m4/ax_cxx_compile_stdcxx.m4

   1 # SPDX-License-Identifier: FSFAP
   2 #
   3 # ===========================================================================
   4 #  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
   5 # ===========================================================================
   6 #
   7 # SYNOPSIS
   8 #
   9 #   AX_CXX_COMPILE_STDCXX(VERSION, [ext|noext], [mandatory|optional])
  10 #
  11 # DESCRIPTION
  12 #
  13 #   Check for baseline language coverage in the compiler for the specified
  14 #   version of the C++ standard.  If necessary, add switches to CXX and
  15 #   CXXCPP to enable support.  VERSION may be '11', '14', '17', or '20' for
  16 #   the respective C++ standard version.
  17 #
  18 #   The second argument, if specified, indicates whether you insist on an
  19 #   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
  20 #   -std=c++11).  If neither is specified, you get whatever works, with
  21 #   preference for no added switch, and then for an extended mode.
  22 #
  23 #   The third argument, if specified 'mandatory' or if left unspecified,
  24 #   indicates that baseline support for the specified C++ standard is
  25 #   required and that the macro should error out if no mode with that
  26 #   support is found.  If specified 'optional', then configuration proceeds
  27 #   regardless, after defining HAVE_CXX${VERSION} if and only if a
  28 #   supporting mode is found.
  29 #
  30 # LICENSE
  31 #
  32 #   Copyright (c) 2008 Benjamin Kosnik <bkoz@redhat.com>
  33 #   Copyright (c) 2012 Zack Weinberg <zackw@panix.com>
  34 #   Copyright (c) 2013 Roy Stogner <roystgnr@ices.utexas.edu>
  35 #   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov@google.com>
  36 #   Copyright (c) 2015 Paul Norman <penorman@mac.com>
  37 #   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
  38 #   Copyright (c) 2016, 2018 Krzesimir Nowak <qdlacz@gmail.com>
  39 #   Copyright (c) 2019 Enji Cooper <yaneurabeya@gmail.com>
  40 #   Copyright (c) 2020 Jason Merrill <jason@redhat.com>
  41 #   Copyright (c) 2021 Jörn Heusipp <osmanx@problemloesungsmaschine.de>
  42 #
  43 #   Copying and distribution of this file, with or without modification, are
  44 #   permitted in any medium without royalty provided the copyright notice
  45 #   and this notice are preserved.  This file is offered as-is, without any
  46 #   warranty.
  47
  48 #serial 18
  49
  50 dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
  51 dnl  (serial version number 13).
  52
  53 AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
  54   m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
  55         [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
  56         [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
  57         [$1], [20], [ax_cxx_compile_alternatives="20"],
  58         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
  59   m4_if([$2], [], [],
  60         [$2], [ext], [],
  61         [$2], [noext], [],
  62         [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX])])dnl
  63   m4_if([$3], [], [ax_cxx_compile_cxx$1_required=true],
  64         [$3], [mandatory], [ax_cxx_compile_cxx$1_required=true],
  65         [$3], [optional], [ax_cxx_compile_cxx$1_required=false],
  66         [m4_fatal([invalid third argument `$3' to AX_CXX_COMPILE_STDCXX])])
  67   AC_LANG_PUSH([C++])dnl
  68   ac_success=no
  69
  70   m4_if([$2], [], [dnl
  71     AC_CACHE_CHECK(whether $CXX supports C++$1 features by default,
  72                    ax_cv_cxx_compile_cxx$1,
  73       [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
  74         [ax_cv_cxx_compile_cxx$1=yes],
  75         [ax_cv_cxx_compile_cxx$1=no])])
  76     if test x$ax_cv_cxx_compile_cxx$1 = xyes; then
  77       ac_success=yes
  78     fi])
  79
  80   m4_if([$2], [noext], [], [dnl
  81   if test x$ac_success = xno; then
  82     for alternative in ${ax_cxx_compile_alternatives}; do
  83       switch="-std=gnu++${alternative}"
  84       cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
  85       AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
