Re: configure adds -std=gnu++11 to CXX variable

On 2024-05-27 03:35, Florian Weimer wrote:
> Does this turn on experimental language modes by default?  That's
> probably not what we want.

What do C++ developers want these days? Autoconf should have a 
reasonable default, and C++11 is surely not a good default anymore.

It would be easy to discourage use of C++23 in the near future by using 
a stricter test, such as the attached patch (which I've not installed). 
Even GCC 14.1 fails the test in the new patch, so 'configure' will fall 
back on C++20. I hope GCC 15 will succeed on it but of course there's no 
guarantee. Although this new test covers a DR and is not specific to 
C++23 (and there seems to be some reluctance to implement the DR, I 
assume because it invalidates some older code), I expect any compiler 
passing both this and the __cplusplus>=202302 check would be good enough.

Would this patch be preferable to the current Autoconf master?


> It would be better to have an option to raise the C++ mode to at least a
> certain revision, and otherwise use the default.

That option is already available. For example, a builder who doesn't 
want C++23 can use './configure ac_cv_prog_cxx_cxx23=no', and a 
developer can discourage C++23 by putting ': ${ac_cv_prog_cxx_cxx23=no}' 
early in configure.ac.

As I mentioned earlier, I volunteered to document this sort of thing if 
Zack doesn't come up with something nicer soon.

From b2f28ce66ea1618b50e14085059ce512d7245300 Mon Sep 17 00:00:00 2001
From: Paul Eggert <[email protected]>
Date: Mon, 27 May 2024 11:56:06 -0700
Subject: [PATCH] Add P1787R6 test to AC_PROG_CXX C++23 check

* lib/autoconf/c.m4 (_AC_CXX_CXX23_TEST_PROGRAM): Check more
carefully for C++23 support, by checking for P1787R6, which even
GCC 14.1 and Clang 18.1 have not implemented.
---
 lib/autoconf/c.m4 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index a0a2b487..157dcb12 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -2856,6 +2856,19 @@ AC_DEFUN([_AC_CXX_CXX23_TEST_PROGRAM],
 # error "Compiler does not advertise C++23 conformance"
 #endif
 
+/* Check support for P1787R6: Declarations and where to find them
+   <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html>.
+   See <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98939#c9>.  */
+template <typename T> struct A {
+  void f(int);
+  template <typename U> void f(U);
+};
+template <typename T> struct B {
+  template <typename T2> struct C { };
+};
+template <typename T, template <typename X> class TT = T::C> struct E { };
+E<B<int> > db;
+
 int
 main ()
 {
-- 
2.45.1