rework concept checking to be more in line with Concepts Lite, fixes #63

ericniebler · Nov 3, 2014 · e1db4ec · e1db4ec
1 parent a9a4b20
commit e1db4ec
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Showing 79 changed files with 353 additions and 248 deletions.
diff --git a/TODO.md b/TODO.md
@@ -20,3 +20,10 @@
   - Make `inplace_merge` work with forward iterators
   - Make the sorting algorithms work with forward iterators
   - Study the impact of allowing ForwardIterator to return proxies
+
+* Maybe iterators are not necessarily countable. Is there a relation between
+  the ability to be able to subtract two iterators to find the distance, and
+  with the existence of a DistanceType associated type? Think of:
+  - counted iterators (subtractable regarless of traversal category)
+  - repeat_view iterators (*not* subtractable but could be random access otherwise)
+  - infinite ranges (only countable with an infinite precision integer which we lack)
diff --git a/include/range/v3/algorithm/adjacent_find.hpp b/include/range/v3/algorithm/adjacent_find.hpp
@@ -59,7 +59,7 @@ namespace ranges
                 typename I = range_iterator_t<Rng>,
                 typename V = iterator_value_t<I>,
                 CONCEPT_REQUIRES_(
-                    ForwardIterable<Rng>() &&
+                    ForwardIterable<Rng &>() &&
                     Invokable<P, V>() &&
                     InvokableRelation<C, concepts::Invokable::result_t<P, V>>()
                 )>

diff --git a/include/range/v3/algorithm/copy.hpp b/include/range/v3/algorithm/copy.hpp
@@ -51,7 +51,7 @@ namespace ranges
             template<typename Rng, typename O, typename P = ident,
                 typename I = range_iterator_t<Rng>,
                 CONCEPT_REQUIRES_(
-                    InputIterable<Rng>() &&
+                    InputIterable<Rng &>() &&
                     WeaklyIncrementable<O>() &&
                     IndirectlyProjectedCopyable<I, P, O>()
                 )>

diff --git a/include/range/v3/algorithm/copy_backward.hpp b/include/range/v3/algorithm/copy_backward.hpp
@@ -46,7 +46,7 @@ namespace ranges
             template<typename Rng, typename O, typename P = ident,
                 typename I = range_iterator_t<Rng>,
                 CONCEPT_REQUIRES_(
-                    BidirectionalIterable<Rng>() &&
+                    BidirectionalIterable<Rng &>() &&
                     BidirectionalIterator<O>() &&
                     IndirectlyProjectedCopyable<I, P, O>()
                 )>

diff --git a/include/range/v3/algorithm/copy_if.hpp b/include/range/v3/algorithm/copy_if.hpp
@@ -60,7 +60,7 @@ namespace ranges
                 typename V = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V>,
                 CONCEPT_REQUIRES_(
-                    InputIterable<Rng>() &&
+                    InputIterable<Rng &>() &&
                     WeaklyIncrementable<O>() &&
                     InvokablePredicate<F, X>() &&
                     IndirectlyProjectedCopyable<I, P, O>()

diff --git a/include/range/v3/algorithm/equal.hpp b/include/range/v3/algorithm/equal.hpp
@@ -83,7 +83,7 @@ namespace ranges
             template<typename Rng0, typename I1Ref,
                 typename C = equal_to, typename P0 = ident, typename P1 = ident,
                 typename I0 = range_iterator_t<Rng0>,
-                typename I1 = detail::uncvref_t<I1Ref>,
+                typename I1 = uncvref_t<I1Ref>,
                 CONCEPT_REQUIRES_(
                     Iterable<Rng0>() &&
                     WeaklyComparable<I0, I1, C, P0, P1>()

diff --git a/include/range/v3/algorithm/equal_range.hpp b/include/range/v3/algorithm/equal_range.hpp
@@ -39,7 +39,7 @@ namespace ranges
 
             template<typename Rng, typename V, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(Iterable<Rng>() && BinarySearchable<I, V, C, P>())>
+                CONCEPT_REQUIRES_(Iterable<Rng &>() && BinarySearchable<I, V, C, P>())>
             range<I>
             operator()(Rng & rng, V const & val, C pred = C{}, P proj = P{}) const
             {

diff --git a/include/range/v3/algorithm/fill.hpp b/include/range/v3/algorithm/fill.hpp
@@ -35,7 +35,7 @@ namespace ranges
 
             template<typename Rng, typename V,
                 typename O = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(OutputIterable<Rng, V>())>
+                CONCEPT_REQUIRES_(OutputIterable<Rng &, V>())>
             O operator()(Rng & rng, V const & val) const
             {
                 return (*this)(begin(rng), end(rng), val);

