Fix unused, extra-semi, mismatched-tags and bitwise warnings in headers

- argsort_iter only reads data_end in assertions. Mark it
  [[maybe_unused]] and drop the (void) idx_end cast, which was dead code
  since idx_end is passed to std::iota.
- The flat argmin and argmax overloads declared a value_type alias they
  never used. Remove it.
- initializer_shape does not read its argument when the shape has no
  dimensions. Mark it [[maybe_unused]].
- accumulate_impl wrapped f in a lambda that was never called for an
  empty tuple. Fold over the assignments directly.
- optional_bitwise combined bool flags with &. The operands are values
  that are already computed, so && gives the same result. It is also
  correct for flag types other than bool, where & could clear a set
  flag (2 & true is 0). The simd path keeps & for batch_bool.
- Remove a stray ';' in xreducer_temporary_type<xoptional>.
- index_mapper was declared as a struct and defined as a class. Declare
  it as a class.

Author: gouarin

include/xtensor/core/xshape.hpp

@@ -197,7 +197,7 @@ namespace xt
         };
 
         template <class R, class U, std::size_t... I>
-        constexpr R initializer_shape(U t, std::index_sequence<I...>)
+        constexpr R initializer_shape([[maybe_unused]] U t, std::index_sequence<I...>)
         {
             using size_type = typename R::value_type;
             return {size_type(initializer_shape_impl<I>::value(t))...};

include/xtensor/misc/xsort.hpp

@@ -294,7 +294,7 @@ namespace xt
         template <class ConstRandomIt, class RandomIt, class Compare, class Method>
         inline void argsort_iter(
             ConstRandomIt data_begin,
-            ConstRandomIt data_end,
+            [[maybe_unused]] ConstRandomIt data_end,
             RandomIt idx_begin,
             RandomIt idx_end,
             Compare comp,
@@ -303,7 +303,6 @@ namespace xt
         {
             XTENSOR_ASSERT(std::distance(data_begin, data_end) >= 0);
             XTENSOR_ASSERT(std::distance(idx_begin, idx_end) == std::distance(data_begin, data_end));
-            (void) idx_end;  // TODO(C++17) [[maybe_unused]] only used in assertion.
 
             std::iota(idx_begin, idx_end, 0);
             switch (method)
@@ -1237,7 +1236,6 @@ namespace xt
     template <layout_type L = XTENSOR_DEFAULT_TRAVERSAL, class E>
     inline auto argmin(const xexpression<E>& e)
     {
-        using value_type = typename E::value_type;
         auto&& ed = eval(e.derived_cast());
         auto begin = ed.template begin<L>();
         auto end = ed.template end<L>();
@@ -1267,7 +1265,6 @@ namespace xt
     template <layout_type L = XTENSOR_DEFAULT_TRAVERSAL, class E>
     inline auto argmax(const xexpression<E>& e)
     {
-        using value_type = typename E::value_type;
         auto&& ed = eval(e.derived_cast());
         auto begin = ed.template begin<L>();
         auto end = ed.template end<L>();

include/xtensor/optional/xoptional.hpp

@@ -199,7 +199,7 @@ namespace xt
             template <class U, class... Args>
             constexpr result_type apply_impl(const U& t, const Args&... args) const
             {
-                return t & apply_impl(args...);
+                return t && apply_impl(args...);
             }
 
             template <class B>
@@ -242,7 +242,6 @@ namespace xt
         struct xreducer_temporary_type<xtl::xoptional<T, B>>
         {
             using type = xtl::xoptional<std::decay_t<T>, bool>;
-            ;
         };
     }
 

include/xtensor/utils/xutils.hpp

@@ -193,11 +193,7 @@ namespace xt
         )
         {
             R res = init;
-            auto wrapper = [&](const auto& i, const auto& j)
-            {
-                res = f(i, j);
-            };
-            (wrapper(res, std::get<I>(t)), ...);
+            ((res = f(res, std::get<I>(t))), ...);
             return res;
         }
     }

include/xtensor/views/index_mapper.hpp

@@ -18,7 +18,7 @@ namespace xt
 {
 
     template <class UndefinedView>
-    struct index_mapper;
+    class index_mapper;
 
     /**
      * @enum access_t