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1# ===========================================================================
2#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
3# ===========================================================================
4#
5# SYNOPSIS
6#
7#   AX_CXX_COMPILE_STDCXX(VERSION, [ext|noext], [mandatory|optional])
8#
9# DESCRIPTION
10#
11#   Check for baseline language coverage in the compiler for the specified
12#   version of the C++ standard.  If necessary, add switches to CXX and
13#   CXXCPP to enable support.  VERSION may be '11', '14', '17', or '20' for
14#   the respective C++ standard version.
15#
16#   The second argument, if specified, indicates whether you insist on an
17#   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
18#   -std=c++11).  If neither is specified, you get whatever works, with
19#   preference for no added switch, and then for an extended mode.
20#
21#   The third argument, if specified 'mandatory' or if left unspecified,
22#   indicates that baseline support for the specified C++ standard is
23#   required and that the macro should error out if no mode with that
24#   support is found.  If specified 'optional', then configuration proceeds
25#   regardless, after defining HAVE_CXX${VERSION} if and only if a
26#   supporting mode is found.
27#
28# LICENSE
29#
30#   Copyright (c) 2008 Benjamin Kosnik <bkoz@redhat.com>
31#   Copyright (c) 2012 Zack Weinberg <zackw@panix.com>
32#   Copyright (c) 2013 Roy Stogner <roystgnr@ices.utexas.edu>
33#   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov@google.com>
34#   Copyright (c) 2015 Paul Norman <penorman@mac.com>
35#   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
36#   Copyright (c) 2016, 2018 Krzesimir Nowak <qdlacz@gmail.com>
37#   Copyright (c) 2019 Enji Cooper <yaneurabeya@gmail.com>
38#   Copyright (c) 2020 Jason Merrill <jason@redhat.com>
39#   Copyright (c) 2021 Jörn Heusipp <osmanx@problemloesungsmaschine.de>
40#
41#   Copying and distribution of this file, with or without modification, are
42#   permitted in any medium without royalty provided the copyright notice
43#   and this notice are preserved.  This file is offered as-is, without any
44#   warranty.
45
46#serial 14
47
48dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
49dnl  (serial version number 13).
50
51AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
52  m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
53        [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
54        [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
55        [$1], [20], [ax_cxx_compile_alternatives="20"],
56        [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
57  m4_if([$2], [], [],
58        [$2], [ext], [],
59        [$2], [noext], [],
60        [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX])])dnl
61  m4_if([$3], [], [ax_cxx_compile_cxx$1_required=true],
62        [$3], [mandatory], [ax_cxx_compile_cxx$1_required=true],
63        [$3], [optional], [ax_cxx_compile_cxx$1_required=false],
64        [m4_fatal([invalid third argument `$3' to AX_CXX_COMPILE_STDCXX])])
65  AC_LANG_PUSH([C++])dnl
66  ac_success=no
67
68  m4_if([$2], [], [dnl
69    AC_CACHE_CHECK(whether $CXX supports C++$1 features by default,
70       ax_cv_cxx_compile_cxx$1,
71      [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
72        [ax_cv_cxx_compile_cxx$1=yes],
73        [ax_cv_cxx_compile_cxx$1=no])])
74    if test x$ax_cv_cxx_compile_cxx$1 = xyes; then
75      ac_success=yes
76    fi])
77
78  m4_if([$2], [noext], [], [dnl
79  if test x$ac_success = xno; then
80    for alternative in ${ax_cxx_compile_alternatives}; do
81      switch="-std=gnu++${alternative}"
82      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
83      AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
84                     $cachevar,
85        [ac_save_CXX="$CXX"
86         CXX="$CXX $switch"
87         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
88          [eval $cachevar=yes],
89          [eval $cachevar=no])
90         CXX="$ac_save_CXX"])
91      if eval test x\$$cachevar = xyes; then
92        CXX="$CXX $switch"
93        if test -n "$CXXCPP" ; then
