ceph/src/jaegertracing/thrift/aclocal/ax_cxx_compile_stdcxx.m4

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