Make json_view.hpp pass include-what-you-use

ci_single_binaries runs IWYU with --error on every header. For json_view.hpp
it suggested adding <array> (std::array is used), the headers that json.hpp
already provides (abi_config, abi_macros, input_adapters, json_pointer,
cpp_future, string_concat, value_t, json_fwd), and <version> for
std::nullptr_t, and removing <cstddef>.

- include <array>
- keep <cstddef> (nullptr_t, size_t; IWYU attributes them to <version> and <cstring>)
- tell IWYU not to suggest the headers that json.hpp provides: the amalgamated
  json_view.hpp only includes json.hpp, so including them here would duplicate
  their definitions
- export json.hpp, keep macro_unscope.hpp, and drop the unused forward declaration of the document class

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

include/nlohmann/json_view.hpp

@@ -24,7 +24,8 @@
 #ifndef INCLUDE_NLOHMANN_JSON_VIEW_HPP_
 #define INCLUDE_NLOHMANN_JSON_VIEW_HPP_
 
-#include <cstddef> // size_t
+#include <array> // array
+#include <cstddef> // nullptr_t, size_t // IWYU pragma: keep
 #include <cstring> // memcpy, strlen
 #include <iterator> // distance, input_iterator_tag, iterator_traits
 #include <memory> // unique_ptr
@@ -32,7 +33,21 @@
 #include <type_traits> // enable_if, integral_constant, is_base_of, is_integral, is_same, remove_cv, remove_extent
 #include <utility> // forward, move
 
-#include <nlohmann/json.hpp>
+#include <nlohmann/json.hpp> // IWYU pragma: export
+
+// json.hpp provides the library's types and macros used below (it includes
+// the headers that define them); json_view.hpp must not include them again,
+// because the amalgamated json_view.hpp only includes json.hpp
+// IWYU pragma: no_include <version>
+// IWYU pragma: no_include "nlohmann/detail/abi_config.hpp"
+// IWYU pragma: no_include "nlohmann/detail/abi_macros.hpp"
+// IWYU pragma: no_include "nlohmann/detail/input/input_adapters.hpp"
+// IWYU pragma: no_include "nlohmann/detail/json_pointer.hpp"
+// IWYU pragma: no_include "nlohmann/detail/meta/cpp_future.hpp"
+// IWYU pragma: no_include "nlohmann/detail/string_concat.hpp"
+// IWYU pragma: no_include "nlohmann/detail/value_t.hpp"
+// IWYU pragma: no_include "nlohmann/json.hpp"
+// IWYU pragma: no_include "nlohmann/json_fwd.hpp"
 
 // the view builds on internals of the library: both must be the same version
 #if NLOHMANN_JSON_VERSION_MAJOR != 3 || NLOHMANN_JSON_VERSION_MINOR != 12 || NLOHMANN_JSON_VERSION_PATCH != 0
@@ -50,9 +65,6 @@
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
-template<typename BasicJsonType>
-class basic_json_document;
-
 /*!
 @brief read-only handle to one value of a basic_json_document
 
@@ -607,6 +619,6 @@ using ordered_json_view = basic_json_view<ordered_json>;
 
 NLOHMANN_JSON_NAMESPACE_END
 
-#include <nlohmann/detail/view/macro_unscope.hpp>
+#include <nlohmann/detail/view/macro_unscope.hpp> // IWYU pragma: keep
 
 #endif  // INCLUDE_NLOHMANN_JSON_VIEW_HPP_