Revert "QStringBuilder: remove unneeded specializations"

This reverts commit 3c6c3eccd1f91bd1ae0a518318ef264f8eff63f5.

Reason for revert: They do appear to be needed, and removing them
changes behavior: QTBUG-114206

Pick-to: 6.6
Fixes: QTBUG-114206
Task-number: QTBUG-114238
Change-Id: Iac75bbc1ef14fe89f4282bd58fe996f9a09b8506
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 65 insertions, 0 deletions

diff --git a/src/corelib/text/qstringbuilder.h b/src/corelib/text/qstringbuilder.h
index f7900f4f58..79b854cdb4 100644
--- a/src/corelib/text/qstringbuilder.h
+++ b/src/corelib/text/qstringbuilder.h
@@ -100,6 +100,45 @@ public:
     const B &b;
 };
 
+// This specialization is here for backwards compatibility: appending
+// two null strings must give back a null string, so we're special
+// casing this one out.
+template <>
+class QStringBuilder <QString, QString> : public QStringBuilderBase<QStringBuilder<QString, QString>, QString>
+{
+    public:
+        QStringBuilder(const QString &a_, const QString &b_) : a(a_), b(b_) {}
+        QStringBuilder(const QStringBuilder &other) : a(other.a), b(other.b) {}
+
+        operator QString() const
+        { QString r(a); r += b; return r; }
+
+        const QString &a;
+        const QString &b;
+
+    private:
+        QStringBuilder &operator=(const QStringBuilder &) = delete;
+};
+
+// Ditto, but see QTBUG-114238
+template <>
+class QStringBuilder <QByteArray, QByteArray> : public QStringBuilderBase<QStringBuilder<QByteArray, QByteArray>, QByteArray>
+{
+    public:
+        QStringBuilder(const QByteArray &a_, const QByteArray &b_) : a(a_), b(b_) {}
+        QStringBuilder(const QStringBuilder &other) : a(other.a), b(other.b) {}
+
+        operator QByteArray() const
+        { QByteArray r(a); r += b; return r; }
+
+        const QByteArray &a;
+        const QByteArray &b;
+
+    private:
+        QStringBuilder &operator=(const QStringBuilder &) = delete;
+};
+
+
 template <> struct QConcatenable<char> : private QAbstractConcatenable
 {
     typedef char type;
diff --git a/tests/auto/corelib/text/qstringbuilder/qstringbuilder1/stringbuilder.cpp b/tests/auto/corelib/text/qstringbuilder/qstringbuilder1/stringbuilder.cpp
index ed9460b92b..dc2aa9eb39 100644
--- a/tests/auto/corelib/text/qstringbuilder/qstringbuilder1/stringbuilder.cpp
+++ b/tests/auto/corelib/text/qstringbuilder/qstringbuilder1/stringbuilder.cpp
@@ -77,6 +77,28 @@ void checkItWorksWithFreeSpaceAtBegin(const String &chunk, const Separator &sepa
     QCOMPARE(str, expected);
 }
 
+template <typename String>
+void checkNullVsEmpty(const String &empty, const char *failureReason = nullptr)
+{
+    String a;
+    String b;
+    QVERIFY(a.isNull());
+    QVERIFY(b.isNull());
+    String result = a P b;
+    QVERIFY(result.isNull());
+
+    String d = empty;
+    QVERIFY(d.isEmpty());
+    QVERIFY(!d.isNull());
+    result = a P d;
+    QVERIFY(result.isEmpty());
+    if (failureReason)
+        QEXPECT_FAIL("", failureReason, Continue);
+    QVERIFY(!result.isNull());
+
+    result = a P a P a;
+    QVERIFY(!result.isNull());
+}
 
 void runScenario()
 {
@@ -357,6 +379,10 @@ void runScenario()
         QCOMPARE(ba2, QByteArray(withZero + withZero + withZero));
     }
 
+    // null vs. empty
+    checkNullVsEmpty(QStringLiteral(""));
+    checkNullVsEmpty(QByteArrayLiteral(""), "QTBUG-114238: inconsistent isEmpty/isNull between QString and QByteArray concatenation");
+
     checkItWorksWithFreeSpaceAtBegin(QByteArray(UTF8_LITERAL), "1234");
     if (QTest::currentTestFailed())
         return;