71 lines
2.7 KiB
ReStructuredText
71 lines
2.7 KiB
ReStructuredText
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.. title:: clang-tidy - modernize-use-constraints
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modernize-use-constraints
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=========================
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Replace ``std::enable_if`` with C++20 requires clauses.
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``std::enable_if`` is a SFINAE mechanism for selecting the desired function or
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class template based on type traits or other requirements. ``enable_if`` changes
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the meta-arity of the template, and has other
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`adverse side effects
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<https://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0225r0.html>`_
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in the code. C++20 introduces concepts and constraints as a cleaner language
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provided solution to achieve the same outcome.
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This check finds some common ``std::enable_if`` patterns that can be replaced
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by C++20 requires clauses. The tool can replace some of these patterns
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automatically, otherwise, the tool will emit a diagnostic without a
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replacement. The tool can detect the following ``std::enable_if`` patterns
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1. ``std::enable_if`` in the return type of a function
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2. ``std::enable_if`` as the trailing template parameter for function templates
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Other uses, for example, in class templates for function parameters, are not
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currently supported by this tool. Other variants such as ``boost::enable_if``
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are not currently supported by this tool.
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Below are some examples of code using ``std::enable_if``.
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.. code-block:: c++
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// enable_if in function return type
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template <typename T>
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std::enable_if_t<T::some_trait, int> only_if_t_has_the_trait() { ... }
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// enable_if in the trailing template parameter
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template <typename T, std::enable_if_t<T::some_trait, int> = 0>
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void another_version() { ... }
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template <typename T>
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typename std::enable_if<T::some_value, Obj>::type existing_constraint() requires (T::another_value) {
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return Obj{};
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}
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template <typename T, std::enable_if_t<T::some_trait, int> = 0>
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struct my_class {};
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The tool will replace the above code with,
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.. code-block:: c++
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// warning: use C++20 requires constraints instead of enable_if [modernize-use-constraints]
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template <typename T>
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int only_if_t_has_the_trait() requires T::some_trait { ... }
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// warning: use C++20 requires constraints instead of enable_if [modernize-use-constraints]
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template <typename T>
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void another_version() requires T::some_trait { ... }
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// The tool will emit a diagnostic for the following, but will
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// not attempt to replace the code.
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// warning: use C++20 requires constraints instead of enable_if [modernize-use-constraints]
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template <typename T>
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typename std::enable_if<T::some_value, Obj>::type existing_constraint() requires (T::another_value) {
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return Obj{};
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}
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// The tool will not emit a diagnostic or attempt to replace the code.
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template <typename T, std::enable_if_t<T::some_trait, int> = 0>
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struct my_class {};
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