QPartialOrdering: add missing conversion from std::{weak,strong}_ordering

QPartialOrdering is suppsed to be a drop-in replacement for
std::partial_ordering, so it has to be convertible from all
std::_ordering types.

It was, however, only convertible from std::partial_ordering, and two
user-defined conversions in a row are not allowed.

Add the missing constructors. They, in turn, can then delegate to the
partial_ordering constructor.

Change-Id: I085d95677b258b4a61aabfd5468c1c43c2212766
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/global/qcompare.h

@@ -706,6 +706,12 @@ public:
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::LegacyUncomparable::Unordered);
     }
 
+    constexpr Q_IMPLICIT QPartialOrdering(std::weak_ordering stdorder) noexcept
+        : QPartialOrdering(std::partial_ordering(stdorder)) {}
+
+    constexpr Q_IMPLICIT QPartialOrdering(std::strong_ordering stdorder) noexcept
+        : QPartialOrdering(std::partial_ordering(stdorder)) {}
+
     constexpr Q_IMPLICIT operator std::partial_ordering() const noexcept
     {
         if (static_cast<QtPrivate::Ordering>(m_order) == QtPrivate::Ordering::Less)

tests/auto/corelib/global/qcompare/tst_qcompare.cpp

@@ -144,6 +144,8 @@ void tst_QCompare::legacyConversions()
         static_assert(QPartialOrdering::Unordered == NS ::partial_ordering::unordered); \
         \
         CHECK_CONVERTS(NS ::partial_ordering, QPartialOrdering); \
+        CHECK_CONVERTS(NS ::weak_ordering,    QPartialOrdering); \
+        CHECK_CONVERTS(NS ::strong_ordering,  QPartialOrdering); \
     } while (false)
 
 #ifdef __cpp_lib_three_way_comparison