diff --git a/include/range/v3/algorithm/find.hpp b/include/range/v3/algorithm/find.hpp
@@ -60,7 +60,7 @@ namespace ranges
                 typename V0 = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V0>,
                 CONCEPT_REQUIRES_(
-                    InputIterable<Rng>() &&
+                    InputIterable<Rng &>() &&
                     Invokable<P, V0>() &&
                     EqualityComparable<X, V1>()
                 )>

diff --git a/include/range/v3/algorithm/find_if.hpp b/include/range/v3/algorithm/find_if.hpp
@@ -63,7 +63,7 @@ namespace ranges
                 typename V = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V>,
                 CONCEPT_REQUIRES_(
-                    InputIterable<Rng>() &&
+                    InputIterable<Rng &>() &&
                     Invokable<P, V>() &&
                     InvokablePredicate<F, X>()
                 )>

diff --git a/include/range/v3/algorithm/find_if_not.hpp b/include/range/v3/algorithm/find_if_not.hpp
@@ -63,7 +63,7 @@ namespace ranges
                 typename V = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V>,
                 CONCEPT_REQUIRES_(
-                    InputIterable<Rng>() &&
+                    InputIterable<Rng &>() &&
                     Invokable<P, V>() &&
                     InvokablePredicate<F, X>()
                 )>

diff --git a/include/range/v3/algorithm/for_each.hpp b/include/range/v3/algorithm/for_each.hpp
@@ -47,7 +47,7 @@ namespace ranges
                 typename I = range_iterator_t<Rng>,
                 typename V = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V>,
-                CONCEPT_REQUIRES_(InputIterable<Rng>() && Invokable<P, V>() && Invokable<F, X>())>
+                CONCEPT_REQUIRES_(InputIterable<Rng &>() && Invokable<P, V>() && Invokable<F, X>())>
             I operator()(Rng &rng, F fun, P proj = P{}) const
             {
                 return (*this)(begin(rng), end(rng), std::move(fun), std::move(proj));

diff --git a/include/range/v3/algorithm/generate.hpp b/include/range/v3/algorithm/generate.hpp
@@ -39,7 +39,7 @@ namespace ranges
             template<typename Rng, typename F,
                 typename O = range_iterator_t<Rng>,
                 CONCEPT_REQUIRES_(Function<F>() &&
-                    OutputIterable<Rng, concepts::Function::result_t<F>>())>
+                    OutputIterable<Rng &, concepts::Function::result_t<F>>())>
             std::pair<O, F> operator()(Rng & rng, F fun) const
             {
                 return (*this)(begin(rng), end(rng), std::move(fun));

diff --git a/include/range/v3/algorithm/heap_algorithm.hpp b/include/range/v3/algorithm/heap_algorithm.hpp
@@ -102,7 +102,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(IsHeapable<I, C, P>() && Iterable<Rng>())>
+                CONCEPT_REQUIRES_(IsHeapable<I, C, P>() && Iterable<Rng &>())>
             I operator()(Rng &rng, C pred = C{}, P proj = P{}) const
             {
                 return detail::is_heap_until_n(begin(rng), distance(rng), std::move(pred), std::move(proj));
@@ -239,7 +239,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(RandomAccessIterable<Rng>() && Sortable<I, C, P>())>
+                CONCEPT_REQUIRES_(RandomAccessIterable<Rng &>() && Sortable<I, C, P>())>
             I operator()(Rng & rng, C pred = C{}, P proj = P{}) const
             {
                 I begin = ranges::begin(rng);
@@ -284,7 +284,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(RandomAccessIterable<Rng>() && Sortable<I, C, P>())>
+                CONCEPT_REQUIRES_(RandomAccessIterable<Rng &>() && Sortable<I, C, P>())>
             I operator()(Rng & rng, C pred = C{}, P proj = P{}) const
             {
                 I begin = ranges::begin(rng);
@@ -314,7 +314,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(RandomAccessIterable<Rng>() && Sortable<I, C, P>())>
+                CONCEPT_REQUIRES_(RandomAccessIterable<Rng &>() && Sortable<I, C, P>())>
             I operator()(Rng & rng, C pred_ = C{}, P proj_ = P{}) const
             {
                 auto &&pred = invokable(pred_);
@@ -347,7 +347,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(RandomAccessIterable<Rng>() && Sortable<I, C, P>())>
+                CONCEPT_REQUIRES_(RandomAccessIterable<Rng &>() && Sortable<I, C, P>())>
             I operator()(Rng & rng, C pred_ = C{}, P proj_ = P{}) const
             {
                 auto &&pred = invokable(pred_);

