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AC_PROG_CXX no longer adjusts C++ language version

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* lib/autoconf/c.m4 (_AC_CXX_CXX98_TEST_GLOBALS)
(_AC_CXX_CXX98_TEST_MAIN, _AC_CXX_CXX11_TEST_GLOBALS)
(_AC_CXX_CXX11_TEST_MAIN, _AC_CXX_CXX98_TEST_PROGRAM)
(_AC_CXX_CXX11_TEST_PROGRAM, _AC_CXX_CXX14_TEST_PROGRAM)
(_AC_CXX_CXX17_TEST_PROGRAM, _AC_CXX_CXX20_TEST_PROGRAM)
(_AC_CXX_CXX23_TEST_PROGRAM, _AC_CXX_CXX98_OPTIONS)
(_AC_CXX_CXX11_OPTIONS, _AC_CXX_CXX14_OPTIONS)
(_AC_CXX_CXX17_OPTIONS, _AC_CXX_CXX20_OPTIONS)
(_AC_CXX_CXX23_OPTIONS, _AC_PROG_CXX_STDCXX_EDITION_TRY)
(_AC_PROG_CXX_STDCXX_EDITION_TRY): Remove.  All uses removed.
This commit is contained in:
Paul Eggert 2024-05-28 09:31:11 -07:00
parent f6522328c7
commit 056518b94e
3 changed files with 5 additions and 476 deletions
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13
NEWS
View File

@ -16,14 +16,11 @@ GNU Autoconf NEWS - User visible changes.
C11 and later. Programs can use AC_C_VARARRAYS and __STDC_NO_VLA__
to use VLAs if available.
*** AC_PROG_CXX now prefers C++23, C++20, C++17, C++14 if available.
Older code may need to be updated, as some older features of C++ are
removed in later standards.
** Notable bug fixes
*** AC_PROG_CXX no longer rejects C++20 compilers
merely because C++20 is not upward-compatible with C++11.
*** AC_PROG_CXX no longer attempts to switch to C++98 or C++11.
Instead, it uses the compiler's default, which you can override
by configuring with something like CXX='g++ -std=gnu++11'.
This reverts to Autoconf 2.69 behavior, and also fixes a bug where
AC_PROG_CXX rejected C++20 compilers.
* Noteworthy changes in release 2.72 (2023-12-22) [release]

29
doc/autoconf.texi
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@ -7821,33 +7821,6 @@ where @var{options} are the appropriate set of options to use by
default. (It is important to use this construct rather than a normal
assignment, so that @code{CXXFLAGS} can still be overridden by the
person building the package. @xref{Preset Output Variables}.)
If necessary, options are added to @code{CXX} to enable support for
ISO Standard C++ features with extensions, preferring the newest edition
of the C++ standard that is supported. Currently the newest edition
Autoconf knows how to detect support for is C++23. After calling
this macro, you can check whether the C++ compiler has been set to
accept standard C++ by inspecting the shell variable @code{ac_prog_cxx_stdcxx}.
Its value will be @samp{cxx23}, @samp{cxx20}, @samp{cxx17},
@samp{cxx14}, @samp{cxx11} or @samp{cxx98},
if the C++ compiler has been set to use the corresponding edition of the
C++ standard, and @samp{no} if the compiler does not support compiling
standard C++ at all.
The tests for standard conformance are not comprehensive. They test
the value of @code{__cplusplus} and a representative sample of the
language features added in each version of the C++ standard. They
do not test the C++ standard library, because this can be extremely
slow, and because the C++ compiler might be generating code for a
``freestanding environment'' (in which most of the C++ standard library
is optional). If you need to know whether a particular C++ standard
header exists, use @code{AC_CHECK_HEADER}.
None of the options that may be added to @code{CXX} by this macro
enable @emph{strict} conformance to the C++ standard. In particular,
system-specific extensions are not disabled. (For example, for GNU
C++, the @option{-std=gnu++@var{nn}} options may be used, but not the
@option{-std=c++@var{nn}} options.)