  86                      $cachevar,
  87         [ac_save_CXX="$CXX"
  88          CXX="$CXX $switch"
  89          AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
  90           [eval $cachevar=yes],
  91           [eval $cachevar=no])
  92          CXX="$ac_save_CXX"])
  93       if eval test x\$$cachevar = xyes; then
  94         CXX="$CXX $switch"
  95         if test -n "$CXXCPP" ; then
  96           CXXCPP="$CXXCPP $switch"
  97         fi
  98         ac_success=yes
  99         break
 100       fi
 101     done
 102   fi])
 103
 104   m4_if([$2], [ext], [], [dnl
 105   if test x$ac_success = xno; then
 106     dnl HP's aCC needs +std=c++11 according to:
 107     dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
 108     dnl Cray's crayCC needs "-h std=c++11"
 109     dnl MSVC needs -std:c++NN for C++17 and later (default is C++14)
 110     for alternative in ${ax_cxx_compile_alternatives}; do
 111       for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}" MSVC; do
 112         if test x"$switch" = xMSVC; then
 113           dnl AS_TR_SH maps both `:` and `=` to `_` so -std:c++17 would collide
 114           dnl with -std=c++17.  We suffix the cache variable name with _MSVC to
 115           dnl avoid this.
 116           switch=-std:c++${alternative}
 117           cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_${switch}_MSVC])
 118         else
 119           cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
 120         fi
 121         AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
 122                        $cachevar,
 123           [ac_save_CXX="$CXX"
 124            CXX="$CXX $switch"
 125            AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
 126             [eval $cachevar=yes],
 127             [eval $cachevar=no])
 128            CXX="$ac_save_CXX"])
 129         if eval test x\$$cachevar = xyes; then
 130           CXX="$CXX $switch"
 131           if test -n "$CXXCPP" ; then
 132             CXXCPP="$CXXCPP $switch"
 133           fi
 134           ac_success=yes
 135           break
 136         fi
 137       done
 138       if test x$ac_success = xyes; then
 139         break
 140       fi
 141     done
 142   fi])
 143   AC_LANG_POP([C++])
 144   if test x$ax_cxx_compile_cxx$1_required = xtrue; then
 145     if test x$ac_success = xno; then
 146       AC_MSG_ERROR([*** A compiler with support for C++$1 language features is required.])
 147     fi
 148   fi
 149   if test x$ac_success = xno; then
 150     HAVE_CXX$1=0
 151     AC_MSG_NOTICE([No compiler with C++$1 support was found])
 152   else
 153     HAVE_CXX$1=1
 154     AC_DEFINE(HAVE_CXX$1,1,
 155               [define if the compiler supports basic C++$1 syntax])
 156   fi
 157   AC_SUBST(HAVE_CXX$1)
 158 ])
 159
 160
 161 dnl  Test body for checking C++11 support
 162
 163 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_11],
 164   _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
 165 )
 166
 167 dnl  Test body for checking C++14 support
 168
 169 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
 170   _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
 171   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
 172 )
 173
 174 dnl  Test body for checking C++17 support
 175
 176 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
 177   _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
 178   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
 179   _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
 180 )
 181
 182 dnl  Test body for checking C++20 support
 183
 184 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_20],
 185   _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
 186   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
 187   _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
 188   _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
 189 )
 190
 191
 192 dnl  Tests for new features in C++11
 193
 194 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[
 195
 196 // If the compiler admits that it is not ready for C++11, why torture it?
 197 // Hopefully, this will speed up the test.
