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, 4 insertions, 4 deletions

diff --git a/tests/auto/testlib/selftests/expected_pairdiagnostics.tap b/tests/auto/testlib/selftests/expected_pairdiagnostics.tap
index 9c45880c2d..d6e1a0b22f 100644
--- a/tests/auto/testlib/selftests/expected_pairdiagnostics.tap
+++ b/tests/auto/testlib/selftests/expected_pairdiagnostics.tap
@@ -5,10 +5,10 @@ not ok 2 - testQPair()
   ---
   type: QCOMPARE
   message: Compared values are not the same
-  wanted: "QPair(1,2)" (pair2)
-  found: "QPair(1,1)" (pair1)
-  expected: "QPair(1,2)" (pair2)
-  actual: "QPair(1,1)" (pair1)
+  wanted: "std::pair(1,2)" (pair2)
+  found: "std::pair(1,1)" (pair1)
+  expected: "std::pair(1,2)" (pair2)
+  actual: "std::pair(1,1)" (pair1)
   at: tst_PairDiagnostics::testQPair() (qtbase/tests/auto/testlib/selftests/pairdiagnostics/tst_pairdiagnostics.cpp:51)
   file: qtbase/tests/auto/testlib/selftests/pairdiagnostics/tst_pairdiagnostics.cpp
   line: 51