@end defmac
@defmac AC_PROG_CXXCPP
@ -21715,8 +21688,6 @@ standard. K&R C compilers are no longer of practical interest, though,
and Autoconf assumes at least C89, the first C standard,
which is sometimes called ``C90'' due to a delay in standardization.
C has since gone through the standards C99, C11, C17, and C23, and
C++ has evolved in a similar way, with the standards
C++98, C++11, C++14, C++17, C++20, and C++23.
Autoconf is compatible with all these standards.
Program portability is a huge topic, and this section can only briefly

439
lib/autoconf/c.m4
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@ -719,7 +719,6 @@ else
GXX=
fi
_AC_PROG_CXX_G
_AC_PROG_CXX_STDCXX_EDITION
AC_LANG_POP(C++)dnl
])# AC_PROG_CXX
@ -2542,441 +2541,3 @@ AC_DEFUN([_AC_OPENMP_SAFE_WD],
[AC@&t@_OPENMP clobbers files named 'mp' and 'penmp'.
Aborting configure because one of these files already exists.]))
fi])
# ---------------------------------- #
# 4b. C++ compiler characteristics. #
# ---------------------------------- #
# See the long comment at the beginning of section 4a for rationale
# for these macros, and constraints on how the test programs should
# be written.
#
# The C++98 freestanding headers are:
# <cstdarg> <cstddef> <cstdlib> <exception> <limits> <new> <typeinfo>
# C++11 adds:
# <atomic> <cfloat> <ciso646> <climits> <cstdalign> <cstdbool>
# <cstdint> <initializer_list> <type_traits>
#
# No other headers can safely be included. Therefore, almost no C++
# standard library features are tested for. Use AC_CHECK_HEADER, etc.
# if you need that.
AC_DEFUN([_AC_CXX_CXX98_TEST_GLOBALS],
[m4_divert_text([INIT_PREPARE],
[[# Test code for whether the C++ compiler supports C++98 (global declarations)
ac_cxx_conftest_cxx98_globals='
// Does the compiler advertise C++98 conformance?
#if !defined __cplusplus || __cplusplus < 199711L
# error "Compiler does not advertise C++98 conformance"
#endif
// These inclusions are to reject old compilers that
// lack the unsuffixed header files.
#include <cstdlib>
#include <exception>
// <cassert> and <cstring> are *not* freestanding headers in C++98.
extern void assert (int);
namespace std {
extern int strcmp (const char *, const char *);
}
// Namespaces, exceptions, and templates were all added after "C++ 2.0".
using std::exception;
using std::strcmp;
namespace {
void test_exception_syntax()
{
try {
throw "test";
} catch (const char *s) {
// Extra parentheses suppress a warning when building autoconf itself,
// due to lint rules shared with more typical C programs.
assert (!(strcmp) (s, "test"));
}
}
template <typename T> struct test_template
{
T const val;
explicit test_template(T t) : val(t) {}
template <typename U> T add(U u) { return static_cast<T>(u) + val; }
};
} // anonymous namespace
'
]])])
AC_DEFUN([_AC_CXX_CXX98_TEST_MAIN],
[m4_divert_text([INIT_PREPARE],
[[# Test code for whether the C++ compiler supports C++98 (body of main)
ac_cxx_conftest_cxx98_main='
assert (argc);
assert (! argv[0]);
{
test_exception_syntax ();
test_template<double> tt (2.0);
assert (tt.add (4) == 6.0);
assert (true && !false);
}
'
]])])
AC_DEFUN([_AC_CXX_CXX11_TEST_GLOBALS],
[m4_divert_text([INIT_PREPARE],
[[# Test code for whether the C++ compiler supports C++11 (global declarations)
ac_cxx_conftest_cxx11_globals='
// Does the compiler advertise C++ 2011 conformance?
