Use the reserve_array helper in the initializer_list insert fix

The previous commit called array_t::reserve() directly on the
temporary buffer used to copy an ilist's values before inserting.
std::deque, a documented ArrayType (tests/src/unit-custom-array-type.cpp),
has no reserve(), so insert(pos, initializer_list) no longer compiled
for it. Use the existing detail::reserve_array() SFINAE helper (already
used by the SAX DOM parser) instead, which leaves array types without
reserve() untouched.

Added a regression check that deque_json::insert(pos, {...}) compiles
and handles the aliasing case from #5656.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -4154,7 +4154,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         // copy the values first: ilist may refer to elements of this array
         array_t values;
-        values.reserve(ilist.size());
+        detail::reserve_array(values, ilist.size(), detail::priority_tag<1> {});
         for (const auto& element : ilist)
         {
             values.push_back(element.moved_or_copied());

single_include/nlohmann/json.hpp

@@ -30235,7 +30235,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         // copy the values first: ilist may refer to elements of this array
         array_t values;
-        values.reserve(ilist.size());
+        detail::reserve_array(values, ilist.size(), detail::priority_tag<1> {});
         for (const auto& element : ilist)
         {
             values.push_back(element.moved_or_copied());

tests/src/unit-custom-array-type.cpp

@@ -117,6 +117,22 @@ TEST_CASE("array type without capacity()")
         CHECK(nested.flatten().unflatten() == nested);
     }
 
+    SECTION("insert(pos, initializer_list) compiles and works without reserve()")
+    {
+        // std::deque has no reserve() either; insert(pos, ilist) must not
+        // require it (regression test for #5656, which also covers an ilist
+        // that refers to elements of the array being inserted into)
+        deque_json j = deque_json::array();
+        j.push_back("a");
+        j.push_back("b");
+        j.push_back("c");
+
+        const deque_json& cj = j;
+        auto it = j.insert(j.begin(), {cj[0], cj[1]});
+        CHECK(*it == deque_json("a"));
+        CHECK(j == deque_json({"a", "b", "a", "b", "c"}));
+    }
+
     SECTION("references stay valid while the array grows")
     {
         deque_json j = deque_json::array();