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

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itamaro merged 5 commits into from
Apr 12, 2026
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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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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 $(PlatformToolset) != 'ClangCL'">
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itamaro marked this conversation as resolved.
<Error Text="MSVC supports tail-calling only for x64."
Condition="$(Platform) != 'x64'" />
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@zooba zooba Apr 14, 2026

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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'" />
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@zooba zooba Apr 14, 2026

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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 zooba Apr 14, 2026

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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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