diff --git a/include/range/v3/algorithm/inplace_merge.hpp b/include/range/v3/algorithm/inplace_merge.hpp
@@ -216,7 +216,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(BidirectionalIterable<Rng>() && Sortable<I, C, P>())>
+                CONCEPT_REQUIRES_(BidirectionalIterable<Rng &>() && Sortable<I, C, P>())>
             I operator()(Rng &rng, I middle, C pred = C{}, P proj = P{}) const
             {
                 return (*this)(begin(rng), std::move(middle), end(rng), std::move(pred), std::move(proj));

diff --git a/include/range/v3/algorithm/is_sorted_until.hpp b/include/range/v3/algorithm/is_sorted_until.hpp
@@ -63,7 +63,7 @@ namespace ranges
                       typename I = range_iterator_t<Rng>,
                       typename V = iterator_value_t<I>,
                       CONCEPT_REQUIRES_(
-                       ForwardIterable<Rng>() && Invokable<P, V>() &&
+                       ForwardIterable<Rng &>() && Invokable<P, V>() &&
                        InvokableRelation<R, concepts::Invokable::result_t<P, V>>())>
             I operator()(Rng &rng, R rel = R{}, P proj = P{}) const
             {

diff --git a/include/range/v3/algorithm/lower_bound.hpp b/include/range/v3/algorithm/lower_bound.hpp
@@ -38,7 +38,7 @@ namespace ranges
 
             template<typename Rng, typename V, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(Iterable<Rng>() && BinarySearchable<I, V, C, P>())>
+                CONCEPT_REQUIRES_(Iterable<Rng &>() && BinarySearchable<I, V, C, P>())>
             I operator()(Rng &rng, V const &val, C pred = C{}, P proj = P{}) const
             {
                 static_assert(!is_infinite<Rng>::value, "Trying to binary search an infinite range");

diff --git a/include/range/v3/algorithm/max_element.hpp b/include/range/v3/algorithm/max_element.hpp
@@ -49,7 +49,7 @@ namespace ranges
                 typename I = range_iterator_t<Rng>,
                 typename V = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V>,
-                CONCEPT_REQUIRES_(ForwardIterable<Rng>() && Invokable<P, V>() &&
+                CONCEPT_REQUIRES_(ForwardIterable<Rng &>() && Invokable<P, V>() &&
                     InvokableRelation<C, X>())>
             I operator()(Rng &rng, C pred = C{}, P proj = P{}) const
             {

diff --git a/include/range/v3/algorithm/merge.hpp b/include/range/v3/algorithm/merge.hpp
@@ -80,8 +80,8 @@ namespace ranges
                 typename I0 = range_iterator_t<Rng0>,
                 typename I1 = range_iterator_t<Rng1>,
                 CONCEPT_REQUIRES_(
-                    Iterable<Rng0>() &&
-                    Iterable<Rng1>() &&
+                    Iterable<Rng0 &>() &&
+                    Iterable<Rng1 &>() &&
                     Mergeable<I0, I1, O, C, P0, P1>()
                 )>
             std::tuple<I0, I1, O>

diff --git a/include/range/v3/algorithm/min_element.hpp b/include/range/v3/algorithm/min_element.hpp
@@ -49,7 +49,7 @@ namespace ranges
                 typename I = range_iterator_t<Rng>,
                 typename V = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V>,
-                CONCEPT_REQUIRES_(ForwardIterable<Rng>() && Invokable<P, V>() &&
+                CONCEPT_REQUIRES_(ForwardIterable<Rng &>() && Invokable<P, V>() &&
                     InvokableRelation<C, X>())>
             I operator()(Rng &rng, C pred = C{}, P proj = P{}) const
             {

diff --git a/include/range/v3/algorithm/minmax_element.hpp b/include/range/v3/algorithm/minmax_element.hpp
@@ -84,7 +84,7 @@ namespace ranges
                 typename I = range_iterator_t<Rng>,
                 typename V = iterator_value_t<I>,
                 typename X = concepts::Invokable::result_t<P, V>,
-                CONCEPT_REQUIRES_(ForwardIterable<Rng>() && Invokable<P, V>() &&
+                CONCEPT_REQUIRES_(ForwardIterable<Rng &>() && Invokable<P, V>() &&
                     InvokableRelation<C, X>())>
             std::pair<I, I> operator()(Rng &rng, C pred = C{}, P proj = P{}) const
             {