 198
 199 #ifndef __cplusplus
 200
 201 #error "This is not a C++ compiler"
 202
 203 // MSVC always sets __cplusplus to 199711L in older versions; newer versions
 204 // only set it correctly if /Zc:__cplusplus is specified as well as a
 205 // /std:c++NN switch:
 206 // https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
 207 #elif __cplusplus < 201103L && !defined _MSC_VER
 208
 209 #error "This is not a C++11 compiler"
 210
 211 #else
 212
 213 namespace cxx11
 214 {
 215
 216   namespace test_static_assert
 217   {
 218
 219     template <typename T>
 220     struct check
 221     {
 222       static_assert(sizeof(int) <= sizeof(T), "not big enough");
 223     };
 224
 225   }
 226
 227   namespace test_final_override
 228   {
 229
 230     struct Base
 231     {
 232       virtual ~Base() {}
 233       virtual void f() {}
 234     };
 235
 236     struct Derived : public Base
 237     {
 238       virtual ~Derived() override {}
 239       virtual void f() override {}
 240     };
 241
 242   }
 243
 244   namespace test_double_right_angle_brackets
 245   {
 246
 247     template < typename T >
 248     struct check {};
 249
 250     typedef check<void> single_type;
 251     typedef check<check<void>> double_type;
 252     typedef check<check<check<void>>> triple_type;
 253     typedef check<check<check<check<void>>>> quadruple_type;
 254
 255   }
 256
 257   namespace test_decltype
 258   {
 259
 260     int
 261     f()
 262     {
 263       int a = 1;
 264       decltype(a) b = 2;
 265       return a + b;
 266     }
 267
 268   }
 269
 270   namespace test_type_deduction
 271   {
 272
 273     template < typename T1, typename T2 >
 274     struct is_same
 275     {
 276       static const bool value = false;
 277     };
 278
 279     template < typename T >
 280     struct is_same<T, T>
 281     {
 282       static const bool value = true;
 283     };
 284
 285     template < typename T1, typename T2 >
 286     auto
 287     add(T1 a1, T2 a2) -> decltype(a1 + a2)
 288     {
 289       return a1 + a2;
 290     }
 291
 292     int
 293     test(const int c, volatile int v)
 294     {
 295       static_assert(is_same<int, decltype(0)>::value == true, "");
 296       static_assert(is_same<int, decltype(c)>::value == false, "");
 297       static_assert(is_same<int, decltype(v)>::value == false, "");
 298       auto ac = c;
 299       auto av = v;
 300       auto sumi = ac + av + 'x';
 301       auto sumf = ac + av + 1.0;
 302       static_assert(is_same<int, decltype(ac)>::value == true, "");
 303       static_assert(is_same<int, decltype(av)>::value == true, "");
 304       static_assert(is_same<int, decltype(sumi)>::value == true, "");
 305       static_assert(is_same<int, decltype(sumf)>::value == false, "");
 306       static_assert(is_same<int, decltype(add(c, v))>::value == true, "");
 307       return (sumf > 0.0) ? sumi : add(c, v);
 308     }
 309
 310   }
 311
 312   namespace test_noexcept
 313   {
 314
 315     int f() { return 0; }
 316     int g() noexcept { return 0; }
 317
 318     static_assert(noexcept(f()) == false, "");
 319     static_assert(noexcept(g()) == true, "");
 320
 321   }
 322
 323   namespace test_constexpr
 324   {
 325
 326     template < typename CharT >
 327     unsigned long constexpr
 328     strlen_c_r(const CharT *const s, const unsigned long acc) noexcept
 329     {
 330       return *s ? strlen_c_r(s + 1, acc + 1) : acc;
 331     }
 332
 333     template < typename CharT >
 334     unsigned long constexpr
 335     strlen_c(const CharT *const s) noexcept
 336     {
 337       return strlen_c_r(s, 0UL);
 338     }
 339
 340     static_assert(strlen_c("") == 0UL, "");
 341     static_assert(strlen_c("1") == 1UL, "");
 342     static_assert(strlen_c("example") == 7UL, "");
 343     static_assert(strlen_c("another\0example") == 7UL, "");
