clang-tidy - bugprone-incorrect-enable-if — Extra Clang Tools 22.1.3 documentation

# Extra Clang Tools 22.1.3 documentation

## clang-tidy - bugprone-incorrect-enable-if

# bugprone-incorrect-enable-if

Detects incorrect usages of `std::enable_if` that don’t name the nested `type` type.

In C++11 introduced `std::enable_if` as a convenient way to leverage SFINAE. One form of using `std::enable_if` is to declare an unnamed template type parameter with a default type equal to `typename std::enable_if<condition>::type`. If the author forgets to name the nested type `type`, then the code will always consider the candidate template even if the condition is not met.

Below are some examples of code using `std::enable_if` correctly and incorrect examples that this check flags.

    template <typename T, typename = typename std::enable_if<T::some_trait>::type>
    void valid_usage() { ... }

    template <typename T, typename = std::enable_if_t<T::some_trait>>
    void valid_usage_with_trait_helpers() { ... }

    // The below code is not a correct application of SFINAE. Even if
    // T::some_trait is not true, the function will still be considered in the
    // set of function candidates. It can either incorrectly select the function
    // when it should not be a candidates, and/or lead to hard compile errors
    // if the body of the template does not compile if the condition is not
    // satisfied.
    template <typename T, typename = std::enable_if<T::some_trait>>
    void invalid_usage() { ... }

    // The tool suggests the following replacement for 'invalid_usage':
    template <typename T, typename = typename std::enable_if<T::some_trait>::type>
    void fixed_invalid_usage() { ... }

C++14 introduced the trait helper `std::enable_if_t` which reduces the likelihood of this error. C++20 introduces constraints, which generally supersede the use of `std::enable_if`. See modernize-type-traits for another tool that will replace `std::enable_if` with `std::enable_if_t`, and see modernize-use-constraints for another tool that replaces `std::enable_if` with C++20 constraints. Consider these newer mechanisms where possible.