fix issues related to union types

src/arg_parser.cr

@@ -187,7 +187,7 @@ module ArgParser
   #
   # Note: You can override this method to change the way validation errors are handled.
   def on_validation_error(key : String, value, errors : Array(String))
-    add_validation_error(key, erorrs)
+    add_validation_error(key, errors)
     raise ValidationError.new(key, value, errors)
   end
 
@@ -198,9 +198,9 @@ module ArgParser
     raise ConversionError.new(key, value, type)
   end
 
-  def add_validation_error(key : String, value, errors : Array(String))
-    @errors[key] ||= [] of String
-    @errors[key].concat errors
+  def add_validation_error(key : String, errors : Array(String))
+    @_validation_errors[key] ||= [] of String
+    @_validation_errors[key].concat errors
   end
 
   # https://en.wikipedia.org/wiki/Quotation_mark#Summary_table

src/arg_parser/from_arg.cr

@@ -123,3 +123,24 @@ struct UUID
     UUID.new(arg)
   end
 end
+
+struct Nil
+  def self.from_arg(arg : String)
+    nil
+  end
+end
+
+def Union.from_arg(arg : String)
+  {% begin %}
+    {% for type in T %}
+      begin
+        %val = {{type}}.from_arg(arg)
+        return %val if %val.is_a?({{type}})
+      rescue
+      end
+
+    {% end %}
+
+    raise ArgParser::Error.new("Argument '#{arg}' cannot be converted to any of the union types: {{T}}")
+  {% end %}
+end