 344
 345   }
 346
 347   namespace test_rvalue_references
 348   {
 349
 350     template < int N >
 351     struct answer
 352     {
 353       static constexpr int value = N;
 354     };
 355
 356     answer<1> f(int&)       { return answer<1>(); }
 357     answer<2> f(const int&) { return answer<2>(); }
 358     answer<3> f(int&&)      { return answer<3>(); }
 359
 360     void
 361     test()
 362     {
 363       int i = 0;
 364       const int c = 0;
 365       static_assert(decltype(f(i))::value == 1, "");
 366       static_assert(decltype(f(c))::value == 2, "");
 367       static_assert(decltype(f(0))::value == 3, "");
 368     }
 369
 370   }
 371
 372   namespace test_uniform_initialization
 373   {
 374
 375     struct test
 376     {
 377       static const int zero {};
 378       static const int one {1};
 379     };
 380
 381     static_assert(test::zero == 0, "");
 382     static_assert(test::one == 1, "");
 383
 384   }
 385
 386   namespace test_lambdas
 387   {
 388
 389     void
 390     test1()
 391     {
 392       auto lambda1 = [](){};
 393       auto lambda2 = lambda1;
 394       lambda1();
 395       lambda2();
 396     }
 397
 398     int
 399     test2()
 400     {
 401       auto a = [](int i, int j){ return i + j; }(1, 2);
 402       auto b = []() -> int { return '0'; }();
 403       auto c = [=](){ return a + b; }();
 404       auto d = [&](){ return c; }();
 405       auto e = [a, &b](int x) mutable {
 406         const auto identity = [](int y){ return y; };
 407         for (auto i = 0; i < a; ++i)
 408           a += b--;
 409         return x + identity(a + b);
 410       }(0);
 411       return a + b + c + d + e;
 412     }
 413
 414     int
 415     test3()
 416     {
 417       const auto nullary = [](){ return 0; };
 418       const auto unary = [](int x){ return x; };
 419       using nullary_t = decltype(nullary);
 420       using unary_t = decltype(unary);
 421       const auto higher1st = [](nullary_t f){ return f(); };
 422       const auto higher2nd = [unary](nullary_t f1){
 423         return [unary, f1](unary_t f2){ return f2(unary(f1())); };
 424       };
 425       return higher1st(nullary) + higher2nd(nullary)(unary);
 426     }
 427
 428   }
 429
 430   namespace test_variadic_templates
 431   {
 432
 433     template <int...>
 434     struct sum;
 435
 436     template <int N0, int... N1toN>
 437     struct sum<N0, N1toN...>
 438     {
 439       static constexpr auto value = N0 + sum<N1toN...>::value;
 440     };
 441
 442     template <>
 443     struct sum<>
 444     {
 445       static constexpr auto value = 0;
 446     };
 447
 448     static_assert(sum<>::value == 0, "");
 449     static_assert(sum<1>::value == 1, "");
 450     static_assert(sum<23>::value == 23, "");
 451     static_assert(sum<1, 2>::value == 3, "");
 452     static_assert(sum<5, 5, 11>::value == 21, "");
 453     static_assert(sum<2, 3, 5, 7, 11, 13>::value == 41, "");
 454
 455   }
 456
 457   // http://stackoverflow.com/questions/13728184/template-aliases-and-sfinae
 458   // Clang 3.1 fails with headers of libstd++ 4.8.3 when using std::function
 459   // because of this.
 460   namespace test_template_alias_sfinae
 461   {
 462
 463     struct foo {};
 464
 465     template<typename T>
 466     using member = typename T::member_type;
 467
 468     template<typename T>
 469     void func(...) {}
 470
 471     template<typename T>
 472     void func(member<T>*) {}
 473
 474     void test();
 475
 476     void test() { func<foo>(0); }
 477
 478   }
 479
 480 }  // namespace cxx11
 481
 482 #endif  // __cplusplus >= 201103L
 483
 484 ]])
 485
 486
 487 dnl  Tests for new features in C++14
 488
 489 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_14], [[
 490
 491 // If the compiler admits that it is not ready for C++14, why torture it?
