QSqlDatabase: fix historical mistake of defaultConnection being non-const

This was likely never intended, and was a mistake in the API design,
missing the second const after the pointer. It should have been:
    static const char *const defaultConnection;

This commit fixes that mistake by replacing the variable with a
constexpr one that can't be modified.

We can actually do better by moving the name to the header, so it will
get emitted in each library, application, or plugin instead of trying to
load from QtSql.

[ChangeLog][Potentially Source-Incompatible Changes] The static
QSqlDatabase::connectionName member variable has changed from a
non-const pointer to a constexpr array. Modifying the pointer was most
likely a mistake in code and would produce a misbehaving application.

Change-Id: Icf2eddc865c2d6dde0c7fffda06e55e215e5f26d
Reviewed-by: Marc Mutz <[email protected]>
Reviewed-by: Christian Ehrlicher <[email protected]>

2 files changed, 9 insertions, 3 deletions

diff --git a/src/sql/kernel/qsqldatabase.cpp b/src/sql/kernel/qsqldatabase.cpp
index 0a54529679e..bb7d5a1ea53 100644
--- a/src/sql/kernel/qsqldatabase.cpp
+++ b/src/sql/kernel/qsqldatabase.cpp
@@ -36,8 +36,6 @@ using namespace Qt::StringLiterals;
 Q_GLOBAL_STATIC_WITH_ARGS(QFactoryLoader, loader,
                           (QSqlDriverFactoryInterface_iid, "/sqldrivers"_L1))
 
-const char *QSqlDatabase::defaultConnection = "qt_sql_default_connection";
-
 namespace {
     struct QtSqlGlobals
     {
diff --git a/src/sql/kernel/qsqldatabase.h b/src/sql/kernel/qsqldatabase.h
index e1e5b631472..c2595ce658d 100644
--- a/src/sql/kernel/qsqldatabase.h
+++ b/src/sql/kernel/qsqldatabase.h
@@ -34,7 +34,13 @@ public:
     QSqlDriver *createObject() const override { return new T; }
 };
 
-class Q_SQL_EXPORT QSqlDatabase
+struct QSqlDatabaseDefaultConnectionName
+{
+    // separate class because of the static inline constexpr variable
+    static constexpr const char defaultConnection[] = "qt_sql_default_connection";
+};
+
+class Q_SQL_EXPORT QSqlDatabase : public QSqlDatabaseDefaultConnectionName
 {
     Q_GADGET
     Q_PROPERTY(QSql::NumericalPrecisionPolicy numericalPrecisionPolicy READ numericalPrecisionPolicy WRITE setNumericalPrecisionPolicy)
@@ -87,7 +93,9 @@ public:
 
     QSqlDriver* driver() const;
 
+#if QT_SQL_REMOVED_SINCE(6, 10)
     static const char *defaultConnection;
+#endif
 
     static QSqlDatabase addDatabase(const QString& type,
                                  const QString& connectionName = QLatin1StringView(defaultConnection));