Q_DECLARE_PRIVATE gets used in the declaration of the public class, where the private class is typically visible only as a forward-decl, with no knowledge of what it's based on; consequently, the macro is obliged to use reinterpret_cast<>, which is subject to warnings when the compiler *can* see both types and their alignments differ. The same applies to Q_DECLARE_PRIVATE_D. So suppress gcc's -Wcast-align around the d_func() return statements. (If we get similar problems with other compilers we can add their suppressions likewise; but, for now, we've only seen this on MIPS64, where we use gcc.) This tripped over one use of Q_DECLARE_PRIVATE in a private Q_SLOTS: section; for some reason, gcc didn't like the semicolon on the friend declaration. Changing the context to plain private fixed that. Fixes: QTBUG-72885 Change-Id: I5edc11d46bd4eb820713adede79d53191a7e2736 Reviewed-by: Liang Qi <liang.qi@qt.io> Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io> Reviewed-by: Boxiang Sun <daetalusun@gmail.com>