Hi, The issue here is that MinGW GCC is issuing a warning and that we compile with -Werror to treat warnings as errors.
c++ - Importing inline functions in MinGW - Stack Overflow<https://stackoverflow.com/questions/11546403/importing-inline-functions-in-mingw> has a similar case. LLVM-MinGW using clang is also issuing warnings, but these warnings can be suppressed via -Wno-ignored-attributes. No idea why GCC has the warning not part of the -Wignored-attributes like clang does. Cheers, Cristian ________________________________ From: Development <development-boun...@qt-project.org> on behalf of Marc Mutz via Development <development@qt-project.org> Sent: Wednesday, August 23, 2023 16:27 To: Qt development mailing list <development@qt-project.org> Subject: [Development] On the use of the inline keyword Hi, Every now and then we get a sporadic MinGW error because someone writes the moral equivalent of the following: class Q_FOO_EXPORT QMeep { ~~~~ QBar bar() const; ~~~~ }; inline QBar QMeep::bar() const; Resulting in something like this: qmeep.h: error: 'QBar QMeep::bar() const' redeclared without dllimport attribute after being referenced with dll linkage [-Werror] The last time I remember this came up was in 5.8 (https://bugreports.qt.io/browse/QTBUG-56459). It's so sporadic that Ivan didn't manage to repro when I pointed this out as a potential problem in API review, until it suddenly hit today in code that was unchanged since Qt 5.0: https://testresults.qt.io/coin/integration/qt/qtbase/tasks/1698267962 I don't claim to know what's the cause (it's probably use of the function in inline implementation), but I do know the fix, and that's to put `inline` on the _declaration_, but _not_ the definition. While it doesn't hurt to put it on the definition, it's exactly this practice that lets other platforms compile this code that MinGW then out of a sudden starts to complain about. By _not_ putting `inline` on the definition, only on the declaration, we cause all platforms to complain if we get it wrong ("multiple definition errors at link time" or "inline function not defined"). So, if we want this as a minimal-complexity rule: - the `inline` keyword goes _only_ on declarations, never on definitions - the `inline` keyword should be omitted on the following declarations: - of constexpr functions (they're implicitly inline) - of consteval functions (ditto) - of in-class-body member function definitions (they, too, are implicitly inline) Please add this to your things to look out for in reviews. Thanks, Marc -- Marc Mutz <marc.m...@qt.io> Principal Software Engineer The Qt Company Erich-Thilo-Str. 10 12489 Berlin, Germany www.qt.io Geschäftsführer: Mika Pälsi, Juha Varelius, Jouni Lintunen Sitz der Gesellschaft: Berlin, Registergericht: Amtsgericht Charlottenburg, HRB 144331 B -- Development mailing list Development@qt-project.org https://lists.qt-project.org/listinfo/development
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