dist/m4/ax_cxx_compile_stdcxx.m4

# SPDX-License-Identifier: FSFAP
# ===========================================================================
#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
# ===========================================================================
#
# SYNOPSIS
#
#   AX_CXX_COMPILE_STDCXX(VERSION, [ext|noext], [mandatory|optional])
#
# DESCRIPTION
#
#   Check for baseline language coverage in the compiler for the specified
#   version of the C++ standard.  If necessary, add switches to CXX and
#   CXXCPP to enable support.  VERSION may be '11', '14', '17', or '20' for
#   the respective C++ standard version.
#
#   The second argument, if specified, indicates whether you insist on an
#   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
#   -std=c++11).  If neither is specified, you get whatever works, with
#   preference for no added switch, and then for an extended mode.
#
#   The third argument, if specified 'mandatory' or if left unspecified,
#   indicates that baseline support for the specified C++ standard is
#   required and that the macro should error out if no mode with that
#   support is found.  If specified 'optional', then configuration proceeds
#   regardless, after defining HAVE_CXX${VERSION} if and only if a
#   supporting mode is found.
#
# LICENSE
#
#   Copyright (c) 2008 Benjamin Kosnik <bkoz (a] redhat.com>
#   Copyright (c) 2012 Zack Weinberg <zackw (a] panix.com>
#   Copyright (c) 2013 Roy Stogner <roystgnr (a] ices.utexas.edu>
#   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov (a] google.com>
#   Copyright (c) 2015 Paul Norman <penorman (a] mac.com>
#   Copyright (c) 2015 Moritz Klammler <moritz (a] klammler.eu>
#   Copyright (c) 2016, 2018 Krzesimir Nowak <qdlacz (a] gmail.com>
#   Copyright (c) 2019 Enji Cooper <yaneurabeya (a] gmail.com>
#   Copyright (c) 2020 Jason Merrill <jason (a] redhat.com>
#   Copyright (c) 2021 Jrn Heusipp <osmanx (a] problemloesungsmaschine.de>
#
#   Copying and distribution of this file, with or without modification, are
#   permitted in any medium without royalty provided the copyright notice
#   and this notice are preserved.  This file is offered as-is, without any
#   warranty.

#serial 18

dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
dnl  (serial version number 13).

AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
  m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
        [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
        [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
        [$1], [20], [ax_cxx_compile_alternatives="20"],
        [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
  m4_if([$2], [], [],
        [$2], [ext], [],
        [$2], [noext], [],
        [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX])])dnl
  m4_if([$3], [], [ax_cxx_compile_cxx$1_required=true],
        [$3], [mandatory], [ax_cxx_compile_cxx$1_required=true],
        [$3], [optional], [ax_cxx_compile_cxx$1_required=false],
        [m4_fatal([invalid third argument `$3' to AX_CXX_COMPILE_STDCXX])])
  AC_LANG_PUSH([C++])dnl
  ac_success=no

  m4_if([$2], [], [dnl
    AC_CACHE_CHECK(whether $CXX supports C++$1 features by default,
		   ax_cv_cxx_compile_cxx$1,
      [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
        [ax_cv_cxx_compile_cxx$1=yes],
        [ax_cv_cxx_compile_cxx$1=no])])
    if test x$ax_cv_cxx_compile_cxx$1 = xyes; then
      ac_success=yes
    fi])

  m4_if([$2], [noext], [], [dnl
  if test x$ac_success = xno; then
    for alternative in ${ax_cxx_compile_alternatives}; do
      switch="-std=gnu++${alternative}"
      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
      AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
                     $cachevar,
        [ac_save_CXX="$CXX"
         CXX="$CXX $switch"
         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
          [eval $cachevar=yes],
          [eval $cachevar=no])
         CXX="$ac_save_CXX"])
      if eval test x\$$cachevar = xyes; then
        CXX="$CXX $switch"
        if test -n "$CXXCPP" ; then
          CXXCPP="$CXXCPP $switch"
        fi
        ac_success=yes
        break
      fi
    done
  fi])

