Fix preprocessor warnings about undefined _MSVC_LANG

[mandree]

Stricter compiler/settings, such as found during a build on FreeBSD with clang 14, issue warnings of the kind below.

/usr/local/include/exiv2/value.hpp:1272:31: warning: '_MSVC_LANG' is not defined, evaluates to 0 [-Wundef] fixed-width font helps here-- ^

Fix: Guard use of _MSVC_LANG by a check.

Personally, I found that MSVC has several feature-specific checks in predefined macros which might allow for one standards-based check that matches GCC/clang/MSVC rather than the split check for C++ standard and MSVC language version settings.

See https://en.cppreference.com/w/cpp/feature_test

I am not building Exiv2 on MSVC, so I cannot test/suggest anything here.

[ghost]

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[codecov]

Codecov Report

Merging #2676 (2f2a926) into main (91af090) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2676   +/-   ##
=======================================
  Coverage   63.89%   63.89%           
=======================================
  Files         103      103           
  Lines       22313    22313           
  Branches    10799    10799           
=======================================
  Hits        14258    14258           
  Misses       5830     5830           
  Partials     2225     2225           

Impacted Files	Coverage Δ
include/exiv2/slice.hpp	90.56% <ø> (ø)
include/exiv2/value.hpp	85.11% <ø> (ø)

[mandree]

Sorry about the broken formatting in the commit message... apparently one line from the warning paste got lost as well. I'll push again with a fixed commit log.

Pull request has been modified.

[kmilos]

Thanks.

Personally, I found that MSVC has several feature-specific checks in predefined macros which might allow for one standards-based check that matches GCC/clang/MSVC rather than the split check for C++ standard and MSVC language version settings.

So can we potentially remove this instead of patching, since we require C++17 now anyway? (I can't check MSVC either...)

[neheb]

@kmilos no this is in the headers. We're keeping them C++11 compatible. The whole reason for using MSVC_LANG is because its __cplusplus macro is broken. Unfortunately for the _t/_v stuff, there's no standard feature test macro AFAIK.

[neheb]

@Mergifyio backport 0.28.x

[mergify]

backport 0.28.x

✅ Backports have been created

• #2677 Fix preprocessor warnings about undefined _MSVC_LANG (backport #2676) has been created for branch 0.28.x