libcruft-util/concepts.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2020, Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include <type_traits>
#include <iterator>
#include <utility>
#include <functional>


///////////////////////////////////////////////////////////////////////////////
/// A minimal implementation of the standard concept library for use with
/// compilers that lack it.
///
/// Portions of this code are adapted from the standard and as such do not
/// fall under the top copyright notice.
///
/// clang#xxx: Remove me when clang includes an implementation of these.
namespace cruft::concepts {
    template <typename T>
    concept floating_point = std::is_floating_point_v<T>;

    template <typename T>
    concept integral = std::is_integral_v<T>;

    template <typename T>
    concept signed_integral = integral<T> && std::is_signed_v<T>;

    template <typename T>
    concept unsigned_integral = integral<T> && std::is_unsigned_v<T>;

    template <typename T>
    concept destructible = std::is_nothrow_destructible_v<T>;

    template <class T, typename ...ArgsT>
    concept constructible_from = destructible<T> && std::is_constructible_v<T, ArgsT...>;

    template <class T>
    concept default_constructible = constructible_from<T>;

    template <class T>
    concept copy_constructible = std::is_copy_constructible_v<T>;

    template <class T>
    concept move_constructible = std::is_move_constructible_v<T>;


    template <typename A, typename B>
    concept same_as = std::is_same_v<A, B> and std::is_same_v<B, A>;


    template <typename From, typename To>
    concept convertible_to =
        std::is_convertible_v<From, To> &&
        requires (From (&f)())
    {
        static_cast<To> (f());
    };


    template <
        typename LHS,
        typename RHS
    >
    concept assignable_from =
        std::is_lvalue_reference_v<LHS> &&
        // clang#xxx: common_reference_t is too annoying to implement as
        // a temporary fix for this temporary fix. Reintroduce this when
        // clang knows about common_reference_t.
        // std::common_reference_with<
        //     std::remove_reference_t<LHS> const&,
        //     std::remove_reference_t<RHS> const&
        // > &&
        requires(LHS lhs, RHS&& rhs)
    {
        { lhs = std::forward<RHS>(rhs) } -> same_as<LHS>;
    };


    template <typename T>
    concept swappable = requires (T &&a, T &&b) { std::swap (a, b); };


    template < class T >
    concept movable =
      std::is_object_v<T> &&
      move_constructible<T> &&
      assignable_from<T&, T> &&
      swappable<T>;


    template <typename T>
    concept boolean =
        movable<std::remove_cvref_t<T>> &&
        requires (
            std::remove_reference_t<T> const& b1,
            std::remove_reference_t<T> const& b2,
            bool const a
        )
    {
        {  b1 } -> convertible_to<bool>;
        { !b1 } -> convertible_to<bool>;

        { b1 && b2 } -> same_as<bool>;
        { b1 &&  a } -> same_as<bool>;
        {  a && b2 } -> same_as<bool>;
        { b1 || b2 } -> same_as<bool>;
        { b1 ||  a } -> same_as<bool>;
        {  a || b2 } -> same_as<bool>;

        { b1 == b2 } -> convertible_to<bool>;
        { b1 ==  a } -> convertible_to<bool>;
        {  a == b2 } -> convertible_to<bool>;
        { b1 != b2 } -> convertible_to<bool>;
        { b1 !=  a } -> convertible_to<bool>;
        {  a != b2 } -> convertible_to<bool>;
    };


    template <typename T>
    concept equality_comparable =
        requires (
            std::remove_reference_t<T> const &a,
            std::remove_reference_t<T> const &b
        )
    {
        { a == b } -> boolean;
        { a != b } -> boolean;
        { b == a } -> boolean;
        { b != a } -> boolean;
    };


    template <typename FunctionT, typename ...ArgsT>
    concept invocable =
        requires (FunctionT &&function, ArgsT &&...args)
    {
        std::invoke (
            std::forward<FunctionT> (function),
            std::forward<ArgsT> (args)...
        );
    };

    template <typename FunctionT, typename ...ArgsT>
    concept regular_invocable = invocable<FunctionT, ArgsT...>;

    template <typename FunctionT, typename ...ArgsT>
    concept predicate =
        regular_invocable<FunctionT, ArgsT...> &&
        boolean<std::invoke_result_t<FunctionT, ArgsT...>>;
}


