totally_ordered_wrapper: make conversion from nullptr implicit

It's harmless and enables the pointer-idiomatic assignment of nullptr
without having to resort to reset().

Found in API-Review.

Change-Id: Id3a02f55b3578a29943f729d8d734fc3b1d3af11
Reviewed-by: Rym Bouabid <rym.bouabid@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 62d3b3c680ed2c6ca417f30fb3ca57482339c2c1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qcomparehelpers.h

@@ -648,6 +648,9 @@ class totally_ordered_wrapper
     P ptr;
 public:
     totally_ordered_wrapper() noexcept = default;
+    Q_IMPLICIT constexpr totally_ordered_wrapper(std::nullptr_t)
+        // requires std::is_pointer_v<P>
+        : totally_ordered_wrapper(P{nullptr}) {}
     explicit constexpr totally_ordered_wrapper(P p) noexcept : ptr(p) {}
 
     constexpr P get() const noexcept { return ptr; }

tests/auto/corelib/global/qcomparehelpers/tst_qcomparehelpers.h

@@ -58,6 +58,8 @@ private Q_SLOTS:
     // Add new test cases to tst_qcomparehelpers1.cpp, because minGW already
     // complains about a too large tst_qcomparehelpers.cpp.obj object file
     void compareWithAttributes();
+
+    void totallyOrderedWrapperBasics();
 };
 
 #endif // TST_QCOMPAREHELPERS_H

tests/auto/corelib/global/qcomparehelpers/tst_qcomparehelpers1.cpp

@@ -54,3 +54,10 @@ void tst_QCompareHelpers::compareWithAttributes()
 
 #undef COMPARE
 }
+
+void tst_QCompareHelpers::totallyOrderedWrapperBasics()
+{
+    Qt::totally_ordered_wrapper<int*> pi; // partially-formed
+    pi = nullptr;
+    QCOMPARE_EQ(pi.get(), nullptr);
+}