[BUG] Even after setting language="c++", the compilation still uses /Tcmyext.c

setuptools version

75.8.0

Python version

3.12

OS

Windows11

Additional environment information

VS2022

Description

When compiling a C extension using setuptools, despite setting language="c++" and extra_compile_args=["/TP"] in the setup.py file, MSVC still compiles the file as C, using the /Tcmyext.c option instead of compiling it as C++.

Actual Behavior:
The compilation command still uses /Tcmyext.c and compiles the file as C instead of C++.

Expected behavior

MSVC should compile the myext.c file as C++ according to the /TP option and language="c++" setting.

How to Reproduce

myext.c

#define PY_SSIZE_T_CLEAN
#include <Python.h>

#define THREAD_LOCAL thread_local

THREAD_LOCAL int counter = 0;  

static PyObject* increment(PyObject* self, PyObject* args) {
    counter += 1;
    return PyLong_FromLong(counter);
}

static PyMethodDef MyMethods[] = {
    {"increment", increment, METH_NOARGS, "Increment thread-local counter"},
    {NULL, NULL, 0, NULL}
};

static struct PyModuleDef mymodule = {
    PyModuleDef_HEAD_INIT, "myext", NULL, -1, MyMethods
};

PyMODINIT_FUNC PyInit_myext(void) {
    return PyModule_Create(&mymodule);
}

setup.py

from setuptools import setup, Extension

module = Extension(
    "myext",
    sources=["myext.c"],  # C file
    extra_compile_args=["/TP"],  # Force C++ compilation
    language="c++"  # Specify language as C++
)

setup(
    name="MyExtension",
    ext_modules=[module],
)

Run the compilation command:

python setup.py build_ext --inplace

Output

"C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\bin\HostX86\x64\cl.exe" /c /nologo /O2 /W3 /GL /DNDEBUG /MD -IC:\Users\foo\AppData\Local\Programs\Python\Python312\include -IC:\Users\foo\AppData\Local\Programs\Python\Python312\Include "-IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\include" "-IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\ATLMFC\include" "-IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Auxiliary\VS\include" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\um" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\shared" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\winrt" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\cppwinrt" /Tcmyext.c /Fobuild\temp.win-amd64-cpython-312\Release\myext.obj /TP
myext.c
myext.c(11): error C2054: expected '(' to follow 'thread_local'
myext.c(11): error C2085: 'counter': not in formal parameter list
myext.c(11): error C2143: syntax error: missing ';' before '='
myext.c(14): error C2065: 'counter': undeclared identifier
myext.c(15): error C2065: 'counter': undeclared identifier

Reproduce the issue with setup.py and myext.c using python setup.py build_ext --inplace, then trace the build_ext path that constructs the MSVC command. Confirm how language="c++", /TP, and the /Tcmyext.c option interact; done means the extension is compiled as C++ on the reported Windows environment and the behavior has regression coverage.