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We should fail to compile if _Py_TAIL_CALL_INTERP is set but preserve_none and musttail do not exist. #140513

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We should fail to compile if _Py_TAIL_CALL_INTERP is set but preserve_none and musttail do not exist.#140513
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3.15bugs and security fixesbuildThe build process and cross-buildeasyinterpreter-core(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or error

Description

@Fidget-Spinner
Fidget-Spinner
opened on Oct 23, 2025

Bug report

Bug description:

The code should look something like this:

#if _Py_TAIL_CALL_INTERP && (defined(__clang__) || defined(__GNUC__))
#    if !_Py__has_attribute(preserve_none) || !!_Py__has_attribute(musttail)
#        error "This compiler does not have support for efficient tail calling."
#    endif
#endif

CPython versions tested on:

CPython main branch, 3.15, 3.14

Operating systems tested on:

No response

Linked PRs

Activity

  1. Fidget-Spinner
    added
    type-bugAn unexpected behavior, bug, or error
    on Oct 23, 2025
  2. StanFromIreland
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    buildThe build process and cross-build
    on Oct 23, 2025
  3. efimov-mikhail
    added
    3.14bugs and security fixes
    3.15bugs and security fixes
    on Oct 23, 2025
  4. Fidget-Spinner
    added
    easy
    on Oct 23, 2025

  5. XChaitanyaX commented on Oct 24, 2025

    @XChaitanyaX
    XChaitanyaX
    on Oct 24, 2025
    Contributor

    Can I have a pr for this?

    !!_Py__has_attribute(musttail)

    double exclamation !! mark?? Shouldn't it be single?

  6. chris-eibl commented on Oct 24, 2025

    @chris-eibl
    chris-eibl
    on Oct 24, 2025
    Member

    Maybe we can already handle that "earlier" in configure?

  7. Fidget-Spinner commented on Oct 24, 2025

    @Fidget-Spinner
    Fidget-Spinner
    on Oct 24, 2025
    MemberAuthor

    Maybe we can already handle that "earlier" in configure?

    This works too but I'd rather bloat up our C code rather than the configure code. The configure script is already pretty complex so I don't like adding more to it.

  8. chris-eibl commented on Oct 24, 2025

    @chris-eibl
    chris-eibl
    on Oct 24, 2025
    Member

    Yeah, and we could handle MSVC (#139962) here then in a similar way, too.

    E.g. include a check for _MSC_VER like #if defined(_MSC_VER) && (_MSC_VER < 1950)?

  9. Fidget-Spinner
    added a commit that references this issue on Nov 1, 2025

    gh-140513: Fail to compile if `_Py_TAIL_CALL_INTERP` is set but `pres…

    2f60b8f
  10. Fidget-Spinner
    removed
    3.14bugs and security fixes
    on Nov 1, 2025

  11. Fidget-Spinner commented on Nov 1, 2025

    @Fidget-Spinner
    Fidget-Spinner
    on Nov 1, 2025
    MemberAuthor

    We missed the train for 3.14. Backporting now would break GCC 15.

  12. Fidget-Spinner
    closed this as completedon Nov 1, 2025
  13. Fidget-Spinner
    reopened this on Nov 2, 2025

  14. Fidget-Spinner commented on Nov 2, 2025

    @Fidget-Spinner
    Fidget-Spinner
    on Nov 2, 2025
    MemberAuthor

    We need to backport this to 3.14 as well. The macro name has changed there. Can someone take care of the manual backport please?

  15. chris-eibl commented on Nov 2, 2025

    @chris-eibl
    chris-eibl
    on Nov 2, 2025
    Member

    Yeah, #139120 did not get backported.

    I think it would be best to start fresh over against 3.14, cherry picker would be no help?
    Also because the MSVC part shouldn't be part of it or needs extra care, too?

    We missed the train for 3.14. Backporting now would break GCC 15.
    which has musttail but no preserve_none

    How to handle that? Something like

    // GCC 15 has support for musttail, but not for preserve_none.
// Because we previously have allowed this, let's continue to do so.
#   if defined(__clang__) || defined(__GNUC__)
#       if !_Py__has_attribute(preserve_none) || !_Py__has_attribute(musttail)
#           if (__GNUC__ != 15)
#               error "This compiler does not have support for efficient tail calling."
#           endif
#       endif
#   elif defined(_MSC_VER)
#       error "tail calling is not supported for MSVC."
#   endif

    https://godbolt.org/z/d74ozM1oE

  16. chris-eibl commented on Nov 2, 2025

    @chris-eibl
    chris-eibl
    on Nov 2, 2025
    Member

    Or better break for GCC 15 like on main?

  17. Fidget-Spinner commented on Nov 2, 2025

    @Fidget-Spinner
    Fidget-Spinner
    on Nov 2, 2025
    MemberAuthor

    Yes just break GCC 15.

  18. chris-eibl commented on Nov 2, 2025

    @chris-eibl
    chris-eibl
    on Nov 2, 2025
    Member

    Ok, I'll take care about the manual backport.

  19. chris-eibl
    added a commit that references this issue on Nov 2, 2025

    pythongh-140513: Fail to compile if `_Py_TAIL_CALL_INTERP` is set but…

    a6d52fe
  20. Fidget-Spinner
    added a commit that references this issue on Nov 4, 2025

    [3.14] gh-140513: Fail to compile if _Py_TAIL_CALL_INTERP is set but …

    f0eb7d4
  21. Fidget-Spinner
    closed this as completedon Nov 4, 2025
  22. StanFromIreland
    added a commit that references this issue on Dec 6, 2025

    pythongh-140513: Fail to compile if `_Py_TAIL_CALL_INTERP` is set but…

    c211943
  23. itamaro
    added a commit that references this issue on Apr 12, 2026

    GH-148047: Check early whether tail-calling is possible for MSVC buil…

    cbd81d5
  24. ljfp
    added a commit that references this issue on Apr 25, 2026

    pythonGH-148047: Check early whether tail-calling is possible for MSV…

    ad923b5
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