json_view: no byte range in exceptions about a removed value

#5990 dropped the found flag from diagnostics(), reasoning that a value
value_path() does not find leaves no tokens, like the root. But the node
of a value that is no longer part of the document still holds the bytes
it had in the source, so with JSON_DIAGNOSTICS and
JSON_DIAGNOSTIC_POSITIONS its exceptions gained "(bytes 1-4)" for text
that is no longer there. The test #5997 added for exactly this case
(unit-json_view_positions.cpp, "views of values that are no longer part
of the document") has failed on develop since both were merged.

Restore the flag: a value that is not found gets no context, the root
keeps its byte range.

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

include/nlohmann/detail/view/builder.hpp

@@ -1168,20 +1168,20 @@ indent_done:
             if (negative)
             {
                 ++p;
-                if (NLOHMANN_VIEW_UNLIKELY(cur() < '0' || cur() > '9'))
-                {
-                    return fail(error_code::number_after_minus);
-                }
             }
             const unsigned char* const int_start = p;
             if (cur() == '0')
             {
                 ++p;
             }
+            else if (!negative || NLOHMANN_VIEW_LIKELY(cur() >= '1' && cur() <= '9'))
+            {
+                // (without a sign, the dispatch saw a digit)
+                p = skip_digits(p + 1, e);
+            }
             else
             {
-                // (a digit: the dispatch or the check after the sign saw it)
+                return fail(error_code::number_after_minus);
-                p = skip_digits(p + 1, e);
             }
             const auto int_digits = static_cast<std::size_t>(p - int_start);
             std::size_t frac_digits = 0;

include/nlohmann/detail/view/errors.hpp

@@ -230,7 +230,8 @@ 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 (also for the root)
+/// "(bytes a-b) " with JSON_DIAGNOSTIC_POSITIONS (also for the root); with
+/// JSON_DIAGNOSTICS, nothing for a value that is no longer part of the document
 ///
 /// 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
@@ -251,10 +252,12 @@ std::string diagnostics(const error_site site)
     std::string context;
 #if JSON_DIAGNOSTICS
     std::vector<std::string> tokens;
+    // a value that is no longer part of the document has no path, and its
+    // node still holds the bytes it had in the source: no context at all
     const bool found = edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens);
     if (!found)
     {
-        // a value that is not found has no path, and with it no bytes
+        // context is still empty; returning it keeps the copy elided (-Wnrvo)
         return context;
     }
     if (!tokens.empty())

single_include/nlohmann/json_view.hpp

@@ -2473,20 +2473,20 @@ indent_done:
             if (negative)
             {
                 ++p;
-                if (NLOHMANN_VIEW_UNLIKELY(cur() < '0' || cur() > '9'))
-                {
-                    return fail(error_code::number_after_minus);
-                }
             }
             const unsigned char* const int_start = p;
             if (cur() == '0')
             {
                 ++p;
             }
+            else if (!negative || NLOHMANN_VIEW_LIKELY(cur() >= '1' && cur() <= '9'))
+            {
+                // (without a sign, the dispatch saw a digit)
+                p = skip_digits(p + 1, e);
+            }
             else
             {
-                // (a digit: the dispatch or the check after the sign saw it)
+                return fail(error_code::number_after_minus);
-                p = skip_digits(p + 1, e);
             }
             const auto int_digits = static_cast<std::size_t>(p - int_start);
             std::size_t frac_digits = 0;
@@ -5059,7 +5059,8 @@ 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 (also for the root)
+/// "(bytes a-b) " with JSON_DIAGNOSTIC_POSITIONS (also for the root); with
+/// JSON_DIAGNOSTICS, nothing for a value that is no longer part of the document
 ///
 /// 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
@@ -5080,10 +5081,12 @@ std::string diagnostics(const error_site site)
     std::string context;
 #if JSON_DIAGNOSTICS
     std::vector<std::string> tokens;
+    // a value that is no longer part of the document has no path, and its
+    // node still holds the bytes it had in the source: no context at all
     const bool found = edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens);
     if (!found)
     {
-        // a value that is not found has no path, and with it no bytes
+        // context is still empty; returning it keeps the copy elided (-Wnrvo)
         return context;
     }
     if (!tokens.empty())

tests/src/unit-locale-cpp.cpp

@@ -505,8 +505,8 @@ struct locale_switching_token
 {
     using value_type = char;
 
-    locale_switching_token(std::string text_, std::string switch_to_)
+    locale_switching_token(std::string text_, const char* switch_to_)
-        : text(std::move(text_)), switch_to(std::move(switch_to_))
+        : text(std::move(text_)), switch_to(switch_to_)
     {}
 
     std::size_t size() const
@@ -523,13 +523,13 @@ struct locale_switching_token
     {
         if (!switched)
         {
-            switched = std::setlocale(LC_NUMERIC, switch_to.c_str()) != nullptr;
+            switched = std::setlocale(LC_NUMERIC, switch_to) != nullptr;
         }
         return &text[0];
     }
 
     std::string text;
-    std::string switch_to;
+    const char* switch_to;
     bool switched = false;
 };