Long live std::pair!

Make QPair an alias for std::pair, and qMakePair just a forwarder
towards std::make_pair.

Why? Fundamentally to ditch a bunch of NIH code; gain for free
structured bindings, std::tuple and std::reference_wrapper
compatibility, and so on.

Breakages:

* Some that code manually forward declares QPair.
We don't care about it (<QContainerFwd> is the proper way).

* Some code that overloads on std::pair and QPair. Luckily
it's mostly centralized: debug, metatypes, testing macros.
Just remove the QPair overload.

* Usages of qMakePair forcing the template type parameters.
There are a handful of these in qtbase, but only one was actually
broken.

* std::pair is NOT (and will never likely be) trivially copiable.
This is agreed to be a mistake done by practically all implementations
in C++11, can can't be fixed without breaking ABI.
Some code using QPair assuming it's trivially copiable may break;
exactly one occurrence was in qtbase.

* QMetaType logic extracts the type names in two different ways,
one by looking at the source code string (e.g. extracted by moc)
and one via some ad-hoc reflection in C++. We need to make
"QPair" (as spelled in the source code) be the same as "std::pair"
(gathered via reflection, which will see through the alias)
when compared. The way it's already done e.g. for QList is
by actually replacing the moc-extracted name with the name
of the actual type used in C++; do the same here.
On libc++, std::pair is actually in an inline namespace --
i.e. std::__1::pair; the reflection will extract and store
"std::__1::pair" so we need an ad-hoc fix to QMetaType.

[ChangeLog][QtCore][QPair] QPair is now an alias to std::pair,
and does not exist as a class in Qt any more. This may break
code such as functions overloaded for both QPair and std::pair.
Usually, the overload taking a QPair can be safely discarded,
leaving only the one taking a std::pair. QPair API has not changed,
and qMakePair is still available for compatibility (although
new code is encouraged to use std::pair and std::make_pair
directly instead).

Change-Id: I7725c751bf23946cde577b1406e86a336c0a3dcf
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

2 files changed, 5 insertions, 6 deletions

diff --git a/tests/auto/corelib/tools/qpair/qpair.pro b/tests/auto/corelib/tools/qpair/qpair.pro
index d684a24a57..39757d3e0f 100644
--- a/tests/auto/corelib/tools/qpair/qpair.pro
+++ b/tests/auto/corelib/tools/qpair/qpair.pro
@@ -3,5 +3,4 @@ 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
+contains(QT_CONFIG, c++2a): CONFIG += c++2a
diff --git a/tests/auto/corelib/tools/qpair/tst_qpair.cpp b/tests/auto/corelib/tools/qpair/tst_qpair.cpp
index 3c972329bc..0a158e5860 100644
--- a/tests/auto/corelib/tools/qpair/tst_qpair.cpp
+++ b/tests/auto/corelib/tools/qpair/tst_qpair.cpp
@@ -39,7 +39,7 @@ private Q_SLOTS:
     void testConstexpr();
     void testConversions();
     void taskQTBUG_48780_pairContainingCArray();
-    void testDeducationRules();
+    void testDeductionRules();
 };
 
 class C { C() {} char _[4]; };
@@ -203,9 +203,9 @@ void tst_QPair::taskQTBUG_48780_pairContainingCArray()
     Q_UNUSED(pair);
 }
 
-void tst_QPair::testDeducationRules()
+void tst_QPair::testDeductionRules()
 {
-#if defined(__cpp_deduction_guides) && __cpp_deduction_guides >= 201606
+#if defined(__cpp_deduction_guides) && __cpp_deduction_guides >= 201907L
     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);
@@ -224,7 +224,7 @@ void tst_QPair::testDeducationRules()
     QCOMPARE(p3.first, "string");
     QCOMPARE(p3.second, 2);
 #else
-    QSKIP("Unsupported");
+    QSKIP("Unsupported (requires C++20's CTAD for aliases)");
 #endif
 }