Keep the byte range in exceptions about the root value (#5987)

With JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS both set, exception::diagnostics()
and the json_view diagnostics() returned early for a value without a path (the
root), dropping "(bytes a-b)". Build the "(path) " prefix only when there are
tokens and always append the byte range.

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

include/nlohmann/detail/exceptions.hpp

@@ -198,18 +198,19 @@ class exception : public std::exception
             // all other parent types (primitives) contribute no token
         }
 
-        if (tokens.empty())
+        // the root value has no path, but still has a byte range
+        std::string prefix;
+        if (!tokens.empty())
         {
-            return "";
+            const auto str = std::accumulate(tokens.rbegin(), tokens.rend(), std::string{},
+                                             [](const std::string & a, const std::string & b)
+            {
+                return concat(a, '/', detail::escape(b));
+            });
+            prefix = concat('(', str, ") ");
         }
 
-        auto str = std::accumulate(tokens.rbegin(), tokens.rend(), std::string{},
-                                   [](const std::string & a, const std::string & b)
-        {
-            return concat(a, '/', detail::escape(b));
-        });
-
-        return concat('(', str, ") ", get_byte_positions(leaf_element));
+        return concat(prefix, get_byte_positions(leaf_element));
 #else
         return get_byte_positions(leaf_element);
 #endif

include/nlohmann/detail/view/errors.hpp

@@ -234,8 +234,7 @@ void value_range(const document_data& d, const node* n, std::size_t& start, std:
 
 /// the context basic_json puts in front of the message of an exception about
 /// a value: "(/path) " with JSON_DIAGNOSTICS (none for the root), then
-/// "(bytes a-b) " with JSON_DIAGNOSTIC_POSITIONS (with JSON_DIAGNOSTICS, only
-/// if there is a path)
+/// "(bytes a-b) " with JSON_DIAGNOSTIC_POSITIONS (also for the root)
 ///
 /// This and the throw helpers that take an error_site are templates, so that
 /// the path and range code is instantiated only where an exception can be
@@ -257,18 +256,17 @@ std::string diagnostics(const error_site site)
 #if JSON_DIAGNOSTICS
     std::vector<std::string> tokens;
     const bool found = edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens);
-    if (!found || tokens.empty())
+    if (found && !tokens.empty())
     {
-        // context is still empty; returning it keeps the copy elided (-Wnrvo)
-        return context;
+        // the root value has no path, but still has a byte range
+        std::string path;
+        for (const std::string& token : tokens)
+        {
+            path.push_back('/');
+            path += detail::escape(token);
+        }
+        context = concat('(', path, ") ");
     }
-    std::string path;
-    for (const std::string& token : tokens)
-    {
-        path.push_back('/');
-        path += detail::escape(token);
-    }
-    context = concat('(', path, ") ");
 #endif
 #if JSON_DIAGNOSTIC_POSITIONS
     std::size_t start = 0;