a611a9f537 (in Qt 5) added support for
mixed-type comparisons through QCOMPARE. That commit added a new
overload for qCompare taking two types, T1 and T2; but it also left the
same-type qCompare(T, T) overload around, guarded by a Qt 6 version
check.
The mixed-type version is however not a generalization of the same-type
one, because it won't work if one of the arguments doesn't participate
in FTAD. Case in point: braced-init-lists. In Qt 5 this worked:
QCOMPARE(some_container, {42});
but in Qt 6 it does not work any more. The mixed-type overload fails
deduction (can't deduce T2); in Qt 5 the same-type overload deduced
T=SomeContainer, and {42} was used to select a constructor for
SomeContainer.
--
There's a partial, straightforward workaround for this: default T2 to
T1 in the mized-type overload. In that case T2 has a "fallback" if it
cannot be deduced. This is partial because of course doesn't address
the case in which T1 cannot be deduced, but I don't think that is
common at all.
QList is special here, because it has qCompare overloads that makes it
comparable with arrays, initializer lists and more. I don't like that
very much -- we should probably have a qCompare(input_range,
input_range) overload, but that's an exercise for C++20.
Change-Id: I344ba33167829984978cd8d649a1904349a9edab
Pick-to: 6.5 6.6
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
5560723e9d