From 15b0bf141b4b4f123ab391b4011ca9826ad29ad0 Mon Sep 17 00:00:00 2001
From: Chris Eibl <138194463+chris-eibl@users.noreply.github.com>
Date: Fri, 27 Mar 2026 08:31:57 +0100
Subject: [PATCH 1/2] check tail-calling support in pythoncore.vcxproj

---
 PCbuild/pythoncore.vcxproj | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index 61bee29c0af3d6..9356a66dfb4642 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -749,4 +749,13 @@
   <Target Name="_DeletePyBuildDirTxt" BeforeTargets="PrepareForBuild">
     <Delete Files="$(OutDir)pybuilddir.txt" />
   </Target>
+
+  <Target Name="_CheckTailCalling" BeforeTargets="PrepareForBuild" Condition="'$(UseTailCallInterp)' == 'true' and $(PlatformToolset) != 'ClangCL'">
+    <Error Text="MSVC supports tail-calling only for x64."
+           Condition="$(Platform) != 'x64'" />
+    <Error Text="Platform toolset >= v145 is required for tail-calling."
+           Condition="$(PlatformToolset.Replace('v', '0')) &lt; '145'" />
+    <Error Text="MSVC requires optimization to be enabled for tail-calling."
+           Condition="$(Configuration) == 'Debug'" />
+  </Target>
 </Project>

From 8768351c9d325a7ad8ab55c616b6367719ae5d1b Mon Sep 17 00:00:00 2001
From: Chris Eibl <138194463+chris-eibl@users.noreply.github.com>
Date: Fri, 3 Apr 2026 20:11:14 +0200
Subject: [PATCH 2/2] blurb it

---
 .../next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst   | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst

diff --git a/Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst b/Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst
new file mode 100644
index 00000000000000..a6f35efade38c5
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst
@@ -0,0 +1,2 @@
+Check early whether tail-calling is possible for MSVC builds on Windows.
+Patch by Chris Eibl.