From 4054759aecd06313a775c8c71748ec52ca7dc27d Mon Sep 17 00:00:00 2001
From: Mikhail Svetkin <mikhail.svetkin@qt.io>
Date: Fri, 8 Mar 2019 15:38:33 +0100
Subject: [PATCH] core: Add deduction guides for QPair

[ChangeLog][QtCore] Added support of deduction guides for QPair

Change-Id: I41a798390dc2c925b0f8432ba12aa345724de2d7
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>
---
 src/corelib/tools/qpair.h                    |  5 ++++
 tests/auto/corelib/tools/qpair/qpair.pro     |  3 +++
 tests/auto/corelib/tools/qpair/tst_qpair.cpp | 26 ++++++++++++++++++++
 3 files changed, 34 insertions(+)

diff --git a/src/corelib/tools/qpair.h b/src/corelib/tools/qpair.h
index 94977fc826..1af43d0a68 100644
--- a/src/corelib/tools/qpair.h
+++ b/src/corelib/tools/qpair.h
@@ -97,6 +97,11 @@ struct QPair
     T2 second;
 };
 
+#if defined(__cpp_deduction_guides) && __cpp_deduction_guides >= 201606
+template<class T1, class T2>
+QPair(T1, T2) -> QPair<T1, T2>;
+#endif
+
 template <typename T1, typename T2>
 void swap(QPair<T1, T2> &lhs, QPair<T1, T2> &rhs) Q_DECL_NOEXCEPT_EXPR(noexcept(lhs.swap(rhs)))
 { lhs.swap(rhs); }
diff --git a/tests/auto/corelib/tools/qpair/qpair.pro b/tests/auto/corelib/tools/qpair/qpair.pro
index 659be887d3..d684a24a57 100644
--- a/tests/auto/corelib/tools/qpair/qpair.pro
+++ b/tests/auto/corelib/tools/qpair/qpair.pro
@@ -2,3 +2,6 @@ CONFIG += testcase
 TARGET = tst_qpair
 QT = core testlib
 SOURCES = tst_qpair.cpp
+
+# Force C++17 if available (needed due to Q_COMPILER_DEDUCTION_GUIDES)
+contains(QT_CONFIG, c++1z): CONFIG += c++1z
diff --git a/tests/auto/corelib/tools/qpair/tst_qpair.cpp b/tests/auto/corelib/tools/qpair/tst_qpair.cpp
index dedc353e67..3c972329bc 100644
--- a/tests/auto/corelib/tools/qpair/tst_qpair.cpp
+++ b/tests/auto/corelib/tools/qpair/tst_qpair.cpp
@@ -39,6 +39,7 @@ private Q_SLOTS:
     void testConstexpr();
     void testConversions();
     void taskQTBUG_48780_pairContainingCArray();
+    void testDeducationRules();
 };
 
 class C { C() {} char _[4]; };
@@ -202,5 +203,30 @@ void tst_QPair::taskQTBUG_48780_pairContainingCArray()
     Q_UNUSED(pair);
 }
 
+void tst_QPair::testDeducationRules()
+{
+#if defined(__cpp_deduction_guides) && __cpp_deduction_guides >= 201606
+    QPair p1{1, 2};
+    static_assert(std::is_same<decltype(p1)::first_type, decltype(1)>::value);
+    static_assert(std::is_same<decltype(p1)::second_type, decltype(2)>::value);
+    QCOMPARE(p1.first, 1);
+    QCOMPARE(p1.second, 2);
+
+    QPair p2{QString("string"), 2};
+    static_assert(std::is_same<decltype(p2)::first_type, QString>::value);
+    static_assert(std::is_same<decltype(p2)::second_type, decltype(2)>::value);
+    QCOMPARE(p2.first, "string");
+    QCOMPARE(p2.second, 2);
+
+    QPair p3(p2);
+    static_assert(std::is_same<decltype(p3)::first_type, decltype(p2)::first_type>::value);
+    static_assert(std::is_same<decltype(p3)::second_type, decltype(p2)::second_type>::value);
+    QCOMPARE(p3.first, "string");
+    QCOMPARE(p3.second, 2);
+#else
+    QSKIP("Unsupported");
+#endif
+}
+
 QTEST_APPLESS_MAIN(tst_QPair)
 #include "tst_qpair.moc"