diff --git a/include/range/v3/algorithm/mismatch.hpp b/include/range/v3/algorithm/mismatch.hpp
@@ -91,7 +91,7 @@ namespace ranges
             template<typename Rng1, typename I2Ref, typename C = equal_to, typename P1 = ident, typename P2 = ident,
                 typename I1 = range_iterator_t<Rng1>,
                 typename I2 = detail::decay_t<I2Ref>, // [*] See below
-                CONCEPT_REQUIRES_(InputIterable<Rng1>() && Mismatchable1<I1, I2, C, P1, P2>())>
+                CONCEPT_REQUIRES_(InputIterable<Rng1 &>() && Mismatchable1<I1, I2, C, P1, P2>())>
             std::pair<I1, I2> operator()(Rng1 & rng1, I2Ref &&begin2,
                                          C pred = C{}, P1 proj1 = P1{}, P2 proj2 = P2{}) const
             {
@@ -102,7 +102,7 @@ namespace ranges
             template<typename Rng1, typename Rng2, typename C = equal_to, typename P1 = ident, typename P2 = ident,
                 typename I1 = range_iterator_t<Rng1>,
                 typename I2 = range_iterator_t<Rng2>,
-                CONCEPT_REQUIRES_(InputIterable<Rng1>() && InputIterable<Rng2>() &&
+                CONCEPT_REQUIRES_(InputIterable<Rng1 &>() && InputIterable<Rng2 &>() &&
                     Mismatchable2<I1, I2, C, P1, P2>())>
             std::pair<I1, I2> operator()(Rng1 &rng1, Rng2 &rng2,
                                          C pred = C{}, P1 proj1 = P1{}, P2 proj2 = P2{}) const

diff --git a/include/range/v3/algorithm/move.hpp b/include/range/v3/algorithm/move.hpp
@@ -44,7 +44,7 @@ namespace ranges
 
             template<typename Rng, typename O, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(InputIterable<Rng>() && WeaklyIncrementable<O>() &&
+                CONCEPT_REQUIRES_(InputIterable<Rng &>() && WeaklyIncrementable<O>() &&
                     IndirectlyProjectedMovable<I, P, O>())>
             std::pair<I, O> operator()(Rng &rng, O out, P proj = P{}) const
             {

diff --git a/include/range/v3/algorithm/move_backward.hpp b/include/range/v3/algorithm/move_backward.hpp
@@ -43,7 +43,7 @@ namespace ranges
 
             template<typename Rng, typename O, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(BidirectionalIterable<Rng>() && BidirectionalIterator<O>() &&
+                CONCEPT_REQUIRES_(BidirectionalIterable<Rng &>() && BidirectionalIterator<O>() &&
                     IndirectlyProjectedMovable<I, P, O>())>
             std::pair<I, O> operator()(Rng &rng, O out, P proj = P{}) const
             {

diff --git a/include/range/v3/algorithm/partial_sort.hpp b/include/range/v3/algorithm/partial_sort.hpp
@@ -54,7 +54,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(Sortable<I, C, P>() && RandomAccessIterable<Rng>())>
+                CONCEPT_REQUIRES_(Sortable<I, C, P>() && RandomAccessIterable<Rng &>())>
             I operator()(Rng & rng, I middle, C pred = C{}, P proj = P{}) const
             {
                 return (*this)(begin(rng), std::move(middle), end(rng), std::move(pred), std::move(proj));

diff --git a/include/range/v3/algorithm/partial_sort_copy.hpp b/include/range/v3/algorithm/partial_sort_copy.hpp
@@ -77,7 +77,7 @@ namespace ranges
                 typename I = range_iterator_t<InRng>,
                 typename O = range_iterator_t<OutRng>,
                 CONCEPT_REQUIRES_(PartialSortCopyConcept<I, O, C, PI, PO>() &&
-                    Iterable<InRng>() && Iterable<OutRng>())>
+                    Iterable<InRng>() && Iterable<OutRng &>())>
             O operator()(InRng && in_rng, OutRng & out_rng, C pred = C{}, PI in_proj = PI{},
                 PO out_proj = PO{}) const
             {

diff --git a/include/range/v3/algorithm/partition.hpp b/include/range/v3/algorithm/partition.hpp
@@ -106,7 +106,7 @@ namespace ranges
 
             template<typename Rng, typename C, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(Partitionable<I, C, P>() && Iterable<Rng>())>
+                CONCEPT_REQUIRES_(Partitionable<I, C, P>() && Iterable<Rng &>())>
             I operator()(Rng &rng, C pred, P proj = P{}) const
             {
                 return partition_fn::impl(begin(rng), end(rng), std::move(pred),

