From 1219dbcd123a8001785a1e58452d9abfdfc43471 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Mon, 2 Oct 2023 18:27:51 +0200
Subject: [PATCH] QPointer: ensure construction from nullptr is constinit'able

Construction from nullptr wasn't, before, because it was using the
QPointer(T*) constructor, which cannot be constexpr. Add a constexpr
QPointer(std::nullptr_t) constructor to enable this use-case.

This requires to mark the (T*) constructor as Q_WEAK_OVERLOAD,
otherwise legacy construction from a literal 0 would be ambiguous.

No documentation changes needed, as the set of valid expressions
(apart from constinit'ing) has not changed. Mention the nullptr ctor,
though, without \since.

Add a test to confirm that contruction from derived still works.

Change-Id: If9d5281f6eca0c408a69f03fecba64a70a0c9cf0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/kernel/qpointer.cpp                     | 1 +
 src/corelib/kernel/qpointer.h                       | 3 +++
 tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp | 7 +++++++
 3 files changed, 11 insertions(+)

diff --git a/src/corelib/kernel/qpointer.cpp b/src/corelib/kernel/qpointer.cpp
index bb6fcc287e2..b82f736170a 100644
--- a/src/corelib/kernel/qpointer.cpp
+++ b/src/corelib/kernel/qpointer.cpp
@@ -76,6 +76,7 @@
 
 /*!
     \fn template <class T> QPointer<T>::QPointer()
+    \fn template <class T> QPointer<T>::QPointer(std::nullptr_t)
 
     Constructs a guarded pointer with value \nullptr.
 
diff --git a/src/corelib/kernel/qpointer.h b/src/corelib/kernel/qpointer.h
index 8d63f2bc0ed..39e4ba3e0f2 100644
--- a/src/corelib/kernel/qpointer.h
+++ b/src/corelib/kernel/qpointer.h
@@ -30,6 +30,9 @@ public:
     Q_NODISCARD_CTOR
     QPointer() noexcept = default;
     Q_NODISCARD_CTOR
+    constexpr QPointer(std::nullptr_t) noexcept : QPointer{} {}
+    Q_WEAK_OVERLOAD
+    Q_NODISCARD_CTOR
     inline QPointer(T *p) : wp(p, true) { }
     // compiler-generated copy/move ctor/assignment operators are fine!
     // compiler-generated dtor is fine!
diff --git a/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp b/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp
index bade09be047..e0612948a12 100644
--- a/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp
+++ b/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp
@@ -39,15 +39,22 @@ private slots:
 // check that nullptr QPointer construction is Q_CONSTINIT:
 [[maybe_unused]] Q_CONSTINIT static QPointer<QFile> s_file1;
 [[maybe_unused]] Q_CONSTINIT static QPointer<QFile> s_file2 = {};
+[[maybe_unused]] Q_CONSTINIT static QPointer<QFile> s_file3 = nullptr;
+[[maybe_unused]] Q_CONSTINIT static QPointer<QFile> s_file4 = 0; // legacy nullptr
 
 void tst_QPointer::constructors()
 {
+    struct Derived : QObject {};
+    Derived derived;
+
     QPointer<QObject> p1;
     QPointer<QObject> p2(this);
     QPointer<QObject> p3(p2);
+    QPointer<QObject> p4 = &derived;
     QCOMPARE(p1, QPointer<QObject>(0));
     QCOMPARE(p2, QPointer<QObject>(this));
     QCOMPARE(p3, QPointer<QObject>(this));
+    QCOMPARE(p4, &derived);
 }
 
 void tst_QPointer::ctad()