case_expression_type_is_not_switch_expression_subtype

The switch case expression type '{0}' must be a subtype of the switch expression type '{1}'.

The analyzer produces this diagnostic when the expression following case in a switch statement has a static type that isn't a subtype of the static type of the expression following switch.

The following code produces this diagnostic because 1 is an int, which isn't a subtype of String (the type of s):

void f(String s) {
  switch (s) {
    case 1:
      break;
  }
}

If the value of the case expression is wrong, then change the case expression so that it has the required type:

void f(String s) {
  switch (s) {
    case '1':
      break;
  }
}

If the value of the case expression is correct, then change the switch expression to have the required type:

void f(int s) {
  switch (s) {
    case 1:
      break;
  }
}