 492 // Hopefully, this will speed up the test.
 493
 494 #ifndef __cplusplus
 495
 496 #error "This is not a C++ compiler"
 497
 498 #elif __cplusplus < 201402L && !defined _MSC_VER
 499
 500 #error "This is not a C++14 compiler"
 501
 502 #else
 503
 504 namespace cxx14
 505 {
 506
 507   namespace test_polymorphic_lambdas
 508   {
 509
 510     int
 511     test()
 512     {
 513       const auto lambda = [](auto&&... args){
 514         const auto istiny = [](auto x){
 515           return (sizeof(x) == 1UL) ? 1 : 0;
 516         };
 517         const int aretiny[] = { istiny(args)... };
 518         return aretiny[0];
 519       };
 520       return lambda(1, 1L, 1.0f, '1');
 521     }
 522
 523   }
 524
 525   namespace test_binary_literals
 526   {
 527
 528     constexpr auto ivii = 0b0000000000101010;
 529     static_assert(ivii == 42, "wrong value");
 530
 531   }
 532
 533   namespace test_generalized_constexpr
 534   {
 535
 536     template < typename CharT >
 537     constexpr unsigned long
 538     strlen_c(const CharT *const s) noexcept
 539     {
 540       auto length = 0UL;
 541       for (auto p = s; *p; ++p)
 542         ++length;
 543       return length;
 544     }
 545
 546     static_assert(strlen_c("") == 0UL, "");
 547     static_assert(strlen_c("x") == 1UL, "");
 548     static_assert(strlen_c("test") == 4UL, "");
 549     static_assert(strlen_c("another\0test") == 7UL, "");
 550
 551   }
 552
 553   namespace test_lambda_init_capture
 554   {
 555
 556     int
 557     test()
 558     {
 559       auto x = 0;
 560       const auto lambda1 = [a = x](int b){ return a + b; };
 561       const auto lambda2 = [a = lambda1(x)](){ return a; };
 562       return lambda2();
 563     }
 564
 565   }
 566
 567   namespace test_digit_separators
 568   {
 569
 570     constexpr auto ten_million = 100'000'000;
 571     static_assert(ten_million == 100000000, "");
 572
 573   }
 574
 575   namespace test_return_type_deduction
 576   {
 577
 578     auto f(int& x) { return x; }
 579     decltype(auto) g(int& x) { return x; }
 580
 581     template < typename T1, typename T2 >
 582     struct is_same
 583     {
 584       static constexpr auto value = false;
 585     };
 586
 587     template < typename T >
 588     struct is_same<T, T>
 589     {
 590       static constexpr auto value = true;
 591     };
 592
 593     int
 594     test()
 595     {
 596       auto x = 0;
 597       static_assert(is_same<int, decltype(f(x))>::value, "");
 598       static_assert(is_same<int&, decltype(g(x))>::value, "");
 599       return x;
 600     }
 601
 602   }
 603
 604 }  // namespace cxx14
 605
 606 #endif  // __cplusplus >= 201402L
 607
 608 ]])
 609
 610
 611 dnl  Tests for new features in C++17
 612
 613 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
 614
 615 // If the compiler admits that it is not ready for C++17, why torture it?
 616 // Hopefully, this will speed up the test.
