Fix xt::drop with variables (#2912)

# Checklist

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- [x] Tests have been added for new features or bug fixes.
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# Description

Address #2867

Changed `xtl::is_integral<T>::value` to
`xtl::is_integral<std::decay_t<T>>::value` in the `drop()` function
template. When passing a variable like `size_t index = 1`, `T` deduces
to `unsigned long&`, a reference type, which `is_integral` rejects.
Decaying strips the reference so the integral branch is correctly taken.

---------

Co-authored-by: Alexis Placet <2400067+Alex-PLACET@users.noreply.github.com>
Co-authored-by: Johan Mabille <johan.mabille@gmail.com>

Author: Alex-PLACET

include/xtensor/views/xslice.hpp

@@ -484,7 +484,7 @@ namespace xt
     template <class R = std::ptrdiff_t, class T>
     inline auto drop(T&& indices)
     {
-        if constexpr (xtl::is_integral<T>::value)
+        if constexpr (xtl::is_integral<std::remove_cvref_t<T>>::value)
         {
             using slice_type = xdrop_slice<R>;
             using container_type = typename slice_type::container_type;

test/test_xoperation.cpp

@@ -858,6 +858,29 @@ namespace xt
             EXPECT_EQ(expected1, res3);
             EXPECT_EQ(expected2, res4);
         }
+
+        TEST_CASE("divide_4d")
+        {
+            using T4 = xt::xtensor<float, 4, xt::layout_type::row_major>;
+            using shape_type = typename T4::shape_type;
+
+            shape_type shape = {2, 3, 2, 2};
+            T4 a(shape, 4.5f);
+            T4 b(shape, 1.3f);
+
+            EXPECT_EQ((a / b)(0, 0, 0, 0), a(0, 0, 0, 0) / b(0, 0, 0, 0));
+
+            float sb = 1.2f;
+            EXPECT_EQ((a / sb)(0, 0, 0, 0), a(0, 0, 0, 0) / sb);
+
+            float sa = 4.6f;
+            EXPECT_EQ((sa / b)(0, 0, 0, 0), sa / b(0, 0, 0, 0));
+
+            // self-divide assignment: a = a / b (no zeros in b)
+            auto a_before = a(1, 2, 1, 1);
+            a = a / b;
+            EXPECT_EQ(a(1, 2, 1, 1), a_before / b(1, 2, 1, 1));
+        }
     }
 
 #undef XOPERATION_TEST_TYPES

test/test_xview.cpp

@@ -1861,4 +1861,23 @@ namespace xt
 
         XT_ASSERT_THROW(const auto col = xt::col(arr, 0), std::invalid_argument);
     }
+
+    TEST(xview, drop_on_1dim_array)
+    {
+        auto my_array = xt::xtensor<double, 1>({1, 2, 3});
+
+        // drop(1) creates a view excluding index 1
+        auto v1 = xt::view(my_array, xt::drop(1));
+        EXPECT_EQ(v1, (xt::xtensor<double, 1>{1, 3}));
+
+        // Assign through the drop view
+        xt::view(my_array, xt::drop(1)) = 0.;
+        EXPECT_EQ(my_array, (xt::xtensor<double, 1>{0, 2, 0}));
+
+        // Reset, then test drop with a variable (the original compilation issue)
+        my_array = xt::xtensor<double, 1>({1, 2, 3});
+        const size_t index = 1;
+        auto v2 = xt::view(my_array, xt::drop(index));
+        EXPECT_EQ(v2, (xt::xtensor<double, 1>{1, 3}));
+    }
 }