QScopeGuard: Fix build failures with qScopeGuard()

Partially reverts 4f077b7e5ff1081afc0e362bdab6522c2b7ee43b.

Can't overload with forwarding references and lvalue references. Use a single
forwarding reference overload, but take care of not trying to create a
QScopeGuard of reference type and forward instead of moving.

Add tests to ensure calling with both lvalues and rvalues is possible.

Change-Id: Ia034afe0a8feb08246c2c7c154a85cae37421c98
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

2 files changed, 19 insertions, 12 deletions

diff --git a/src/corelib/tools/qscopeguard.h b/src/corelib/tools/qscopeguard.h
index 4d2e715df1..6a5bc6cc61 100644
--- a/src/corelib/tools/qscopeguard.h
+++ b/src/corelib/tools/qscopeguard.h
@@ -43,6 +43,8 @@
 
 #include <QtCore/qglobal.h>
 
+#include <type_traits>
+#include <utility>
 
 QT_BEGIN_NAMESPACE
 
@@ -98,18 +100,9 @@ template <typename F>
 #if __has_cpp_attribute(nodiscard)
 Q_REQUIRED_RESULT
 #endif
-QScopeGuard<F> qScopeGuard(F &&f)
+QScopeGuard<typename std::decay<F>::type> qScopeGuard(F &&f)
 {
-    return QScopeGuard<F>(std::move(f));
-}
-
-template <typename F>
-#if __has_cpp_attribute(nodiscard)
-Q_REQUIRED_RESULT
-#endif
-QScopeGuard<F> qScopeGuard(const F &f)
-{
-    return QScopeGuard<F>(f);
+    return QScopeGuard<typename std::decay<F>::type>(std::forward<F>(f));
 }
 
 QT_END_NAMESPACE
diff --git a/tests/auto/corelib/tools/qscopeguard/tst_qscopeguard.cpp b/tests/auto/corelib/tools/qscopeguard/tst_qscopeguard.cpp
index 24c3023183..e5393f694e 100644
--- a/tests/auto/corelib/tools/qscopeguard/tst_qscopeguard.cpp
+++ b/tests/auto/corelib/tools/qscopeguard/tst_qscopeguard.cpp
@@ -112,12 +112,19 @@ void tst_QScopeGuard::constructionFromLvalue()
 {
 #ifdef __cpp_deduction_guides
     Callable::resetCounts();
-    Callable callable;
     {
+        Callable callable;
         QScopeGuard guard(callable);
     }
     QCOMPARE(Callable::copied, 1);
     QCOMPARE(Callable::moved, 0);
+    Callable::resetCounts();
+    {
+        Callable callable;
+        auto guard = qScopeGuard(callable);
+    }
+    QCOMPARE(Callable::copied, 1);
+    QCOMPARE(Callable::moved, 0);
 #else
     QSKIP("This test requires C++17 Class Template Argument Deduction support enabled in the compiler.");
 #endif
@@ -133,6 +140,13 @@ void tst_QScopeGuard::constructionFromRvalue()
     }
     QCOMPARE(Callable::copied, 0);
     QCOMPARE(Callable::moved, 1);
+    Callable::resetCounts();
+    {
+        Callable callable;
+        auto guard = qScopeGuard(std::move(callable));
+    }
+    QCOMPARE(Callable::copied, 0);
+    QCOMPARE(Callable::moved, 1);
 #else
     QSKIP("This test requires C++17 Class Template Argument Deduction support enabled in the compiler.");
 #endif