non_exhaustive_switch_expression

The type '{0}' is not exhaustively matched by the switch cases since it doesn't match '{1}'.

The analyzer produces this diagnostic when a switch expression is missing a case for one or more of the possible values that could flow through it.

The following code produces this diagnostic because the switch expression doesn't have a case for the value E.three:

enum E { one, two, three }

String f(E e) => switch (e) {
    E.one => 'one',
    E.two => 'two',
  };

If the missing values are distinctly meaningful to the switch expression, then add a case for each of the values missing a match:

enum E { one, two, three }

String f(E e) => switch (e) {
    E.one => 'one',
    E.two => 'two',
    E.three => 'three',
  };

If the missing values don't need to be matched, then add a wildcard pattern that returns a simple default:

enum E { one, two, three }

String f(E e) => switch (e) {
    E.one => 'one',
    E.two => 'two',
    _ => 'unknown',
  };

Be aware that a wildcard pattern will handle any values added to the type in the future. You will lose the ability to have the compiler warn you if the switch needs to be updated to account for newly added types.