94          CXXCPP="$CXXCPP $switch"
95        fi
96        ac_success=yes
97        break
98      fi
99    done
100  fi])
101
102  m4_if([$2], [ext], [], [dnl
103  if test x$ac_success = xno; then
104    dnl HP's aCC needs +std=c++11 according to:
105    dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
106    dnl Cray's crayCC needs "-h std=c++11"
107    for alternative in ${ax_cxx_compile_alternatives}; do
108      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}"; do
109        cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
110        AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
111                       $cachevar,
112          [ac_save_CXX="$CXX"
113           CXX="$CXX $switch"
114           AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
115            [eval $cachevar=yes],
116            [eval $cachevar=no])
117           CXX="$ac_save_CXX"])
118        if eval test x\$$cachevar = xyes; then
119          CXX="$CXX $switch"
120          if test -n "$CXXCPP" ; then
121            CXXCPP="$CXXCPP $switch"
122          fi
123          ac_success=yes
124          break
125        fi
126      done
127      if test x$ac_success = xyes; then
128        break
129      fi
130    done
131  fi])
132  AC_LANG_POP([C++])
133  if test x$ax_cxx_compile_cxx$1_required = xtrue; then
134    if test x$ac_success = xno; then
135      AC_MSG_ERROR([*** A compiler with support for C++$1 language features is required.])
136    fi
137  fi
138  if test x$ac_success = xno; then
139    HAVE_CXX$1=0
140    AC_MSG_NOTICE([No compiler with C++$1 support was found])
141  else
142    HAVE_CXX$1=1
143    AC_DEFINE(HAVE_CXX$1,1,
144              [define if the compiler supports basic C++$1 syntax])
145  fi
146  AC_SUBST(HAVE_CXX$1)
147])
148
149
150dnl  Test body for checking C++11 support
151
152m4_define([_AX_CXX_COMPILE_STDCXX_testbody_11],
153  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
154)
155
156dnl  Test body for checking C++14 support
157
158m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
159  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
160  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
161)
162
163dnl  Test body for checking C++17 support
164
165m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
166  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
167  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
168  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
169)
170
171dnl  Test body for checking C++20 support
172
173m4_define([_AX_CXX_COMPILE_STDCXX_testbody_20],
174  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
175  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
176  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
177  _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
178)
179
180
181dnl  Tests for new features in C++11
182
183m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[
184
185// If the compiler admits that it is not ready for C++11, why torture it?
186// Hopefully, this will speed up the test.
187
188#ifndef __cplusplus
189
190#error "This is not a C++ compiler"
191
192#elif __cplusplus < 201103L
193
194#error "This is not a C++11 compiler"
195
196#else
197
198namespace cxx11
199{
200
201  namespace test_static_assert
202  {
203
204    template <typename T>
205    struct check
206    {
207      static_assert(sizeof(int) <= sizeof(T), "not big enough");
208    };
209
210  }
211
212  namespace test_final_override
213  {
214
215    struct Base
216    {
217      virtual ~Base() {}
218      virtual void f() {}
219    };
220
221    struct Derived : public Base
222    {
223      virtual ~Derived() override {}
224      virtual void f() override {}
225    };
226
227  }
228
229  namespace test_double_right_angle_brackets
230  {
231
232    template < typename T >
233    struct check {};
234
235    typedef check<void> single_type;
236    typedef check<check<void>> double_type;
237    typedef check<check<check<void>>> triple_type;
238    typedef check<check<check<check<void>>>> quadruple_type;
239
240  }
241
242  namespace test_decltype
243  {
244
245    int
246    f()
247    {
248      int a = 1;
249      decltype(a) b = 2;
