Loosen the requirements on the container passed to QtConcurrent::map*

Using std::begin() and std::end() forces the user to have const begin()
and end() member functions being defined for the passed container. This
is because std::declval<T>() returns rvalue which forces the compiler
to select std::{begin, end}()(const Container &c) overloads and an test
for a presence of const {begin, end}() methods.

Change-Id: I9d96d9f73891ece53224f1741a1334500f7b35ad
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/concurrent/qtconcurrentcompilertest.h

@@ -51,8 +51,8 @@ namespace QtPrivate {
     template <class T, typename = void>
     struct IsIterable : std::false_type {};
     template <class T>
-    struct IsIterable<T, std::void_t<decltype(std::begin(std::declval<T>())),
-        decltype(std::end(std::declval<T>()))>>
+    struct IsIterable<T, std::void_t<decltype(std::declval<T>().begin()),
+        decltype(std::declval<T>().end())>>
         : std::true_type
     { };
 

src/concurrent/qtconcurrentfunctionwrappers.h

@@ -77,7 +77,7 @@ struct Argument
 template <class Sequence>
 struct Argument<Sequence, typename std::enable_if<IsIterableValue<Sequence>>::type>
 {
-    using Type = std::decay_t<decltype(*std::begin(std::declval<Sequence>()))>;
+    using Type = std::decay_t<decltype(*std::declval<Sequence>().begin())>;
 };
 
 template <class Iterator>

tests/auto/concurrent/qtconcurrentmap/functions.h

@@ -159,8 +159,6 @@ public:
     const_iterator cend() const { return data.cend(); }
     iterator begin() { return data.begin(); }
     iterator end() { return data.end(); }
-    const_iterator begin() const { return data.cbegin(); }
-    const_iterator end() const { return data.cend(); }
     bool operator==(const MoveOnlyVector &other) const { return data == other.data; }
 
 private: