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GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows by chris-eibl · Pull Request #148036 · python/cpython · GitHub

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GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows #148036

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Merged
itamaro merged 5 commits into from
Apr 12, 2026
Conversation 17 Commits 5 Checks 53 Files changed
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GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows #148036

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15b0bf1
check tail-calling support in pythoncore.vcxproj
chris-eibl Mar 27, 2026
92a236d
Merge branch 'main' into CheckTailCalling
chris-eibl Apr 3, 2026
3a28aeb
Merge remote-tracking branch 'upstream/main' into CheckTailCalling
chris-eibl Apr 3, 2026
8768351
blurb it
chris-eibl Apr 3, 2026
8e9a47e
update news entry
chris-eibl Apr 5, 2026
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst
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@@ -0,0 +1,2 @@
Fail fast with an explicit and clear error message if tail-calling is not
possible for MSVC builds on Windows. Patch by Chris Eibl.
9 changes: 9 additions & 0 deletions PCbuild/pythoncore.vcxproj
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Expand Up @@ -749,4 +749,13 @@
<Target Name="_DeletePyBuildDirTxt" BeforeTargets="PrepareForBuild">
<Delete Files="$(OutDir)pybuilddir.txt" />
</Target>

<Target Name="_CheckTailCalling" BeforeTargets="PrepareForBuild" Condition="'$(UseTailCallInterp)' == 'true' and $(PlatformToolset) != 'ClangCL'">
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<Error Text="MSVC supports tail-calling only for x64."
Condition="$(Platform) != 'x64'" />

zooba Apr 14, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Not convinced this needs to be an error - if/when MSVC gets support, they may want to test it on CPython themselves before shipping, and we're just going to block them.

As with Debug, make it a warning if you really want, but ultimately it's up to the user to decide whether they're enabling a feature or not. We shouldn't prevent users from doing what they want to do.
<Error Text="Platform toolset >= v145 is required for tail-calling."
Condition="$(PlatformToolset.Replace('v', '0')) &lt; '145'" />

zooba Apr 14, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

We can't do this check - PlatformToolset is an arbitrary string, you can't assume that it's comparable to a number in any meaningful way.
<Error Text="MSVC requires optimization to be enabled for tail-calling."
Condition="$(Configuration) == 'Debug'" />
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zooba Apr 14, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Why would we do this? It makes it annoyingly hard to set tail calling enabled in my environment and then choose between debug/release builds.

A warning is fine, if you really want, but a debug build is inherently without optimisations, so there's no reason to remind users that that's what they're getting.
</Target>
</Project>
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