Assert on missing array indices in const operator[] and document the JSON pointer case

The const operator[] overloads are unchecked by design, and a missing key
or index is undefined behavior. The key overload guards this with a
runtime assertion, but the index overload did not, although the element
access documentation says an assertion fires in both cases. The const
JSON pointer overload inherits both through json_pointer::get_unchecked(),
so a pointer to a missing array index read out of bounds even in debug
builds, and its documentation promised out_of_range.404 for any pointer
that cannot be resolved.

Add JSON_ASSERT(idx < size()) to const operator[](size_type), which also
covers the index leg of the const JSON pointer overload. Document the
undefined behavior for the const JSON pointer overload in operator[].md
and in the runtime assertions page. Release builds are unchanged.

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

include/nlohmann/detail/json_pointer.hpp

@@ -535,6 +535,10 @@ class json_pointer
     @return const reference to the JSON value pointed to by the JSON
     pointer
 
+    @pre Every object key and array index the pointer refers to exists.
+         Like the const operator[] for keys and indices, a missing one is
+         undefined behavior, guarded by a runtime assertion.
+
     @throw parse_error.106   if an array index begins with '0'
     @throw parse_error.109   if an array index was not a number
     @throw out_of_range.402  if the array index '-' is used
@@ -549,7 +553,8 @@ class json_pointer
             {
                 case detail::value_t::object:
                 {
-                    // use unchecked object access
+                    // use unchecked object access; the const operator[]
+                    // asserts that the key exists
                     ptr = &ptr->operator[](reference_token);
                     break;
                 }
@@ -562,7 +567,8 @@ class json_pointer
                         JSON_THROW(detail::out_of_range::create(402, detail::concat("array index '-' (", std::to_string(ptr->m_data.m_value.array->size()), ") is out of range"), ptr));
                     }
 
-                    // use unchecked array access
+                    // use unchecked array access; the const operator[]
+                    // asserts that the index exists
                     ptr = &ptr->operator[](array_index<BasicJsonType>(reference_token));
                     break;
                 }

include/nlohmann/json.hpp

@@ -2866,6 +2866,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // const operator[] only works for arrays
         if (JSON_HEDLEY_LIKELY(is_array()))
         {
+            JSON_ASSERT(idx < m_data.m_value.array->size());
             return m_data.m_value.array->operator[](idx);
         }