250      return a + b;
251    }
252
253  }
254
255  namespace test_type_deduction
256  {
257
258    template < typename T1, typename T2 >
259    struct is_same
260    {
261      static const bool value = false;
262    };
263
264    template < typename T >
265    struct is_same<T, T>
266    {
267      static const bool value = true;
268    };
269
270    template < typename T1, typename T2 >
271    auto
272    add(T1 a1, T2 a2) -> decltype(a1 + a2)
273    {
274      return a1 + a2;
275    }
276
277    int
278    test(const int c, volatile int v)
279    {
280      static_assert(is_same<int, decltype(0)>::value == true, "");
281      static_assert(is_same<int, decltype(c)>::value == false, "");
282      static_assert(is_same<int, decltype(v)>::value == false, "");
283      auto ac = c;
284      auto av = v;
285      auto sumi = ac + av + 'x';
286      auto sumf = ac + av + 1.0;
287      static_assert(is_same<int, decltype(ac)>::value == true, "");
288      static_assert(is_same<int, decltype(av)>::value == true, "");
289      static_assert(is_same<int, decltype(sumi)>::value == true, "");
290      static_assert(is_same<int, decltype(sumf)>::value == false, "");
291      static_assert(is_same<int, decltype(add(c, v))>::value == true, "");
292      return (sumf > 0.0) ? sumi : add(c, v);
293    }
294
295  }
296
297  namespace test_noexcept
298  {
299
300    int f() { return 0; }
301    int g() noexcept { return 0; }
302
303    static_assert(noexcept(f()) == false, "");
304    static_assert(noexcept(g()) == true, "");
305
306  }
307
308  namespace test_constexpr
309  {
310
311    template < typename CharT >
312    unsigned long constexpr
313    strlen_c_r(const CharT *const s, const unsigned long acc) noexcept
314    {
315      return *s ? strlen_c_r(s + 1, acc + 1) : acc;
316    }
317
318    template < typename CharT >
319    unsigned long constexpr
320    strlen_c(const CharT *const s) noexcept
321    {
322      return strlen_c_r(s, 0UL);
323    }
324
325    static_assert(strlen_c("") == 0UL, "");
326    static_assert(strlen_c("1") == 1UL, "");
327    static_assert(strlen_c("example") == 7UL, "");
328    static_assert(strlen_c("another\0example") == 7UL, "");
329
330  }
331
332  namespace test_rvalue_references
333  {
334
335    template < int N >
336    struct answer
337    {
338      static constexpr int value = N;
339    };
340
341    answer<1> f(int&)       { return answer<1>(); }
342    answer<2> f(const int&) { return answer<2>(); }
343    answer<3> f(int&&)      { return answer<3>(); }
344
345    void
346    test()
347    {
348      int i = 0;
349      const int c = 0;
350      static_assert(decltype(f(i))::value == 1, "");
351      static_assert(decltype(f(c))::value == 2, "");
352      static_assert(decltype(f(0))::value == 3, "");
353    }
354
355  }
356
357  namespace test_uniform_initialization
358  {
359
360    struct test
361    {
362      static const int zero {};
363      static const int one {1};
364    };
365
366    static_assert(test::zero == 0, "");
367    static_assert(test::one == 1, "");
368
369  }
370
371  namespace test_lambdas
372  {
373
374    void
375    test1()
376    {
377      auto lambda1 = [](){};
378      auto lambda2 = lambda1;
379      lambda1();
380      lambda2();
381    }
382
383    int
384    test2()
385    {
386      auto a = [](int i, int j){ return i + j; }(1, 2);
387      auto b = []() -> int { return '0'; }();
388      auto c = [=](){ return a + b; }();
389      auto d = [&](){ return c; }();
390      auto e = [a, &b](int x) mutable {
391        const auto identity = [](int y){ return y; };
392        for (auto i = 0; i < a; ++i)
393          a += b--;
394        return x + identity(a + b);
395      }(0);
396      return a + b + c + d + e;
397    }
398
399    int
400    test3()
401    {
402      const auto nullary = [](){ return 0; };
403      const auto unary = [](int x){ return x; };
404      using nullary_t = decltype(nullary);
405      using unary_t = decltype(unary);
406      const auto higher1st = [](nullary_t f){ return f(); };
407      const auto higher2nd = [unary](nullary_t f1){
408        return [unary, f1](unary_t f2){ return f2(unary(f1())); };
409      };
410      return higher1st(nullary) + higher2nd(nullary)(unary);
