Short live QT_ENABLE_P0846_SEMANTICS_FOR!

In C++17, unqualified lookup doesn't find function templates that
require ADL from a call with explicit template arguments, unless
another function template of that name is in scope (otherwise, the <
is parsed as operator less-than instead).

P0846, merged for C++20, fixes this to repeat the name lookup, parsing
the < as indicating a template.

We have API in Qt (Tuple Protocol for some types, e.g. QPoint) that
work for the purpose of Structured Bindings, but don't work for manual
unqualified calls when P0846 semantics are missing, and we're adding
more, to QVariant, so add a macro to handle the issue.

The macro simply declares a function template overload of the given
name for a throw-away struct, thereby bringing, for that one name,
P0846 semantics into C++17.

When we require C++20, we can drop this again.

Amends:
- fb6b7869e8bdda94f7e791db7f281f3bb6e0e004 for QPoint(F)
- 8ae9431c792f14a32909ac013a1383547d6bcfa8 for QMargins(F)
- 0e22001a3bb070d4e9956e89543ec0e5ac6f23f8 for the rest

[ChangeLog][QtCore][QSize/F, QMargins/F, QPoint/F] Fixed manual
get<I>() calls (Tuple Protocol) in C++17 mode.

Task-number: QTBUG-111598
Change-Id: I2ffaef12c5bb6d82f75ce78a7c03c6789dfa0691
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/tests/auto/corelib/tools/qsize/tst_qsize.cpp b/tests/auto/corelib/tools/qsize/tst_qsize.cpp
index 54ffd9fcfd..77198c4576 100644
--- a/tests/auto/corelib/tools/qsize/tst_qsize.cpp
+++ b/tests/auto/corelib/tools/qsize/tst_qsize.cpp
@@ -1,6 +1,28 @@
 // Copyright (C) 2022 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
 
+#include <QSize>
+#ifdef QVARIANT_H
+# error "This test requires qsize.h to not include qvariant.h"
+#endif
+
+// don't assume <type_traits>
+template <typename T, typename U>
+constexpr inline bool my_is_same_v = false;
+template <typename T>
+constexpr inline bool my_is_same_v<T, T> = true;
+
+#define CHECK(cvref) \
+    static_assert(my_is_same_v<decltype(get<0>(std::declval<QSize cvref >())), int cvref >); \
+    static_assert(my_is_same_v<decltype(get<1>(std::declval<QSize cvref >())), int cvref >)
+
+CHECK(&);
+CHECK(const &);
+CHECK(&&);
+CHECK(const &&);
+
+#undef CHECK
+
 #include <QTest>
 #include <qsize.h>