compilers/c++: Add MSVC option to make the __cplusplus define accurate

Otherwise it always returns the value for c++98, starting with MSVC 2017
15.7 or later. Earlier versions are not affected by this mis-feature.

See: https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-160

This was originally applied as 0b97d585480e973d8b149618901f7a4ddfa1a906
but later reverted because it made the CI red. Try it again, now.

Original-patch-by: Dylan Baker <dylan@pnwbakers.com>
Co-authored-by: Dylan Baker <dylan@pnwbakers.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/mesonbuild/compilers/cpp.py b/mesonbuild/compilers/cpp.py
index fe09b6b..4c24767 100644
--- a/mesonbuild/compilers/cpp.py
+++ b/mesonbuild/compilers/cpp.py
@@ -734,6 +734,16 @@ class VisualStudioCPPCompiler(CPP11AsCPP14Mixin, VisualStudioLikeCPPCompilerMixi
             del args[i]
         return args
 
+    def get_always_args(self) -> T.List[str]:
+        args = super().get_always_args()
+
+        # By default, MSVC has a broken __cplusplus define that pretends to be c++98:
+        # https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-160
+        # Pass the flag to enable a truthful define, if possible.
+        if version_compare(self.version, '>= 15.7') and '/Zc:__cplusplus' not in args:
+            return args + ['/Zc:__cplusplus']
+        return args
+
 class ClangClCPPCompiler(CPP11AsCPP14Mixin, VisualStudioLikeCPPCompilerMixin, ClangClCompiler, CPPCompiler):
 
     id = 'clang-cl'