Fix operator checks for containers whose value_type equals themselves.
It does not make sense to recurse on value_type in that case. Thanks to
std::disjunction having short-circuiting semantics, we can avoid that
issue by checking first whether T is T::value_type.
As a drive-by, check for value_type typedef before checking for
begin/end in is_container. This works around an issue in gcc <= 8.1,
which fails to correctly SFINAE the case where begin and end are private
methods.
Pick-to: 6.0 6.1
Fixes: QTBUG-89456
Change-Id: I27305a7cfe050f13a279c07f00bc229c01daa25b
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
ce83e56cfe