Fix off-by-one error in forward reference checking

Author: odersky

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1055,7 +1055,7 @@ object messages {
            |
            |Define `${definition.name}` before it is used,
            |or move the definition of `${value.name}` so it does not appear between
-           |the declartion of `${definition.name}` and its use,
+           |the declaration of `${definition.name}` and its use,
            |or define `${value.name}` as lazy.
            |""".stripMargin
   }

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

@@ -789,7 +789,7 @@ class RefChecks extends MiniPhase { thisTransformer =>
       val sym = tree.symbol
       if (sym.exists && sym.owner.isTerm && !sym.is(Lazy))
         currentLevel.levelAndIndex.get(sym) match {
-          case Some((level, symIdx)) if symIdx < level.maxIndex =>
+          case Some((level, symIdx)) if symIdx <= level.maxIndex =>
             ctx.error(ForwardReferenceExtendsOverDefinition(sym, level.refSym), level.refPos)
           case _ =>
         }

tests/neg/i1992.scala

@@ -0,0 +1,9 @@
+object Test {
+  def main(args: Array[String]) = {
+    val x: Int => Unit =
+      y => println(x) //  error: `x` is a forward reference
+    implicit val z: String => Unit =
+      y => println(implicitly[String => Unit]) // error: `z` is a forward reference
+  }
+}
+