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, 11 insertions, 14 deletions

diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index d8b547c8e2..d964c16915 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -240,8 +240,7 @@ inline Q_DECL_CONSTEXPR int qMetaTypeId();
 
 #define QT_FOR_EACH_AUTOMATIC_TEMPLATE_2ARG(F) \
     F(QHash, class) \
-    F(QMap, class) \
-    F(QPair, struct)
+    F(QMap, class)
 
 #define QT_FOR_EACH_AUTOMATIC_TEMPLATE_SMART_POINTER(F) \
     F(QSharedPointer) \
@@ -1392,15 +1391,6 @@ template<typename From>
 struct QPairVariantInterfaceConvertFunctor;
 
 template<typename T, typename U>
-struct QPairVariantInterfaceConvertFunctor<QPair<T, U> >
-{
-    QPairVariantInterfaceImpl operator()(const QPair<T, U>& f) const
-    {
-        return QPairVariantInterfaceImpl(&f);
-    }
-};
-
-template<typename T, typename U>
 struct QPairVariantInterfaceConvertFunctor<std::pair<T, U> >
 {
     QPairVariantInterfaceImpl operator()(const std::pair<T, U>& f) const
@@ -1670,8 +1660,6 @@ namespace QtPrivate
         }
     };
     template<typename T, typename U>
-    struct IsPair<QPair<T, U> > : IsMetaTypePair<QPair<T, U> > {};
-    template<typename T, typename U>
     struct IsPair<std::pair<T, U> > : IsMetaTypePair<std::pair<T, U> > {};
 
     template<typename T>
@@ -2209,7 +2197,6 @@ Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE(QHash)
 Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE(QMap)
 Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE(std::map)
 
-Q_DECLARE_METATYPE_TEMPLATE_2ARG(QPair)
 Q_DECLARE_METATYPE_TEMPLATE_2ARG(std::pair)
 
 #define Q_DECLARE_METATYPE_TEMPLATE_SMART_POINTER_ITER(TEMPLATENAME) \
@@ -2532,6 +2519,16 @@ public:
             // Replace QList by QVector
             appendStr("QVector");
         }
+
+        if (skipToken(begin, end, "QPair")) {
+            // replace QPair by std::pair
+#ifdef _LIBCPP_VERSION
+            appendStr("std::" QT_STRINGIFY(_LIBCPP_ABI_NAMESPACE) "::pair");
+#else
+            appendStr("std::pair");
+#endif
+        }
+
         if (!hasMiddleConst) {
             // Normalize the integer types
             int numLong = 0;