non_constant_default_value

The default value of an optional parameter must be constant.

The analyzer produces this diagnostic when an optional parameter, either named or positional, has a default value that isn't a compile-time constant.

The following code produces this diagnostic:

var defaultValue = 3;

void f([int value = defaultValue]) {}

If the default value can be converted to be a constant, then convert it:

const defaultValue = 3;

void f([int value = defaultValue]) {}

If the default value needs to change over time, then apply the default value inside the function:

var defaultValue = 3;

void f([int? value]) {
  value ??= defaultValue;
}

Unless stated otherwise, the documentation on this site reflects Dart 3.8.0. Page last updated on 2025-05-08. View source or report an issue.