///////////////////////////////////////////////////////////////////////////////
// C++ named requirements
namespace cruft::concepts {
    /// Corresponds to the "Container" named requirement.
    template <class T>
    concept container =
        default_constructible<T> &&
        copy_constructible<T> &&
        move_constructible<T> &&
        destructible<T> &&
        equality_comparable<T> &&
        requires (T a, T b)
    {
        typename T::value_type;
        typename T::reference;
        typename T::const_reference;
        typename T::iterator;
        typename T::const_iterator;
        typename T::difference_type;
        typename T::size_type;

        { a = b } -> same_as<T&>;
        { a = std::move (b) } -> same_as<T&>;

        { a.begin  () } -> same_as<typename T::iterator>;
        { a.end    () } -> same_as<typename T::iterator>;
        { a.cbegin () } -> same_as<typename T::const_iterator>;
        { a.cend   () } -> same_as<typename T::const_iterator>;

        { a.swap (b)  } -> same_as<void>;
        { std::swap (a, b) } -> same_as<void>;

        { a.size () } -> same_as<typename T::size_type>;
        { a.max_size () } -> same_as<typename T::size_type>;
        { a.empty () } -> boolean;
    };


    template <typename T>
    concept move_assignable =
        requires (T a, T b)
    {
        { a = std::move (b) } -> same_as<T&>;
    };


    template <typename T>
    concept copy_assignable =
        move_assignable<T> &&
        requires (T a, T b)
    {
        { a = b } -> same_as<T&>;
    };


    template <typename T>
    concept legacy_iterator =
        copy_constructible<T> &&
        copy_assignable<T> &&
        destructible<T> &&
        swappable<T> &&
        requires (T t)
    {
        typename std::iterator_traits<T>::value_type;
        typename std::iterator_traits<T>::difference_type;
        typename std::iterator_traits<T>::reference;
        typename std::iterator_traits<T>::pointer;
        typename std::iterator_traits<T>::iterator_category;

        {  *t };
        { ++t } -> same_as<T&>;
    };

    template <typename T>
    concept legacy_input_iterator =
        legacy_iterator<T> &&
        equality_comparable<T> &&
        requires (T a, T b)
    {
        typename std::iterator_traits<T>::reference;
        typename std::iterator_traits<T>::value_type;
        { a != b } -> boolean;

        { ++a } -> same_as<T&>;
        { a++ };
    };
}


///////////////////////////////////////////////////////////////////////////////
// Trivial wrappers around traits
namespace cruft::concepts {
    template <typename T>
    concept arithmetic = std::is_arithmetic_v<T>;

    template <typename T>
    concept scalar = std::is_scalar_v<T>;

    template <typename T>
    concept enumeration = std::is_enum_v<T>;

    template <typename T>
    concept pointer = std::is_pointer_v<T>;

    template <typename T>
    concept reference = std::is_reference_v<T>;
}


///////////////////////////////////////////////////////////////////////////////
// Some handy non-standard concepts
#include <tuple>

namespace cruft::concepts {
    /// Tests if the type has all typedefs required for use with
    /// std::iterator_traits.
    template <typename T>
    concept supports_iterator_traits = requires
    {
        typename T::difference_type;
        typename T::value_type;
        typename T::reference;
        typename T::iterator_category;

        // C++20 defines `pointer` as void if it's not present.
        #if __cplusplus <= 201703L
        typename T::pointer;
        #endif
    };




    /// A type that supports arithmetic operators.
    template <typename T>
    concept numeric = requires (T t)
    {
        {  t * t } -> convertible_to<T>;
        {  t / t } -> convertible_to<T>;
        {  t - t } -> convertible_to<T>;
        {  t + t } -> convertible_to<T>;
    };


    /// Anything that can be looped over using begin/end
    template <typename T>
    concept iterable = requires (T t)
    {
        { std::begin  (t) } -> legacy_iterator;
        { std::end    (t) } -> legacy_iterator;
        { std::cbegin (t) } -> legacy_iterator;
        { std::cend   (t) } -> legacy_iterator;
    };


    /// A class that supports tuple manipulators.
    template <typename T>
    concept tuple = requires (T a, T b)
    {
        // We should be checking this, but it fails for zero length tuples and
        // it's kind of a low priority right now.
        // { std::tuple_element<0,T> {} };
        { std::tuple_size<T>::value } -> convertible_to<std::size_t>;
        { std::tuple_cat (a, b) };
    };
}