.C files are now treated as C++ code

mesonbuild/compilers/compilers.py

@@ -51,7 +51,7 @@ lib_suffixes = ('a', 'lib', 'dll', 'dll.a', 'dylib', 'so')  # type: T.Tuple[str,
 # This means we can't include .h headers here since they could be C, C++, ObjC, etc.
 lang_suffixes = {
     'c': ('c',),
-    'cpp': ('cpp', 'cc', 'cxx', 'c++', 'hh', 'hpp', 'ipp', 'hxx', 'ino', 'ixx'),
+    'cpp': ('cpp', 'cc', 'cxx', 'c++', 'hh', 'hpp', 'ipp', 'hxx', 'ino', 'ixx', 'C'),
     'cuda': ('cu',),
     # f90, f95, f03, f08 are for free-form fortran ('f90' recommended)
     # f, for, ftn, fpp are for fixed-form fortran ('f' or 'for' recommended)
@@ -510,7 +510,9 @@ class Compiler(metaclass=abc.ABCMeta):
     def can_compile(self, src: 'mesonlib.FileOrString') -> bool:
         if isinstance(src, mesonlib.File):
             src = src.fname
-        suffix = os.path.splitext(src)[1].lower()
+        suffix = os.path.splitext(src)[1]
+        if suffix != '.C':
+            suffix = suffix.lower()
         return bool(suffix) and suffix[1:] in self.can_compile_suffixes
 
     def get_id(self) -> str:

mesonbuild/mesonlib/universal.py

@@ -350,10 +350,12 @@ class FileMode:
         return perms
 
 dot_C_dot_H_warning = """You are using .C or .H files in your project. This is deprecated.
-         Currently, Meson treats this as C code, but this
-            might change in the future, breaking your build.
-         You code also might be already broken on gcc and clang.
-         See https://github.com/mesonbuild/meson/pull/8239 for the discussions."""
+         Currently, Meson treats this as C++ code, but they
+            used to be treated as C code.
+         Note that the situation is a bit more complex if you are using the
+         Visual Studio compiler, as it treats .C files as C code, unless you add
+         the /TP compiler flag, but this is unreliable.
+         See https://github.com/mesonbuild/meson/pull/8747 for the discussions."""
 class File:
     def __init__(self, is_built: bool, subdir: str, fname: str):
         if fname.endswith(".C") or fname.endswith(".H"):

test cases/common/2 cpp/cpp.C

@@ -0,0 +1,6 @@
+#include<iostream>
+
+int main(void) {
+  std::cout << "C++ seems to be working." << std::endl;
+  return 0;
+}

test cases/common/2 cpp/meson.build

@@ -32,3 +32,10 @@ else
 endif
 
 assert(exe_disabled, 'Executable was not disabled.')
+
+if cpp.get_id() == 'msvc'
+  exe = executable('cppprog', 'cpp.C', cpp_args : '/TP')
+else
+  exe = executable('cppprog', 'cpp.C')
+endif
+test('cpptest', exe)