Constant values from a deferred library can't be used in patternsDRT-W1568

The analyzer produces this diagnostic when a pattern contains a value declared in a different library, and that library is imported using a deferred import. Constants are evaluated at compile time, but values from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

Example

Given a file a.dart that defines the constant zero:

const zero = 0;

The following code produces this diagnostic because the constant pattern a.zero is imported using a deferred import:

import 'a.dart' deferred as a;

void f(int x) {
  switch (x) {
    case a.zero:
      // ...
      break;
  }
}

Common fixes

If you need to reference the constant from the imported library, then remove the deferred keyword:

import 'a.dart' as a;

void f(int x) {
  switch (x) {
    case a.zero:
      // ...
      break;
  }
}

If you need to reference the constant from the imported library and also need the imported library to be deferred, then rewrite the switch statement as a sequence of if statements:

import 'a.dart' deferred as a;

void f(int x) {
  if (x == a.zero) {
    // ...
  }
}

If you don't need to reference the constant, then replace the case expression:

void f(int x) {
  switch (x) {
    case 0:
      // ...
      break;
  }
}