  m4_if([$2], [ext], [], [dnl
  if test x$ac_success = xno; then
    dnl HP's aCC needs +std=c++11 according to:
    dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
    dnl Cray's crayCC needs "-h std=c++11"
    dnl MSVC needs -std:c++NN for C++17 and later (default is C++14)
    for alternative in ${ax_cxx_compile_alternatives}; do
      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}" MSVC; do
        if test x"$switch" = xMSVC; then
          dnl AS_TR_SH maps both `:` and `=` to `_` so -std:c++17 would collide
          dnl with -std=c++17.  We suffix the cache variable name with _MSVC to
          dnl avoid this.
          switch=-std:c++${alternative}
          cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_${switch}_MSVC])
        else
          cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
        fi
        AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
                       $cachevar,
          [ac_save_CXX="$CXX"
           CXX="$CXX $switch"
           AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
            [eval $cachevar=yes],
            [eval $cachevar=no])
           CXX="$ac_save_CXX"])
        if eval test x\$$cachevar = xyes; then
          CXX="$CXX $switch"
          if test -n "$CXXCPP" ; then
            CXXCPP="$CXXCPP $switch"
          fi
          ac_success=yes
          break
        fi
      done
      if test x$ac_success = xyes; then
        break
      fi
    done
  fi])
  AC_LANG_POP([C++])
  if test x$ax_cxx_compile_cxx$1_required = xtrue; then
    if test x$ac_success = xno; then
      AC_MSG_ERROR([*** A compiler with support for C++$1 language features is required.])
    fi
  fi
  if test x$ac_success = xno; then
    HAVE_CXX$1=0
    AC_MSG_NOTICE([No compiler with C++$1 support was found])
  else
    HAVE_CXX$1=1
    AC_DEFINE(HAVE_CXX$1,1,
              [define if the compiler supports basic C++$1 syntax])
  fi
  AC_SUBST(HAVE_CXX$1)
])


dnl  Test body for checking C++11 support

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_11],
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
)

dnl  Test body for checking C++14 support

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
)

dnl  Test body for checking C++17 support

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
)

dnl  Test body for checking C++20 support

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_20],
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
  _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
)


dnl  Tests for new features in C++11

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[

// If the compiler admits that it is not ready for C++11, why torture it?
// Hopefully, this will speed up the test.

#ifndef __cplusplus

#error "This is not a C++ compiler"

// MSVC always sets __cplusplus to 199711L in older versions; newer versions
// only set it correctly if /Zc:__cplusplus is specified as well as a
// /std:c++NN switch:
// https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
#elif __cplusplus < 201103L && !defined _MSC_VER

#error "This is not a C++11 compiler"

#else

namespace cxx11
{

  namespace test_static_assert
  {

    template <typename T>
    struct check
    {
      static_assert(sizeof(int) <= sizeof(T), "not big enough");
    };

  }

  namespace test_final_override
  {

    struct Base
    {
      virtual ~Base() {}
      virtual void f() {}
    };

    struct Derived : public Base
    {
      virtual ~Derived() override {}
      virtual void f() override {}
    };

  }

  namespace test_double_right_angle_brackets
  {

    template < typename T >
    struct check {};

    typedef check<void> single_type;
    typedef check<check<void>> double_type;
    typedef check<check<check<void>>> triple_type;
    typedef check<check<check<check<void>>>> quadruple_type;

  }

  namespace test_decltype
  {

    int
    f()
    {
      int a = 1;
      decltype(a) b = 2;
      return a + b;
    }

  }

  namespace test_type_deduction
  {

    template < typename T1, typename T2 >
    struct is_same
    {
      static const bool value = false;
    };

    template < typename T >
    struct is_same<T, T>
    {
      static const bool value = true;
    };

    template < typename T1, typename T2 >
    auto
    add(T1 a1, T2 a2) -> decltype(a1 + a2)
    {
      return a1 + a2;
    }

    int
    test(const int c, volatile int v)
    {
      static_assert(is_same<int, decltype(0)>::value == true, "");
      static_assert(is_same<int, decltype(c)>::value == false, "");
      static_assert(is_same<int, decltype(v)>::value == false, "");
      auto ac = c;
      auto av = v;
      auto sumi = ac + av + 'x';
      auto sumf = ac + av + 1.0;
      static_assert(is_same<int, decltype(ac)>::value == true, "");
      static_assert(is_same<int, decltype(av)>::value == true, "");
      static_assert(is_same<int, decltype(sumi)>::value == true, "");
      static_assert(is_same<int, decltype(sumf)>::value == false, "");
      static_assert(is_same<int, decltype(add(c, v))>::value == true, "");
      return (sumf > 0.0) ? sumi : add(c, v);
    }