diff --git a/include/range/v3/algorithm/partition_copy.hpp b/include/range/v3/algorithm/partition_copy.hpp
@@ -67,7 +67,7 @@ namespace ranges
 
             template<typename Rng, typename O0, typename O1, typename C, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(PartitionCopyable<I, O0, O1, C, P>() && Iterable<Rng>())>
+                CONCEPT_REQUIRES_(PartitionCopyable<I, O0, O1, C, P>() && Iterable<Rng &>())>
             std::tuple<I, O0, O1> operator()(Rng &rng, O0 o0, O1 o1, C pred, P proj = P{}) const
             {
                 return (*this)(begin(rng), end(rng), std::move(o0), std::move(o1), std::move(pred),

diff --git a/include/range/v3/algorithm/partition_point.hpp b/include/range/v3/algorithm/partition_point.hpp
@@ -71,7 +71,7 @@ namespace ranges
 
             template<typename Rng, typename C, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(PartitionPointable<I, C, P>() && Iterable<Rng>())>
+                CONCEPT_REQUIRES_(PartitionPointable<I, C, P>() && Iterable<Rng &>())>
             I operator()(Rng &rng, C pred, P proj = P{}) const
             {
                 return (*this)(begin(rng), end(rng), std::move(pred), std::move(proj));

diff --git a/include/range/v3/algorithm/permutation.hpp b/include/range/v3/algorithm/permutation.hpp
@@ -164,7 +164,7 @@ namespace ranges
 
             template<typename Rng1, typename I2Ref, typename C = equal_to, typename P1 = ident,
                 typename P2 = ident, typename I1 = range_iterator_t<Rng1>,
-                typename I2 = detail::uncvref_t<I2Ref>,
+                typename I2 = uncvref_t<I2Ref>,
                 CONCEPT_REQUIRES_(ForwardIterable<Rng1>() && IsPermutationable<I1, I2, C, P1, P2>())>
             bool operator()(Rng1 &&rng1, I2Ref &&begin2,
                 C pred = C{}, P1 proj1 = P1{}, P2 proj2 = P2{}) const
@@ -227,7 +227,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(BidirectionalIterable<Rng>() && Sortable<I, C, P>())>
+                CONCEPT_REQUIRES_(BidirectionalIterable<Rng &>() && Sortable<I, C, P>())>
             bool operator()(Rng &rng, C pred = C{}, P proj = P{}) const
             {
                 return (*this)(begin(rng), end(rng), std::move(pred), std::move(proj));
@@ -271,7 +271,7 @@ namespace ranges
 
             template<typename Rng, typename C = ordered_less, typename P = ident,
                 typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(BidirectionalIterable<Rng>() && Sortable<I, C, P>())>
+                CONCEPT_REQUIRES_(BidirectionalIterable<Rng &>() && Sortable<I, C, P>())>
             bool operator()(Rng &rng, C pred = C{}, P proj = P{}) const
             {
                 return (*this)(begin(rng), end(rng), std::move(pred), std::move(proj));

diff --git a/include/range/v3/algorithm/random_shuffle.hpp b/include/range/v3/algorithm/random_shuffle.hpp
@@ -32,10 +32,10 @@ namespace ranges
             struct RandomNumberGenerator
             {
                 template<typename Gen, typename D>
-                auto requires_(Gen && rand, D d) -> decltype(
+                auto requires_(Gen rand, D d) -> decltype(
                     concepts::valid_expr(
-                        concepts::model_of<Integral>(d),
-                        concepts::convertible_to<D>(((Gen &&) rand)(d))
+                        concepts::model_of<Integral, D>(),
+                        concepts::convertible_to<D>(val<Gen>()(d))
                     ));
             };
         }
@@ -87,15 +87,15 @@ namespace ranges
             }
 
             template<typename Rng, typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(RandomAccessIterable<Rng>() &&
+                CONCEPT_REQUIRES_(RandomAccessIterable<Rng &>() &&
                                   Permutable<I>())>
             I operator()(Rng & rng) const
             {
                 return (*this)(begin(rng), end(rng));
             }
 
             template<typename Rng, typename Gen, typename I = range_iterator_t<Rng>,
-                CONCEPT_REQUIRES_(RandomAccessIterable<Rng>() &&
+                CONCEPT_REQUIRES_(RandomAccessIterable<Rng &>() &&
                                   Permutable<I>() &&
                                   RandomNumberGenerator<Gen, iterator_difference_t<I>>())>
             I operator()(Rng & rng, Gen && rand) const