non_constant_case_expression_from_deferred_library

Constant values from a deferred library can't be used as a case expression.

The analyzer produces this diagnostic when the expression in a case clause references a constant from a library that is imported using a deferred import. In order for switch statements to be compiled efficiently, the constants referenced in case clauses need to be available at compile time, and constants from deferred libraries aren't available at compile time.

For more information, check out Lazily loading a library.

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

const zero = 0;

The following code produces this diagnostic because the library a.dart is imported using a deferred import, and the constant a.zero, declared in the imported library, is used in a case clause:

import 'a.dart' deferred as a;
​
void f(int x) {
  switch (x) {
    case a.zero:
      // ...
      break;
  }
}

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;
  }
}