  }

  namespace test_noexcept
  {

    int f() { return 0; }
    int g() noexcept { return 0; }

    static_assert(noexcept(f()) == false, "");
    static_assert(noexcept(g()) == true, "");

  }

  namespace test_constexpr
  {

    template < typename CharT >
    unsigned long constexpr
    strlen_c_r(const CharT *const s, const unsigned long acc) noexcept
    {
      return *s ? strlen_c_r(s + 1, acc + 1) : acc;
    }

    template < typename CharT >
    unsigned long constexpr
    strlen_c(const CharT *const s) noexcept
    {
      return strlen_c_r(s, 0UL);
    }

    static_assert(strlen_c("") == 0UL, "");
    static_assert(strlen_c("1") == 1UL, "");
    static_assert(strlen_c("example") == 7UL, "");
    static_assert(strlen_c("another\0example") == 7UL, "");

  }

  namespace test_rvalue_references
  {

    template < int N >
    struct answer
    {
      static constexpr int value = N;
    };

    answer<1> f(int&)       { return answer<1>(); }
    answer<2> f(const int&) { return answer<2>(); }
    answer<3> f(int&&)      { return answer<3>(); }

    void
    test()
    {
      int i = 0;
      const int c = 0;
      static_assert(decltype(f(i))::value == 1, "");
      static_assert(decltype(f(c))::value == 2, "");
      static_assert(decltype(f(0))::value == 3, "");
    }

  }

  namespace test_uniform_initialization
  {

    struct test
    {
      static const int zero {};
      static const int one {1};
    };

    static_assert(test::zero == 0, "");
    static_assert(test::one == 1, "");

  }

  namespace test_lambdas
  {

    void
    test1()
    {
      auto lambda1 = [](){};
      auto lambda2 = lambda1;
      lambda1();
      lambda2();
    }

    int
    test2()
    {
      auto a = [](int i, int j){ return i + j; }(1, 2);
      auto b = []() -> int { return '0'; }();
      auto c = [=](){ return a + b; }();
      auto d = [&](){ return c; }();
      auto e = [a, &b](int x) mutable {
        const auto identity = [](int y){ return y; };
        for (auto i = 0; i < a; ++i)
          a += b--;
        return x + identity(a + b);
      }(0);
      return a + b + c + d + e;
    }

    int
    test3()
    {
      const auto nullary = [](){ return 0; };
      const auto unary = [](int x){ return x; };
      using nullary_t = decltype(nullary);
      using unary_t = decltype(unary);
      const auto higher1st = [](nullary_t f){ return f(); };
      const auto higher2nd = [unary](nullary_t f1){
        return [unary, f1](unary_t f2){ return f2(unary(f1())); };
      };
      return higher1st(nullary) + higher2nd(nullary)(unary);
    }

  }

  namespace test_variadic_templates
  {

    template <int...>
    struct sum;

    template <int N0, int... N1toN>
    struct sum<N0, N1toN...>
    {
      static constexpr auto value = N0 + sum<N1toN...>::value;
    };

    template <>
    struct sum<>
    {
      static constexpr auto value = 0;
    };

    static_assert(sum<>::value == 0, "");
    static_assert(sum<1>::value == 1, "");
    static_assert(sum<23>::value == 23, "");
    static_assert(sum<1, 2>::value == 3, "");
    static_assert(sum<5, 5, 11>::value == 21, "");
    static_assert(sum<2, 3, 5, 7, 11, 13>::value == 41, "");

  }

  // http://stackoverflow.com/questions/13728184/template-aliases-and-sfinae
  // Clang 3.1 fails with headers of libstd++ 4.8.3 when using std::function
  // because of this.
  namespace test_template_alias_sfinae
  {

    struct foo {};

    template<typename T>
    using member = typename T::member_type;

    template<typename T>
    void func(...) {}

    template<typename T>
    void func(member<T>*) {}

    void test();

    void test() { func<foo>(0); }

  }

}  // namespace cxx11

#endif  // __cplusplus >= 201103L

]])


dnl  Tests for new features in C++14

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_14], [[

// If the compiler admits that it is not ready for C++14, why torture it?
// Hopefully, this will speed up the test.

