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, 15 insertions, 0 deletions

diff --git a/docs/markdown/snippets/msvc_cplusplus_define.md b/docs/markdown/snippets/msvc_cplusplus_define.md
new file mode 100644
index 0000000..2c5648d
--- /dev/null
+++ b/docs/markdown/snippets/msvc_cplusplus_define.md
@@ -0,0 +1,15 @@
+## MSVC now sets the __cplusplus #define accurately
+
+MSVC will always return `199711L` for `__cplusplus`, even when a newer c++
+standard is explicitly requested, unless you pass a specific option to the
+compiler for MSVC 2017 15.7 and newer. Older versions are unaffected by this.
+
+Microsoft's stated rationale is that "a lot of existing code appears to depend
+on the value of this macro matching 199711L", therefore for compatibility with
+such (MSVC-only) code they will require opting in to the standards-conformant
+value.
+
+Meson now always sets the option if it is available, as it is unlikely that
+users want the default behavior, and *impossible* to use the default behavior
+in cross-platform code (which frequently breaks as soon as the first person
+tries to compile using MSVC).