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, 5 deletions

diff --git a/src/opengl/qopengltimerquery.cpp b/src/opengl/qopengltimerquery.cpp
index 44955d48fc..d5ef875efa 100644
--- a/src/opengl/qopengltimerquery.cpp
+++ b/src/opengl/qopengltimerquery.cpp
@@ -143,11 +143,11 @@ bool QOpenGLTimerQueryPrivate::create()
 
     // Check to see if we also need to resolve the functions for EXT_timer_query
     QSurfaceFormat f = context->format();
-    if (f.version() <= qMakePair<int, int>(3, 2)
+    if (f.version() <= qMakePair(3, 2)
         && !context->hasExtension(QByteArrayLiteral("GL_ARB_timer_query"))
         && context->hasExtension(QByteArrayLiteral("GL_EXT_timer_query"))) {
         ext = new QExtTimerQueryHelper(context);
-    } else if (f.version() <= qMakePair<int, int>(3, 2)
+    } else if (f.version() <= qMakePair(3, 2)
                && !context->hasExtension(QByteArrayLiteral("GL_ARB_timer_query"))
                && !context->hasExtension(QByteArrayLiteral("GL_EXT_timer_query"))) {
         qWarning("QOpenGLTimerQuery requires one of:\n"
@@ -530,11 +530,11 @@ bool QOpenGLTimeMonitorPrivate::create()
 
     // Check to see if we also need to resolve the functions for EXT_timer_query
     QSurfaceFormat f = context->format();
-    if (f.version() <= qMakePair<int, int>(3, 2)
+    if (f.version() <= qMakePair(3, 2)
         && !context->hasExtension(QByteArrayLiteral("GL_ARB_timer_query"))
         && context->hasExtension(QByteArrayLiteral("GL_EXT_timer_query"))) {
         ext = new QExtTimerQueryHelper(context);
-    } else if (f.version() <= qMakePair<int, int>(3, 2)
+    } else if (f.version() <= qMakePair(3, 2)
                && !context->hasExtension(QByteArrayLiteral("GL_ARB_timer_query"))
                && !context->hasExtension(QByteArrayLiteral("GL_EXT_timer_query"))) {
         qWarning("QOpenGLTimeMonitor requires one of:\n"
diff --git a/src/opengl/qopenglvertexarrayobject.cpp b/src/opengl/qopenglvertexarrayobject.cpp
index 7f28a0bc61..44a0640567 100644
--- a/src/opengl/qopenglvertexarrayobject.cpp
+++ b/src/opengl/qopenglvertexarrayobject.cpp
@@ -176,7 +176,7 @@ bool QOpenGLVertexArrayObjectPrivate::create()
         vaoFuncsType = NotSupported;
         QSurfaceFormat format = ctx->format();
 #if !QT_CONFIG(opengles2)
-        if (format.version() >= qMakePair<int, int>(3,2)) {
+        if (format.version() >= qMakePair(3,2)) {
             vaoFuncs.core_3_2 = QOpenGLVersionFunctionsFactory::get<QOpenGLFunctions_3_2_Core>(ctx);
             vaoFuncsType = Core_3_2;
             vaoFuncs.core_3_2->glGenVertexArrays(1, &vao);