[patch #9572] ax_cxx_compile_stdcxx.m4: Drop the check for default C++ version support

[Krzesimir Nowak]

URL:
  <http://savannah.gnu.org/patch/?9572>

                 Summary: ax_cxx_compile_stdcxx.m4: Drop the check for default
C++ version support
                 Project: GNU Autoconf Archive
            Submitted by: krnowak
            Submitted on: Wed 21 Feb 2018 02:39:20 PM UTC
                Priority: 5 - Normal
                  Status: None
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any

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Details:

The problem with the check is that it can't detect whether the version is with
extensions or not. So it can't take the "noext" or "ext" option into account.

This may be a problem when an extended version of C++ has a feature that
conflicts with another feature from the C++ standard and we have no way to
tell the macro to add a "-std" flag anyway.

An example:

    using namespace std::complex_literals;
    std::complex<double> c = 1.0;
    c += 1.0i;
    std::cout << "abs" << c << " = " << abs(c) << '\n';

Here, the "1.0i" constant is either a GCC extension if compiled with
-std=gnu++14 or a standard C++ operator if compiled with -std=c++14. If a GCC
extension is used, the snippet fails to compile. Unfortunately, recent GCC
versions default to gnu++14, so this code fails to compile by default.

Dropping the check forces the macro to evaluate compiler flags with taking the
"ext"/"noext" option into account.

Issue spotted by Frédéric Mangano-Tarumi.




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File Attachments:


-------------------------------------------------------
Date: Wed 21 Feb 2018 02:39:20 PM UTC  Name:
0001-ax_cxx_compile_stdcxx.m4-Drop-the-check-for-default-.patch  Size: 3KiB  
By: krnowak

<http://savannah.gnu.org/patch/download.php?file_id=43377>

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