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 0b97d58548
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>

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).

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'

test cases/windows/18 msvc cplusplus define/main.cpp

@@ -0,0 +1,7 @@
+int main() {
+#if __cplusplus == 199711L
+    return 1;
+#else
+    return 0;
+#endif
+}

test cases/windows/18 msvc cplusplus define/meson.build

@@ -0,0 +1,14 @@
+project('msvc __cplusplus', 'cpp', default_options : ['cpp_std=c++14'])
+
+cpp = meson.get_compiler('cpp')
+
+if cpp.get_id() != 'msvc'
+  error('MESON_SKIP_TEST: test is only relevant for msvc')
+elif meson.project_version().version_compare('< 15.7')
+  error('MESON_SKIP_TEST: test is only relevant for msvc versions >= 15.7')
+endif
+
+test(
+  'main',
+  executable('main', 'main.cpp'),
+)

test cases/windows/3 cpp/meson.build

@@ -1,4 +1,4 @@
-project('wincpp', 'cpp')
+project('wincpp', 'cpp', default_options : ['cpp_std=c++14'])
 
 exe = executable('prog', 'prog.cpp')
 test('wincpp', exe)