QFile: make constructors taking a path explicit

This is a level A SIC, as it breaks

  QFile f = "/some/path";

In general, it's not a good idea to have this implicit conversion. A
QFile is not a representation of a path, so the conversion should be
explicit.

I am going to keep the current semantics (implicit conversion) up to and
including Qt 6.8 (LTS). Starting from 6.9, the constructor will be
unconditionally explicit. This is deliberate, and done in order to make
users fix their code while staying in Qt 6, rather than encountering
this issue (and countless many more) if and when they upgrade from Qt 6
to Qt 7. In the meanwhile, users can opt-in to the new semantics by
defining a macro.

[ChangeLog][QtCore][QFile] The QFile constructors that take a path are
going to become unconditionally `explicit` in Qt 6.9. Code like `QFile f
= "/path";` will need to be ported to equivalent one (e.g. `QFile
f{"/path/"}`). This has been done in order to prevent a category of
mistakes when passing strings or paths to functions that actually take a
QFile. Users can opt-in to this change even before Qt 6.9 by defining
the QT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH macro before including any
Qt header.

Change-Id: I065a09b9ce5d24c352664df0d48776545f6a0d8e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

cmake/QtInternalTargets.cmake

@@ -152,6 +152,7 @@ target_link_libraries(PlatformToolInternal INTERFACE PlatformAppInternal)
 
 qt_internal_add_global_definition(QT_NO_JAVA_STYLE_ITERATORS)
 qt_internal_add_global_definition(QT_NO_NARROWING_CONVERSIONS_IN_CONNECT)
+qt_internal_add_global_definition(QT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH)
 
 if(WARNINGS_ARE_ERRORS)
     qt_internal_set_warnings_are_errors_flags(PlatformModuleInternal INTERFACE)

src/corelib/io/qfile.cpp

@@ -226,6 +226,15 @@ QFile::QFile(QObject *parent)
 }
 /*!
     Constructs a new file object to represent the file with the given \a name.
+
+//! [qfile-explicit-constructor-note]
+    \note In versions up to and including Qt 6.8, this constructor is
+    implicit, for backward compatibility. Starting from Qt 6.9 this
+    constructor is unconditionally \c{explicit}. Users can force this
+    constructor to be \c{explicit} even in earlier versions of Qt by
+    defining the \c{QT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH} macro
+    before including any Qt header.
+//! [qfile-explicit-constructor-note]
 */
 QFile::QFile(const QString &name)
     : QFileDevice(*new QFilePrivate, nullptr)
@@ -1145,6 +1154,8 @@ qint64 QFile::size() const
     \since 6.0
 
     Constructs a new file object to represent the file with the given \a name.
+
+    \include qfile.cpp qfile-explicit-constructor-note
 */
 /*!
     \fn QFile::QFile(const std::filesystem::path &name, QObject *parent)

src/corelib/io/qfile.h

@@ -54,6 +54,13 @@ using ForceFilesystemPath = typename std::enable_if_t<std::is_same_v<std::filesy
 class QTemporaryFile;
 class QFilePrivate;
 
+// ### Qt 7: remove this, and make constructors always explicit.
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 9, 0)) || defined(QT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH)
+#  define QFILE_MAYBE_EXPLICIT explicit
+#else
+#  define QFILE_MAYBE_EXPLICIT Q_IMPLICIT
+#endif
+
 class Q_CORE_EXPORT QFile : public QFileDevice
 {
 #ifndef QT_NO_QOBJECT
@@ -63,12 +70,12 @@ class Q_CORE_EXPORT QFile : public QFileDevice
 
 public:
     QFile();
-    QFile(const QString &name);
+    QFILE_MAYBE_EXPLICIT QFile(const QString &name);
 #ifdef Q_QDOC
-    QFile(const std::filesystem::path &name);
+    QFILE_MAYBE_EXPLICIT QFile(const std::filesystem::path &name);
 #elif QT_CONFIG(cxx17_filesystem)
     template<typename T, QtPrivate::ForceFilesystemPath<T> = 0>
-    QFile(const T &name) : QFile(QtPrivate::fromFilesystemPath(name))
+    QFILE_MAYBE_EXPLICIT QFile(const T &name) : QFile(QtPrivate::fromFilesystemPath(name))
     {
     }
 #endif // QT_CONFIG(cxx17_filesystem)