#if !defined __cplusplus || __cplusplus < 201103L
# error "Compiler does not advertise C++11 conformance"
#endif
namespace cxx11test
{
constexpr int get_val() { return 20; }
struct testinit
{
int i;
double d;
};
class delegate
{
public:
delegate(int n) : n(n) {}
delegate(): delegate(2354) {}
virtual int getval() { return this->n; };
protected:
int n;
};
class overridden : public delegate
{
public:
overridden(int n): delegate(n) {}
virtual int getval() override final { return this->n * 2; }
};
class nocopy
{
public:
nocopy(int i): i(i) {}
nocopy() = default;
nocopy(const nocopy&) = delete;
nocopy & operator=(const nocopy&) = delete;
private:
int i;
};
// for testing lambda expressions
template <typename Ret, typename Fn> Ret eval(Fn f, Ret v)
{
return f(v);
}
// for testing variadic templates and trailing return types
template <typename V> auto sum(V first) -> V
{
return first;
}
template <typename V, typename... Args> auto sum(V first, Args... rest) -> V
{
return first + sum(rest...);
}
}
'
]])])
AC_DEFUN([_AC_CXX_CXX11_TEST_MAIN],
[m4_divert_text([INIT_PREPARE],
[[# Test code for whether the C++ compiler supports C++11 (body of main)
ac_cxx_conftest_cxx11_main='
{
// Test auto and decltype
auto a1 = 6538;
auto a2 = 48573953.4;
auto a3 = "String literal";
int total = 0;
for (auto i = a3; *i; ++i) { total += *i; }
decltype(a2) a4 = 34895.034;
}
{
// Test constexpr
short sa[cxx11test::get_val()] = { 0 };
}
{
// Test initializer lists
cxx11test::testinit il = { 4323, 435234.23544 };
}
{
// Test range-based for
int array[] = {9, 7, 13, 15, 4, 18, 12, 10, 5, 3,
14, 19, 17, 8, 6, 20, 16, 2, 11, 1};
for (auto &x : array) { x += 23; }
}
{
// Test lambda expressions
using cxx11test::eval;
assert (eval ([](int x) { return x*2; }, 21) == 42);
double d = 2.0;
assert (eval ([&](double x) { return d += x; }, 3.0) == 5.0);
assert (d == 5.0);
assert (eval ([=](double x) mutable { return d += x; }, 4.0) == 9.0);
assert (d == 5.0);
}
{
// Test use of variadic templates
using cxx11test::sum;
auto a = sum(1);
auto b = sum(1, 2);
auto c = sum(1.0, 2.0, 3.0);
}
{
// Test constructor delegation
cxx11test::delegate d1;
cxx11test::delegate d2();
cxx11test::delegate d3(45);
}
{
// Test override and final
cxx11test::overridden o1(55464);
}
{
// Test nullptr
char *c = nullptr;
}
{
// Test template brackets
test_template<::test_template<int>> v(test_template<int>(12));
}
{
// Unicode literals. Do not test u8"..." as C++20 would reject it.
char16_t const *utf16 = u"UTF-8 string \u2500";
char32_t const *utf32 = U"UTF-32 string \u2500";
}
'
]])])
AC_DEFUN([_AC_CXX_CXX98_TEST_PROGRAM],
[AC_REQUIRE([_AC_CXX_CXX98_TEST_GLOBALS])dnl
AC_REQUIRE([_AC_CXX_CXX98_TEST_MAIN])dnl
m4_divert_text([INIT_PREPARE],
[[# Test code for whether the C compiler supports C++98 (complete).
ac_cxx_conftest_cxx98_program="${ac_cxx_conftest_cxx98_globals}
int
main (int argc, char **argv)
{
int ok = 0;
${ac_cxx_conftest_cxx98_main}
return ok;
}
"
]])])
AC_DEFUN([_AC_CXX_CXX11_TEST_PROGRAM],
[AC_REQUIRE([_AC_CXX_CXX98_TEST_GLOBALS])dnl
AC_REQUIRE([_AC_CXX_CXX98_TEST_MAIN])dnl
AC_REQUIRE([_AC_CXX_CXX11_TEST_GLOBALS])dnl
AC_REQUIRE([_AC_CXX_CXX11_TEST_MAIN])dnl
m4_divert_text([INIT_PREPARE],
[[# Test code for whether the C compiler supports C++11 (complete).
