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index b9fdd7280e..293b36c4d8 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/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)
diff --git a/test/files/neg/delayed-init-ref.check b/test/files/neg/delayed-init-ref.check
new file mode 100644
index 0000000000..42ccabed1b
--- /dev/null
+++ b/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
diff --git a/test/files/neg/delayed-init-ref.flags b/test/files/neg/delayed-init-ref.flags
new file mode 100644
index 0000000000..7949c2afa2
--- /dev/null
+++ b/test/files/neg/delayed-init-ref.flags
@@ -0,0 +1 @@
+-Xlint -Xfatal-warnings
diff --git a/test/files/neg/delayed-init-ref.scala b/test/files/neg/delayed-init-ref.scala
new file mode 100644
index 0000000000..f2aa804e28
--- /dev/null
+++ b/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)
+  }
+}