 617
 618 #ifndef __cplusplus
 619
 620 #error "This is not a C++ compiler"
 621
 622 #elif __cplusplus < 201703L && !defined _MSC_VER
 623
 624 #error "This is not a C++17 compiler"
 625
 626 #else
 627
 628 #include <initializer_list>
 629 #include <utility>
 630 #include <type_traits>
 631
 632 namespace cxx17
 633 {
 634
 635   namespace test_constexpr_lambdas
 636   {
 637
 638     constexpr int foo = [](){return 42;}();
 639
 640   }
 641
 642   namespace test::nested_namespace::definitions
 643   {
 644
 645   }
 646
 647   namespace test_fold_expression
 648   {
 649
 650     template<typename... Args>
 651     int multiply(Args... args)
 652     {
 653       return (args * ... * 1);
 654     }
 655
 656     template<typename... Args>
 657     bool all(Args... args)
 658     {
 659       return (args && ...);
 660     }
 661
 662   }
 663
 664   namespace test_extended_static_assert
 665   {
 666
 667     static_assert (true);
 668
 669   }
 670
 671   namespace test_auto_brace_init_list
 672   {
 673
 674     auto foo = {5};
 675     auto bar {5};
 676
 677     static_assert(std::is_same<std::initializer_list<int>, decltype(foo)>::value);
 678     static_assert(std::is_same<int, decltype(bar)>::value);
 679   }
 680
 681   namespace test_typename_in_template_template_parameter
 682   {
 683
 684     template<template<typename> typename X> struct D;
 685
 686   }
 687
 688   namespace test_fallthrough_nodiscard_maybe_unused_attributes
 689   {
 690
 691     int f1()
 692     {
 693       return 42;
 694     }
 695
 696     [[nodiscard]] int f2()
 697     {
 698       [[maybe_unused]] auto unused = f1();
 699
 700       switch (f1())
 701       {
 702       case 17:
 703         f1();
 704         [[fallthrough]];
 705       case 42:
 706         f1();
 707       }
 708       return f1();
 709     }
 710
 711   }
 712
 713   namespace test_extended_aggregate_initialization
 714   {
 715
 716     struct base1
 717     {
 718       int b1, b2 = 42;
 719     };
 720
 721     struct base2
 722     {
 723       base2() {
 724         b3 = 42;
 725       }
 726       int b3;
 727     };
 728
 729     struct derived : base1, base2
 730     {
 731         int d;
 732     };
 733
 734     derived d1 {{1, 2}, {}, 4};  // full initialization
 735     derived d2 {{}, {}, 4};      // value-initialized bases
 736
 737   }
 738
 739   namespace test_general_range_based_for_loop
 740   {
 741
 742     struct iter
 743     {
 744       int i;
 745
 746       int& operator* ()
 747       {
 748         return i;
 749       }
 750
 751       const int& operator* () const
 752       {
 753         return i;
 754       }
 755
 756       iter& operator++()
 757       {
 758         ++i;
 759         return *this;
 760       }
 761     };
 762
 763     struct sentinel
 764     {
 765       int i;
 766     };
 767
 768     bool operator== (const iter& i, const sentinel& s)
 769     {
 770       return i.i == s.i;
 771     }
 772
 773     bool operator!= (const iter& i, const sentinel& s)
 774     {
 775       return !(i == s);
 776     }
 777
 778     struct range
 779     {
 780       iter begin() const
 781       {
 782         return {0};
 783       }
 784
 785       sentinel end() const
 786       {
 787         return {5};
 788       }
 789     };
 790
 791     void f()
 792     {
 793       range r {};
 794
 795       for (auto i : r)
 796       {
 797         [[maybe_unused]] auto v = i;
 798       }
 799     }
 800
 801   }
 802
 803   namespace test_lambda_capture_asterisk_this_by_value
 804   {
 805
 806     struct t
 807     {
 808       int i;
 809       int foo()
 810       {
 811         return [*this]()
 812         {
 813           return i;
 814         }();
 815       }
