From 390cc8a262468cd6381ef2193d1bc257a36d0917 Mon Sep 17 00:00:00 2001
From: Chris Eibl <138194463+chris-eibl@users.noreply.github.com>
Date: Mon, 23 Mar 2026 20:06:56 +0100
Subject: [PATCH] fix building the jit stencils on Windows when the

interpreter is built with a different clang version.
---
 ...-03-23-20-06-35.gh-issue-146210.C01Rmq.rst |  2 ++
 Tools/jit/_llvm.py                            | 19 ++++++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Build/2026-03-23-20-06-35.gh-issue-146210.C01Rmq.rst

diff --git a/Misc/NEWS.d/next/Build/2026-03-23-20-06-35.gh-issue-146210.C01Rmq.rst b/Misc/NEWS.d/next/Build/2026-03-23-20-06-35.gh-issue-146210.C01Rmq.rst
new file mode 100644
index 00000000000000..ce59a9a3a571b4
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2026-03-23-20-06-35.gh-issue-146210.C01Rmq.rst
@@ -0,0 +1,2 @@
+Fix building the jit stencils on Windows when the interpreter is built with
+a different clang version. Patch by Chris Eibl.
diff --git a/Tools/jit/_llvm.py b/Tools/jit/_llvm.py
index 8b68c1e8636af7..215013aa213305 100644
--- a/Tools/jit/_llvm.py
+++ b/Tools/jit/_llvm.py
@@ -42,9 +42,26 @@ async def _run(tool: str, args: typing.Iterable[str], echo: bool = False) -> str
     async with _CORES:
         if echo:
             print(shlex.join(command))
+
+        if os.name == "nt":
+            env = os.environ.copy()
+            try:
+                # https://github.com/python/cpython/issues/146210
+                # When the Python iterpreter is built with
+                # "/p:PlatformToolset=ClangCL" "/p:LLVMInstallDir=...""
+                # msbuild populates the INCLUDE variable based on this
+                # clang version, which might be different to the one used
+                # for building the jit stencils. Mixing those include paths
+                # can cause mysterious build errors, so we remove the
+                # variable from the environment when invoking LLVM tools.
+                del env["INCLUDE"]
+            except KeyError:
+                pass
+        else:
+            env = None
         try:
             process = await asyncio.create_subprocess_exec(
-                *command, stdout=subprocess.PIPE
+                *command, stdout=subprocess.PIPE, env=env
             )
         except FileNotFoundError:
             return None