avoid_misused_test_matchers

v0.4.0 Warning Testing Rules

Flags expect() calls where the matcher is incompatible with the actual value’s type. For example, using isNull on a non-nullable type, isEmpty on an int, or isList on a String. Misused matchers can cause tests to always pass (hiding bugs) or always fail (making assertions useless).

Why use this rule

A mismatched matcher silently undermines your test suite. expect(42, isNull) will always fail because int is non-nullable, while expect(42, isNotNull) will always pass, giving a false sense of coverage. This rule catches these type mismatches at analysis time before they reach CI.

See also: test package - Matchers

Don’t

expect(someString, isList);       // String is not a List
expect(someNumber, isEmpty);      // int has no isEmpty
expect(someNumber, isNull);       // int is non-nullable
expect(someNumber, isNotNull);    // always true, redundant
expect(someNumber, hasLength(1)); // int has no length
expect(someString, isZero);       // String is not a num
expect(someNumber, isTrue);       // int is not a bool
expect(someString, isMap);        // String is not a Map
expect(true, isNegative);         // bool is not a num

Do

expect(someList, isList);
expect(<String, int>{}, isMap);
expect(nullableValue, isNull);
expect(nullableValue, isNotNull);
expect(someList, isEmpty);
expect(someString, isEmpty);
expect(someList, hasLength(3));
expect(someNumber, isZero);
expect(5, isPositive);
expect(true, isTrue);
expect(false, isFalse);

Configuration

This rule is in the core preset, so it is on with preset: core, preset: recommended or preset: opinionated.

To turn it off:

many_lints.yaml

rules:
  avoid_misused_test_matchers: false

To keep the rule on but skip certain paths, use per-rule exclude.

Related rules

• prefer_test_matchers — Prefer using a Matcher instead of a literal value in expect().
• prefer_expect_later — Use ‘expectLater’ instead of ‘expect’ when testing Futures.
• format_test_name — Hold test descriptions to a house pattern.
• prefer_correct_test_file_name — Name test files so the runner actually runs them.