411    }
412
413  }
414
415  namespace test_variadic_templates
416  {
417
418    template <int...>
419    struct sum;
420
421    template <int N0, int... N1toN>
422    struct sum<N0, N1toN...>
423    {
424      static constexpr auto value = N0 + sum<N1toN...>::value;
425    };
426
427    template <>
428    struct sum<>
429    {
430      static constexpr auto value = 0;
431    };
432
433    static_assert(sum<>::value == 0, "");
434    static_assert(sum<1>::value == 1, "");
435    static_assert(sum<23>::value == 23, "");
436    static_assert(sum<1, 2>::value == 3, "");
437    static_assert(sum<5, 5, 11>::value == 21, "");
438    static_assert(sum<2, 3, 5, 7, 11, 13>::value == 41, "");
439
440  }
441
442  // http://stackoverflow.com/questions/13728184/template-aliases-and-sfinae
443  // Clang 3.1 fails with headers of libstd++ 4.8.3 when using std::function
444  // because of this.
445  namespace test_template_alias_sfinae
446  {
447
448    struct foo {};
449
450    template<typename T>
451    using member = typename T::member_type;
452
453    template<typename T>
454    void func(...) {}
455
456    template<typename T>
457    void func(member<T>*) {}
458
459    void test();
460
461    void test() { func<foo>(0); }
462
463  }
464
465}  // namespace cxx11
466
467#endif  // __cplusplus >= 201103L
468
469]])
470
471
472dnl  Tests for new features in C++14
473
474m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_14], [[
475
476// If the compiler admits that it is not ready for C++14, why torture it?
477// Hopefully, this will speed up the test.
478
479#ifndef __cplusplus
480
481#error "This is not a C++ compiler"
482
483#elif __cplusplus < 201402L
484
485#error "This is not a C++14 compiler"
486
487#else
488
489namespace cxx14
490{
491
492  namespace test_polymorphic_lambdas
493  {
494
495    int
496    test()
497    {
498      const auto lambda = [](auto&&... args){
499        const auto istiny = [](auto x){
500          return (sizeof(x) == 1UL) ? 1 : 0;
501        };
502        const int aretiny[] = { istiny(args)... };
503        return aretiny[0];
504      };
505      return lambda(1, 1L, 1.0f, '1');
506    }
507
508  }
509
510  namespace test_binary_literals
511  {
512
513    constexpr auto ivii = 0b0000000000101010;
514    static_assert(ivii == 42, "wrong value");
515
516  }
517
518  namespace test_generalized_constexpr
519  {
520
521    template < typename CharT >
522    constexpr unsigned long
523    strlen_c(const CharT *const s) noexcept
524    {
525      auto length = 0UL;
526      for (auto p = s; *p; ++p)
527        ++length;
528      return length;
529    }
530
531    static_assert(strlen_c("") == 0UL, "");
532    static_assert(strlen_c("x") == 1UL, "");
533    static_assert(strlen_c("test") == 4UL, "");
534    static_assert(strlen_c("another\0test") == 7UL, "");
535
536  }
537
538  namespace test_lambda_init_capture
539  {
540
541    int
542    test()
543    {
544      auto x = 0;
545      const auto lambda1 = [a = x](int b){ return a + b; };
546      const auto lambda2 = [a = lambda1(x)](){ return a; };
547      return lambda2();
548    }
549
550  }
551
552  namespace test_digit_separators
553  {
554
555    constexpr auto ten_million = 100'000'000;
556    static_assert(ten_million == 100000000, "");
557
558  }
559
560  namespace test_return_type_deduction
561  {
562
563    auto f(int& x) { return x; }
564    decltype(auto) g(int& x) { return x; }
565
566    template < typename T1, typename T2 >
567    struct is_same
568    {
569      static constexpr auto value = false;
570    };
571
572    template < typename T >
573    struct is_same<T, T>
574    {
575      static constexpr auto value = true;
576    };
577
578    int
579    test()
580    {
581      auto x = 0;
582      static_assert(is_same<int, decltype(f(x))>::value, "");
583      static_assert(is_same<int&, decltype(g(x))>::value, "");
584      return x;
585    }
586
587  }
588
589}  // namespace cxx14
590
591#endif  // __cplusplus >= 201402L
592
593]])
594
595
596dnl  Tests for new features in C++17
597
598m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
599
600// If the compiler admits that it is not ready for C++17, why torture it?
