avoid_contradictory_expressions

v0.4.0 Warning Control Flow

Warns when a logical AND (&&) expression contains contradictory comparisons on the same variable, resulting in a condition that always evaluates to false. For example, x == 3 && x == 4 can never be true because x cannot equal both values simultaneously.

Why use this rule

Contradictory conditions create unreachable code that silently does nothing. These are almost always bugs — typically from copy-paste errors where one operand was not updated, or from refactoring that accidentally introduced conflicting constraints. Catching them at analysis time prevents hard-to-debug logic errors.

See also: Effective Dart: Usage

Don’t

void bad(int x, int y) {
  // x cannot equal both 3 and 4
  if (x == 3 && x == 4) {
    print('unreachable');
  }

  // Impossible range — x can't be less than 4 AND greater than 4
  if (x < 4 && x > 4) {
    print('unreachable');
  }

  // Equality contradicts inequality
  if (x == 2 && x != 2) {
    print('unreachable');
  }

  // Same comparison with variable, opposite operators
  if (x == y && x != y) {
    print('unreachable');
  }
}

Do

void good(int x, int y) {
  // Uses OR — x can be 3 or 4
  if (x == 3 || x == 4) {
    print('ok');
  }

  // Consistent range — x between 2 and 4
  if (x < 4 && x > 2) {
    print('ok');
  }

  // Different variables
  if (x == 3 && y == 4) {
    print('ok');
  }
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      avoid_contradictory_expressions: false