src/Pure/System/options.scala

       case None => error("Missing value for option " + quote(name) + " : " + opt.typ.print)
     }
   }
 
   def + (str: String): Options =
-  {
-    str.indexOf('=') match {
-      case -1 => this + (str, None)
-      case i => this + (str.substring(0, i), str.substring(i + 1))
-    }
-  }
+    str match {
+      case Properties.Eq((a, b)) => this + (a, b)
+      case _ => this + (str, None)
+    }
 
   def ++ (specs: List[Options.Spec]): Options =
     specs.foldLeft(this) { case (x, (y, z)) => x + (y, z) }