601// Hopefully, this will speed up the test.
602
603#ifndef __cplusplus
604
605#error "This is not a C++ compiler"
606
607#elif __cplusplus < 201703L
608
609#error "This is not a C++17 compiler"
610
611#else
612
613#include <initializer_list>
614#include <utility>
615#include <type_traits>
616
617namespace cxx17
618{
619
620  namespace test_constexpr_lambdas
621  {
622
623    constexpr int foo = [](){return 42;}();
624
625  }
626
627  namespace test::nested_namespace::definitions
628  {
629
630  }
631
632  namespace test_fold_expression
633  {
634
635    template<typename... Args>
636    int multiply(Args... args)
637    {
638      return (args * ... * 1);
639    }
640
641    template<typename... Args>
642    bool all(Args... args)
643    {
644      return (args && ...);
645    }
646
647  }
648
649  namespace test_extended_static_assert
650  {
651
652    static_assert (true);
653
654  }
655
656  namespace test_auto_brace_init_list
657  {
658
659    auto foo = {5};
660    auto bar {5};
661
662    static_assert(std::is_same<std::initializer_list<int>, decltype(foo)>::value);
663    static_assert(std::is_same<int, decltype(bar)>::value);
664  }
665
666  namespace test_typename_in_template_template_parameter
667  {
668
669    template<template<typename> typename X> struct D;
670
671  }
672
673  namespace test_fallthrough_nodiscard_maybe_unused_attributes
674  {
675
676    int f1()
677    {
678      return 42;
679    }
680
681    [[nodiscard]] int f2()
682    {
683      [[maybe_unused]] auto unused = f1();
684
685      switch (f1())
686      {
687      case 17:
688        f1();
689        [[fallthrough]];
690      case 42:
691        f1();
692      }
693      return f1();
694    }
695
696  }
697
698  namespace test_extended_aggregate_initialization
699  {
700
701    struct base1
702    {
703      int b1, b2 = 42;
704    };
705
706    struct base2
707    {
708      base2() {
709        b3 = 42;
710      }
711      int b3;
712    };
713
714    struct derived : base1, base2
715    {
716        int d;
717    };
718
719    derived d1 {{1, 2}, {}, 4};  // full initialization
720    derived d2 {{}, {}, 4};      // value-initialized bases
721
722  }
723
724  namespace test_general_range_based_for_loop
725  {
726
727    struct iter
728    {
729      int i;
730
731      int& operator* ()
732      {
733        return i;
734      }
735
736      const int& operator* () const
737      {
738        return i;
739      }
740
741      iter& operator++()
742      {
743        ++i;
744        return *this;
745      }
746    };
747
748    struct sentinel
749    {
750      int i;
751    };
752
753    bool operator== (const iter& i, const sentinel& s)
754    {
755      return i.i == s.i;
756    }
757
758    bool operator!= (const iter& i, const sentinel& s)
759    {
760      return !(i == s);
761    }
762
763    struct range
764    {
765      iter begin() const
766      {
767        return {0};
768      }
769
770      sentinel end() const
771      {
772        return {5};
773      }
774    };
775
776    void f()
777    {
778      range r {};
779
780      for (auto i : r)
781      {
782        [[maybe_unused]] auto v = i;
783      }
784    }
785
786  }
787
788  namespace test_lambda_capture_asterisk_this_by_value
789  {
790
791    struct t
792    {
793      int i;
794      int foo()
795      {
796        return [*this]()
797        {
798          return i;
799        }();