ac_cxx_conftest_cxx11_program="${ac_cxx_conftest_cxx98_globals}
${ac_cxx_conftest_cxx11_globals}
int
main (int argc, char **argv)
{
int ok = 0;
${ac_cxx_conftest_cxx98_main}
${ac_cxx_conftest_cxx11_main}
return ok;
}
"
]])])
AC_DEFUN([_AC_CXX_CXX14_TEST_PROGRAM],
[m4_divert_text([INIT_PREPARE],
[[ac_cxx_conftest_cxx14_program='
#if __cplusplus < 201402
# error "Compiler does not advertise C++14 conformance"
#endif
int
main ()
{
auto floating_point_literal_with_single_quotes = 0.123'\''456;
}
'
]])])
AC_DEFUN([_AC_CXX_CXX17_TEST_PROGRAM],
[m4_divert_text([INIT_PREPARE],
[[ac_cxx_conftest_cxx17_program='
#if __cplusplus < 201707
# error "Compiler does not advertise C++17 conformance"
#endif
int
main ()
{
auto u8_expression_with_u8_character_literals = u8'\''x'\'' == u8'\''x'\'';
}
'
]])])
AC_DEFUN([_AC_CXX_CXX20_TEST_PROGRAM],
[m4_divert_text([INIT_PREPARE],
[[ac_cxx_conftest_cxx20_program='
#if __cplusplus < 202002
# error "Compiler does not advertise C++20 conformance"
#endif
#include <compare>
int
main ()
{
auto expression_with_three_way_comparison = 1 <=> 2;
}
'
]])])
AC_DEFUN([_AC_CXX_CXX23_TEST_PROGRAM],
[m4_divert_text([INIT_PREPARE],
[[ac_cxx_conftest_cxx23_program='
#if __cplusplus < 202302
# error "Compiler does not advertise C++23 conformance"
#endif
int
main ()
{
auto expression_with_signed_size_literal = -1z < 0;
}
'
]])])
# _AC_CXX_CXX98_OPTIONS
# ---------------------
# Whitespace-separated list of options that might put the C++ compiler
# into a mode conforming to ISO C++ 1998 with extensions. Do not try
# "strictly conforming" modes (e.g. gcc's -std=c++98); they break some
# systems' header files. If more than one option is needed, put
# shell quotes around the group.
#
# GCC -std=gnu++98
# Intel ICC -std=c++98
# Note: because -std=c++98 puts GCC in strictly conforming mode,
# this option must be tested *after* -std=gnu++98.
# IBM XL C -qlanglvl=extended
# HP aC++ -AA
# Solaris N/A (default)
# Tru64 N/A (default, but -std gnu could be used)
m4_define([_AC_CXX_CXX98_OPTIONS], [
-std=gnu++98
-std=c++98
-qlanglvl=extended
-AA
])
# _AC_CXX_CXX11_OPTIONS
# ---------------------
# Whitespace-separated list of options that might put the C++ compiler
# into a mode conforming to ISO C++ 2011 with extensions. Do not try
# "strictly conforming" modes (e.g. gcc's -std=c++11); they break some
# systems' header files. If more than one option is needed, put
# shell quotes around the group.
#
# GCC -std=gnu++11, -std=gnu++0x
# Intel ICC -std=c++11, -std=c++0x
# Note: because -std=c++11 puts GCC in strictly conforming mode,
# these options must be tested *after* -std=gnu++11.
# IBM XL C -qlanglvl=extended0x (pre-V12.1)
# HP aC++ -AA
# Solaris N/A (no support)
# Tru64 N/A (no support)
m4_define([_AC_CXX_CXX11_OPTIONS], [
-std=gnu++11
-std=gnu++0x
-std=c++11
-std=c++0x
-qlanglvl=extended0x
-AA
])
# Similarly for C++14, C++17, C++20, C++23,
# where -std=gnu++XX should be good enough.
m4_define([_AC_CXX_CXX14_OPTIONS], [-std=gnu++14])
m4_define([_AC_CXX_CXX17_OPTIONS], [-std=gnu++17])
m4_define([_AC_CXX_CXX20_OPTIONS], [-std=gnu++20])
m4_define([_AC_CXX_CXX23_OPTIONS], [-std=gnu++23])
