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>

1 files changed, 3 insertions, 3 deletions

diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index d036be55c6..2688d90b43 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -1640,7 +1640,7 @@ void tst_QMetaType::automaticTemplateRegistration()
     QCOMPARE(QVariant::fromValue(intIntPair).value<IntIntPair>().second, 2);
   }
   {
-    IntUIntPair intUIntPair = qMakePair<int, uint>(4, 2);
+    IntUIntPair intUIntPair = qMakePair(4, 2u);
     QCOMPARE(QVariant::fromValue(intUIntPair).value<IntUIntPair>().first, 4);
     QCOMPARE(QVariant::fromValue(intUIntPair).value<IntUIntPair>().second, (uint)2);
   }
@@ -1727,7 +1727,7 @@ void tst_QMetaType::automaticTemplateRegistration()
     #define FOR_EACH_2ARG_TEMPLATE_TYPE(F, RealName1, RealName2) \
         F(QHash, RealName1, RealName2) \
         F(QMap, RealName1, RealName2) \
-        F(QPair, RealName1, RealName2)
+        F(std::pair, RealName1, RealName2)
 
     #define PRINT_2ARG_TEMPLATE_INTERNAL(RealName1, RealName2) \
         FOR_EACH_2ARG_TEMPLATE_TYPE(CREATE_AND_VERIFY_CONTAINER, RealName1, RealName2)
@@ -1753,7 +1753,7 @@ void tst_QMetaType::automaticTemplateRegistration()
     CREATE_AND_VERIFY_CONTAINER(QVector, void*)
     CREATE_AND_VERIFY_CONTAINER(QVector, const void*)
     CREATE_AND_VERIFY_CONTAINER(QList, void*)
-    CREATE_AND_VERIFY_CONTAINER(QPair, void*, void*)
+    CREATE_AND_VERIFY_CONTAINER(std::pair, void*, void*)
     CREATE_AND_VERIFY_CONTAINER(QHash, void*, void*)
     CREATE_AND_VERIFY_CONTAINER(QHash, const void*, const void*)