800      }
801    };
802
803  }
804
805  namespace test_enum_class_construction
806  {
807
808    enum class byte : unsigned char
809    {};
810
811    byte foo {42};
812
813  }
814
815  namespace test_constexpr_if
816  {
817
818    template <bool cond>
819    int f ()
820    {
821      if constexpr(cond)
822      {
823        return 13;
824      }
825      else
826      {
827        return 42;
828      }
829    }
830
831  }
832
833  namespace test_selection_statement_with_initializer
834  {
835
836    int f()
837    {
838      return 13;
839    }
840
841    int f2()
842    {
843      if (auto i = f(); i > 0)
844      {
845        return 3;
846      }
847
848      switch (auto i = f(); i + 4)
849      {
850      case 17:
851        return 2;
852
853      default:
854        return 1;
855      }
856    }
857
858  }
859
860  namespace test_template_argument_deduction_for_class_templates
861  {
862
863    template <typename T1, typename T2>
864    struct pair
865    {
866      pair (T1 p1, T2 p2)
867        : m1 {p1},
868          m2 {p2}
869      {}
870
871      T1 m1;
872      T2 m2;
873    };
874
875    void f()
876    {
877      [[maybe_unused]] auto p = pair{13, 42u};
878    }
879
880  }
881
882  namespace test_non_type_auto_template_parameters
883  {
884
885    template <auto n>
886    struct B
887    {};
888
889    B<5> b1;
890    B<'a'> b2;
891
892  }
893
894  namespace test_structured_bindings
895  {
896
897    int arr[2] = { 1, 2 };
898    std::pair<int, int> pr = { 1, 2 };
899
900    auto f1() -> int(&)[2]
901    {
902      return arr;
903    }
904
905    auto f2() -> std::pair<int, int>&
906    {
907      return pr;
908    }
909
910    struct S
911    {
912      int x1 : 2;
913      volatile double y1;
914    };
915
916    S f3()
917    {
918      return {};
919    }
920
921    auto [ x1, y1 ] = f1();
922    auto& [ xr1, yr1 ] = f1();
923    auto [ x2, y2 ] = f2();
924    auto& [ xr2, yr2 ] = f2();
925    const auto [ x3, y3 ] = f3();
926
927  }
928
929  namespace test_exception_spec_type_system
930  {
931
932    struct Good {};
933    struct Bad {};
934
935    void g1() noexcept;
936    void g2();
937
938    template<typename T>
939    Bad
940    f(T*, T*);
941
942    template<typename T1, typename T2>
943    Good
944    f(T1*, T2*);
945
946    static_assert (std::is_same_v<Good, decltype(f(g1, g2))>);
947
948  }
949
950  namespace test_inline_variables
951  {
952
953    template<class T> void f(T)
954    {}
955
956    template<class T> inline T g(T)
957    {
958      return T{};
959    }
960
961    template<> inline void f<>(int)
962    {}
963
964    template<> int g<>(int)
965    {
966      return 5;
967    }
968
969  }
970
971}  // namespace cxx17
972
973#endif  // __cplusplus < 201703L
974
975]])
976
977
978dnl  Tests for new features in C++20
979
980m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_20], [[
981
982#ifndef __cplusplus
983
984#error "This is not a C++ compiler"
985
986#elif __cplusplus < 202002L
987
988#error "This is not a C++20 compiler"
989
990#else
991
992#include <version>
993
994namespace cxx20
995{
996
997// As C++20 supports feature test macros in the standard, there is no
998// immediate need to actually test for feature availability on the
999// Autoconf side.
1000
1001}  // namespace cxx20
1002
1003#endif  // __cplusplus < 202002L
1004
1005]])
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