autoconf, clang static analyser and C++17

[Peter Hull <peterhull90@xxxxxxxxx>]

Hi,
I've got a problem running scan-build on some C++ 17 code and I'm not
sure if the cause is me, scan-build or autoconf. I'd welcome any
advice.

I've made a small test case:(still long, sorry)

$ cat configure.ac
#                                               -*- Autoconf -*-
# Process this file with autoconf to produce a configure script.

AC_PREREQ([2.71])
AC_INIT([FULL-PACKAGE-NAME], [VERSION], [BUG-REPORT-ADDRESS])
AC_CONFIG_SRCDIR([me.cc])
AC_CONFIG_MACRO_DIR([m4])

AC_PROG_CXX
AX_CXX_COMPILE_STDCXX([17])

AC_CONFIG_FILES([Makefile])
AC_OUTPUT

$ cat Makefile.in
CXXFLAGS:=@CXXFLAGS@
CXX:=@CXX@

all: me

clean:
        -rm me

$ cat me.cc
#include <string>
#include <string_view>
#include <iostream>

int main() {
  std::string_view s("Hello");
  std::cout << std::string(s) << std::endl;
  return 0;
}


I've put the AX_CXX_COMPILE_STDCXX macro from
https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
into ./m4/

I ran aclocal and autoconf then ./configure && make, and everything is fine.

However scan-build ./configure && scan-build --keep-cc make gives me
errors because it can't find std::string_view

/usr/bin/../libexec/c++-analyzer -g -O2    me.cc   -o me
me.cc:6:19: error: expected ';' after expression
  std::string_view s("Hello");
                  ^
                  ;
me.cc:6:8: error: no member named 'string_view' in namespace 'std'
  std::string_view s("Hello");
  ~~~~~^

which I am assuming is because string_view was new in C++17 and the
analyzer is defaulting to some version prior to that. In fact I can
specify '-std=c++17' explicitly in the CXXFLAGS given to configure and
it works.

However, the purpose of the AX_CXX_COMPILE_STDCXX is to handle that
for me I believe. During normal configuration I see this:

$ ./configure
checking for g++... g++
...lines omitted...
checking for g++ option to enable C++11 features... none needed
checking whether g++ supports C++17 features by default... yes
configure: creating ./config.status
config.status: creating Makefile


$ scan-build ./configure
scan-build: Using '/usr/bin/clang-15' for static analysis
...lines omitted...
checking for /usr/bin/../libexec/c++-analyzer option to enable C++11
features... none needed
checking whether /usr/bin/../libexec/c++-analyzer supports C++17
features by default... yes
configure: creating ./config.status
config.status: creating Makefile

So I think maybe AX_CXX_COMPILE_STDCXX believes that a -std= flag
isn't needed but it is.

Does this make sense? What do you think?

Final bit:
I'm running Fedora 37, autoconf (GNU Autoconf) 2.71,
AX_CXX_COMPILE_STDCXX v15 (2021),  g++ (GCC) 12.2.1, and clang version
15.0.7

Thanks,
Peter