m4/ax_cxx_compile_stdcxx.m4

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