Implement [variant.get] for QVariant [1/2]: get_if()

QVariant supports non-default-constructible payloads, in principle
(QTBUG-105140). And fromValue() works with such types, but value()
insists on providing a wide contract and therefore accidentally
requires default-constructible.

We can now invent other "Qt-ish" API like optional::value_or() or a
value() that returns optional<T>, but we should first get the
interface in that generic code must use, and which at the same time is
the most versatile, because it gives write access to the element
stored in the variant: [variant.get], consisting of get_if(), get(),
and holds_alternative(). The latter is the same as get_if() !=
nullptr, so we won't provide it.

This first patch implements get_if(), adds test for it.

As a Hidden Friend supposed to be called with explicit template
arguments, we run into the problem that wg21.link/P0846 solved for
C++20. Add the usual work-around, and check it works.

The ChangeLog will be on the last patch.

Task-number: QTBUG-111598
Change-Id: I23f57ea2de3946944810c5552c68a7a3060a44f2
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions