PyQt 6.11.0 / Qt 6.11.2: QMetaMethodBuilder layout mismatch breaks slot return types with Clang

[Phil Thompson]

On 16/09/2026 09:11, pyqt-user.folic415 at passinbox.com wrote:
> Hello,
> 
> I encountered a compatibility issue when building PyQt 6.11.0 against
> Qt 6.11.2 for Android using python-for-android.
> I then let my coding harness analyze the issue and it came up with a
> patch and the issue description below. I don't fully understand but it
> might be helpful for you to fix a bug:
> 
> Environment
> 
> - PyQt 6.11.0
> - Qt 6.11.2
> - Python 3.11.15
> - Android NDK r28c, Clang 19
> - x86_64 APK running on an Android 9 emulator
> 
> A Python slot declared with @pyqtSlot(result=str) returns undefined
> when invoked from QML. In my application, passing that result to
> another slot accepting a string produces an empty string and
> subsequently a
> KeyError.
> 
> ### Apparent cause
> 
> Qt 6.11.2 added a QMetaMethod::MethodType _type member to 
> QMetaMethodBuilder:
> 
> https://github.com/qt/qtbase/commit/a598cc13db785acc6fe85ee5f54735557d878bcc
> <https://github.com/qt/qtbase/commit/a598cc13db785acc6fe85ee5f54735557d878bcc>
> 
> 
> However, PyQt’s copied declaration in:
> 
> qpy/QtCore/qpycore_qmetaobjectbuilder.h
> 
> still contains only:
> 
> const QMetaObjectBuilder *_mobj;
> int _index;
> 
> On x86_64, the new field occupies bytes previously considered padding.
> Clang can discard those bytes when receiving a builder returned by Qt.
> Qt subsequently reads an invalid _type, causing setReturnType() to
> silently do nothing. The generated slot metadata therefore remains
> void, and QML receives undefined.
> 
> ### Independent reproduction
> 
> A focused C++ test using PyQt’s copied header and the actual Qt 6.11.2
> library calls:
> 
> auto slot = builder.addSlot("startWizard()");
> slot.setReturnType("QString");
> 
> It then inspects the method produced by builder.toMetaObject().
> 
> With deliberately nonzero backing storage to expose the missing field
> deterministically:
> 
> Compiler                 Original declaration Corrected declaration
> ━━━━━━━━━━━━━━━  ━━━━━━━━━━━━━━━━━━━━━━  ━━━━━━━━━━━━━━━━━━━━━━━
> Clang 19, -O2         Return type void                  Return type 
> QString
> ───────────────  ──────────────────────  ───────────────────────
> GCC, -O2                Return type QString             Return type 
> QString
> 
> GCC happens to preserve the relevant bytes in this reproduction. Clang
> also exhibits the failure with optimizations disabled.
> 
> ### Proposed correction
> 
> Include and add the following member immediately after _index in
> PyQt’s QMetaMethodBuilder declaration:
> 
> #if <#if> QT_VERSION >= QT_VERSION_CHECK(6, 11, 2)
> QMetaMethod::MethodType _type = QMetaMethod::Method;
> #endif <#endif>
> 
> This restores the correct return metadata in the focused Clang
> reproduction. Verification of the patched Android APK is still
> pending.
> 
> I also checked the official 6.11.1.dev2609111110 source snapshot; its
> copied header is unchanged and still lacks this member.
> 
> Could this declaration be updated in an upcoming PyQt release?

Yes, thanks.

Phil