pattern_variable_assignment_inside_guard

Pattern variables can't be assigned inside the guard of the enclosing guarded pattern.

The analyzer produces this diagnostic when a pattern variable is assigned a value inside a guard (when) clause.

The following code produces this diagnostic because the variable a is assigned a value inside the guard clause:

void f(int x) {
  if (x case var a when (a = 1) > 0) {
    print(a);
  }
}

If there's a value you need to capture, then assign it to a different variable:

void f(int x) {
  var b;
  if (x case var a when (b = 1) > 0) {
    print(a + b);
  }
}

If there isn't a value you need to capture, then remove the assignment:

void f(int x) {
  if (x case var a when 1 > 0) {
    print(a);
  }
}