# _AC_PROG_CXX_STDCXX_EDITION_TRY(EDITION)
# ----------------------------------------
# Subroutine of _AC_PROG_CXX_STDCXX_EDITION. Not to be called directly.
#
# Check whether the C++ compiler accepts features of EDITION of the
# C++ standard. EDITION should be a two-digit year (e.g. 98, 11).
# (FIXME: Switch to four-digit years for futureproofing.)
# This is done by compiling the test program defined by
# _AC_C_CXX{EDITION}_TEST_PROGRAM, first with no additional
# command-line options, and then with each of the options
# in the space-separated list defined by _AC_C_CXX{EDITION}_OPTIONS.
#
# If we find a way to make the test program compile, set cache variable
# ac_cv_prog_cxx_cxxEDITION to the options required (if any), and add those
# options to $CXX. Set shell variable ac_prog_cxx_stdcxx to 'cxxEDITION',
# and set shell variable ac_cv_prog_cxx_stdcxx to the options required.
# (Neither of these variables is AC_SUBSTed. ac_cv_prog_cxx_stdcxx used
# to be a cache variable and is preserved with this name for backward
# compatibility.) Otherwise, ac_cv_prog_cxx_cxxEDITION is set to 'no'
# and the other variables are not changed.
#
# If ac_prog_cxx_stdcxx is already set to a value other than 'no',
# the shell code produced by this macro does nothing. This is so
# _AC_PROG_CXX_STDCXX_EDITION can use m4_map to iterate through
# all the editions.
AC_DEFUN([_AC_PROG_CXX_STDCXX_EDITION_TRY],
[AC_LANG_ASSERT([C++])]dnl
[AC_REQUIRE([_AC_CXX_CXX$1_TEST_PROGRAM])]dnl
[AS_IF([test x$ac_prog_cxx_stdcxx = xno],
[AC_MSG_CHECKING([for $CXX option to enable C++$1 features])
AC_CACHE_VAL(ac_cv_prog_cxx_cxx$1,
[ac_cv_prog_cxx_cxx$1=no
ac_save_CXX=$CXX
AC_LANG_CONFTEST([AC_LANG_DEFINES_PROVIDED][$][ac_cxx_conftest_cxx$1_program])
for ac_arg in '' m4_normalize(m4_defn([_AC_CXX_CXX$1_OPTIONS]))
do
CXX="$ac_save_CXX $ac_arg"
_AC_COMPILE_IFELSE([], [ac_cv_prog_cxx_cxx$1=$ac_arg])
test "x$ac_cv_prog_cxx_cxx$1" != "xno" && break
done
rm -f conftest.$ac_ext
CXX=$ac_save_CXX])
AS_IF([test "x$ac_cv_prog_cxx_cxx$1" = xno],
[AC_MSG_RESULT([unsupported])],
[AS_IF([test "x$ac_cv_prog_cxx_cxx$1" = x],
[AC_MSG_RESULT([none needed])],
[AC_MSG_RESULT([$ac_cv_prog_cxx_cxx$1])
CXX="$CXX $ac_cv_prog_cxx_cxx$1"])
ac_cv_prog_cxx_stdcxx=$ac_cv_prog_cxx_cxx$1
ac_prog_cxx_stdcxx=cxx$1])])
])
# _AC_PROG_CXX_STDCXX_EDITION
# ---------------------------
# Detect the most recent edition of the ISO C++ standard that is
# supported by the C++ compiler. Add command-line options to $CXX,
# if necessary, to enable support for this edition. Set the shell
# variable ac_prog_cxx_stdcxx to indicate the edition.
AC_DEFUN([_AC_PROG_CXX_STDCXX_EDITION],
[ac_prog_cxx_stdcxx=no
m4_map([_AC_PROG_CXX_STDCXX_EDITION_TRY], [23, 20, 17, 14, 11, 98])])