 816     };
 817
 818   }
 819
 820   namespace test_enum_class_construction
 821   {
 822
 823     enum class byte : unsigned char
 824     {};
 825
 826     byte foo {42};
 827
 828   }
 829
 830   namespace test_constexpr_if
 831   {
 832
 833     template <bool cond>
 834     int f ()
 835     {
 836       if constexpr(cond)
 837       {
 838         return 13;
 839       }
 840       else
 841       {
 842         return 42;
 843       }
 844     }
 845
 846   }
 847
 848   namespace test_selection_statement_with_initializer
 849   {
 850
 851     int f()
 852     {
 853       return 13;
 854     }
 855
 856     int f2()
 857     {
 858       if (auto i = f(); i > 0)
 859       {
 860         return 3;
 861       }
 862
 863       switch (auto i = f(); i + 4)
 864       {
 865       case 17:
 866         return 2;
 867
 868       default:
 869         return 1;
 870       }
 871     }
 872
 873   }
 874
 875   namespace test_template_argument_deduction_for_class_templates
 876   {
 877
 878     template <typename T1, typename T2>
 879     struct pair
 880     {
 881       pair (T1 p1, T2 p2)
 882         : m1 {p1},
 883           m2 {p2}
 884       {}
 885
 886       T1 m1;
 887       T2 m2;
 888     };
 889
 890     void f()
 891     {
 892       [[maybe_unused]] auto p = pair{13, 42u};
 893     }
 894
 895   }
 896
 897   namespace test_non_type_auto_template_parameters
 898   {
 899
 900     template <auto n>
 901     struct B
 902     {};
 903
 904     B<5> b1;
 905     B<'a'> b2;
 906
 907   }
 908
 909   namespace test_structured_bindings
 910   {
 911
 912     int arr[2] = { 1, 2 };
 913     std::pair<int, int> pr = { 1, 2 };
 914
 915     auto f1() -> int(&)[2]
 916     {
 917       return arr;
 918     }
 919
 920     auto f2() -> std::pair<int, int>&
 921     {
 922       return pr;
 923     }
 924
 925     struct S
 926     {
 927       int x1 : 2;
 928       volatile double y1;
 929     };
 930
 931     S f3()
 932     {
 933       return {};
 934     }
 935
 936     auto [ x1, y1 ] = f1();
 937     auto& [ xr1, yr1 ] = f1();
 938     auto [ x2, y2 ] = f2();
 939     auto& [ xr2, yr2 ] = f2();
 940     const auto [ x3, y3 ] = f3();
 941
 942   }
 943
 944   namespace test_exception_spec_type_system
 945   {
 946
 947     struct Good {};
 948     struct Bad {};
 949
 950     void g1() noexcept;
 951     void g2();
 952
 953     template<typename T>
 954     Bad
 955     f(T*, T*);
 956
 957     template<typename T1, typename T2>
 958     Good
 959     f(T1*, T2*);
 960
 961     static_assert (std::is_same_v<Good, decltype(f(g1, g2))>);
 962
 963   }
 964
 965   namespace test_inline_variables
 966   {
 967
 968     template<class T> void f(T)
 969     {}
 970
 971     template<class T> inline T g(T)
 972     {
 973       return T{};
 974     }
 975
 976     template<> inline void f<>(int)
 977     {}
 978
 979     template<> int g<>(int)
 980     {
 981       return 5;
 982     }
 983
 984   }
 985
 986 }  // namespace cxx17
 987
 988 #endif  // __cplusplus < 201703L && !defined _MSC_VER
 989
 990 ]])
 991
 992
 993 dnl  Tests for new features in C++20
 994
 995 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_20], [[
 996
 997 #ifndef __cplusplus
 998
 999 #error "This is not a C++ compiler"
1000
1001 #elif __cplusplus < 202002L && !defined _MSC_VER
1002
1003 #error "This is not a C++20 compiler"
1004
1005 #else
1006
1007 #include <version>
1008
1009 namespace cxx20
1010 {
1011
1012 // As C++20 supports feature test macros in the standard, there is no
1013 // immediate need to actually test for feature availability on the
1014 // Autoconf side.
1015
1016 }  // namespace cxx20
1017
1018 #endif  // __cplusplus < 202002L && !defined _MSC_VER
1019
1020 ]])