#ifndef __cplusplus

#error "This is not a C++ compiler"

#elif __cplusplus < 201402L && !defined _MSC_VER

#error "This is not a C++14 compiler"

#else

namespace cxx14
{

  namespace test_polymorphic_lambdas
  {

    int
    test()
    {
      const auto lambda = [](auto&&... args){
        const auto istiny = [](auto x){
          return (sizeof(x) == 1UL) ? 1 : 0;
        };
        const int aretiny[] = { istiny(args)... };
        return aretiny[0];
      };
      return lambda(1, 1L, 1.0f, '1');
    }

  }

  namespace test_binary_literals
  {

    constexpr auto ivii = 0b0000000000101010;
    static_assert(ivii == 42, "wrong value");

  }

  namespace test_generalized_constexpr
  {

    template < typename CharT >
    constexpr unsigned long
    strlen_c(const CharT *const s) noexcept
    {
      auto length = 0UL;
      for (auto p = s; *p; ++p)
        ++length;
      return length;
    }

    static_assert(strlen_c("") == 0UL, "");
    static_assert(strlen_c("x") == 1UL, "");
    static_assert(strlen_c("test") == 4UL, "");
    static_assert(strlen_c("another\0test") == 7UL, "");

  }

  namespace test_lambda_init_capture
  {

    int
    test()
    {
      auto x = 0;
      const auto lambda1 = [a = x](int b){ return a + b; };
      const auto lambda2 = [a = lambda1(x)](){ return a; };
      return lambda2();
    }

  }

  namespace test_digit_separators
  {

    constexpr auto ten_million = 100'000'000;
    static_assert(ten_million == 100000000, "");

  }

  namespace test_return_type_deduction
  {

    auto f(int& x) { return x; }
    decltype(auto) g(int& x) { return x; }

    template < typename T1, typename T2 >
    struct is_same
    {
      static constexpr auto value = false;
    };

    template < typename T >
    struct is_same<T, T>
    {
      static constexpr auto value = true;
    };

    int
    test()
    {
      auto x = 0;
      static_assert(is_same<int, decltype(f(x))>::value, "");
      static_assert(is_same<int&, decltype(g(x))>::value, "");
      return x;
    }

  }

}  // namespace cxx14

#endif  // __cplusplus >= 201402L

]])


dnl  Tests for new features in C++17

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[

// If the compiler admits that it is not ready for C++17, why torture it?
// Hopefully, this will speed up the test.

#ifndef __cplusplus

#error "This is not a C++ compiler"

#elif __cplusplus < 201703L && !defined _MSC_VER

#error "This is not a C++17 compiler"

#else

#include <initializer_list>
#include <utility>
#include <type_traits>

namespace cxx17
{

  namespace test_constexpr_lambdas
  {

    constexpr int foo = [](){return 42;}();

  }

  namespace test::nested_namespace::definitions
  {

  }

  namespace test_fold_expression
  {

    template<typename... Args>
    int multiply(Args... args)
    {
      return (args * ... * 1);
    }

    template<typename... Args>
    bool all(Args... args)
    {
      return (args && ...);
    }

  }

  namespace test_extended_static_assert
  {

    static_assert (true);

  }

  namespace test_auto_brace_init_list
  {

    auto foo = {5};
    auto bar {5};

    static_assert(std::is_same<std::initializer_list<int>, decltype(foo)>::value);
    static_assert(std::is_same<int, decltype(bar)>::value);
  }

  namespace test_typename_in_template_template_parameter
  {

    template<template<typename> typename X> struct D;

  }

  namespace test_fallthrough_nodiscard_maybe_unused_attributes
  {

    int f1()
    {
      return 42;
    }

    [[nodiscard]] int f2()
    {
      [[maybe_unused]] auto unused = f1();

      switch (f1())
      {
      case 17:
        f1();
        [[fallthrough]];
      case 42:
        f1();
      }
      return f1();
    }

  }

  namespace test_extended_aggregate_initialization
  {

    struct base1
    {
      int b1, b2 = 42;
    };

    struct base2
    {
      base2() {
        b3 = 42;
      }
      int b3;
    };

    struct derived : base1, base2
    {
        int d;
    };

    derived d1 {{1, 2}, {}, 4};  // full initialization
    derived d2 {{}, {}, 4};      // value-initialized bases

  }

  namespace test_general_range_based_for_loop
  {

    struct iter
    {
      int i;

      int& operator* ()
      {
        return i;
      }

      const int& operator* () const
      {
        return i;
      }

      iter& operator++()
      {
        ++i;
        return *this;
      }
    };

    struct sentinel
    {
      int i;
    };

    bool operator== (const iter& i, const sentinel& s)
    {
      return i.i == s.i;
    }

    bool operator!= (const iter& i, const sentinel& s)
    {
      return !(i == s);
    }

    struct range
    {
      iter begin() const
      {
        return {0};
      }

      sentinel end() const
      {
        return {5};
      }
    };

    void f()
    {
      range r {};

      for (auto i : r)
      {
        [[maybe_unused]] auto v = i;
      }
    }

  }

  namespace test_lambda_capture_asterisk_this_by_value
  {

    struct t
    {
      int i;
      int foo()
      {
        return [*this]()
        {
          return i;
        }();
      }
    };

  }

  namespace test_enum_class_construction
  {

    enum class byte : unsigned char
    {};

    byte foo {42};

  }

  namespace test_constexpr_if
  {

    template <bool cond>
    int f ()
    {
      if constexpr(cond)
      {
        return 13;
      }
      else
      {
        return 42;
      }
    }

  }

  namespace test_selection_statement_with_initializer
  {

    int f()
    {
      return 13;
    }

    int f2()
    {
      if (auto i = f(); i > 0)
      {
        return 3;
      }

      switch (auto i = f(); i + 4)
      {
      case 17:
        return 2;

      default:
        return 1;
      }
    }

  }

  namespace test_template_argument_deduction_for_class_templates
  {

    template <typename T1, typename T2>
    struct pair
    {
      pair (T1 p1, T2 p2)
        : m1 {p1},
          m2 {p2}
      {}

      T1 m1;
      T2 m2;
    };

    void f()
    {
      [[maybe_unused]] auto p = pair{13, 42u};
    }

  }

  namespace test_non_type_auto_template_parameters
  {

    template <auto n>
    struct B
    {};

    B<5> b1;
    B<'a'> b2;

  }

  namespace test_structured_bindings
  {

    int arr[2] = { 1, 2 };
    std::pair<int, int> pr = { 1, 2 };

    auto f1() -> int(&)[2]
    {
      return arr;
    }

    auto f2() -> std::pair<int, int>&
    {
      return pr;
    }

    struct S
    {
      int x1 : 2;
      volatile double y1;
    };

    S f3()
    {
      return {};
    }

    auto [ x1, y1 ] = f1();
    auto& [ xr1, yr1 ] = f1();
    auto [ x2, y2 ] = f2();
    auto& [ xr2, yr2 ] = f2();
    const auto [ x3, y3 ] = f3();

  }

  namespace test_exception_spec_type_system
  {

    struct Good {};
    struct Bad {};

    void g1() noexcept;
    void g2();

    template<typename T>
    Bad
    f(T*, T*);

    template<typename T1, typename T2>
    Good
    f(T1*, T2*);

    static_assert (std::is_same_v<Good, decltype(f(g1, g2))>);

  }

  namespace test_inline_variables
  {

    template<class T> void f(T)
    {}

    template<class T> inline T g(T)
    {
      return T{};
    }

    template<> inline void f<>(int)
    {}

    template<> int g<>(int)
    {
      return 5;
    }

  }

}  // namespace cxx17

#endif  // __cplusplus < 201703L && !defined _MSC_VER

]])


dnl  Tests for new features in C++20

m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_20], [[

#ifndef __cplusplus

#error "This is not a C++ compiler"

#elif __cplusplus < 202002L && !defined _MSC_VER

#error "This is not a C++20 compiler"

#else

#include <version>

namespace cxx20
{

// As C++20 supports feature test macros in the standard, there is no
// immediate need to actually test for feature availability on the
// Autoconf side.

}  // namespace cxx20

#endif  // __cplusplus < 202002L && !defined _MSC_VER

]])