Improve generic factory pair readers

[ehughsbaird]

Summary

Infrastructure "Improve generic factory readers for std::pair"

Purpose of change

#82943 (comment) prompted a discussion about the pair readers, and a request for the { "a": b } format.

Describe the solution

Support the "canonical" array format [ a, b ], as well as the object format { "a": b }, and single format a -> (a, a) in pair_reader. Comment on supported format for other pair readers, and improve the generic_map_reader support for different key types in the pairs.

Testing

tools/load_all_mods.py

Rebased onto #82943 and swapped out the reader added there for pair_reader.

Additional context

I considered adding { "a": b } as a "canonical" pair format the the JSON API recognizes, but I think there should only be one "canonical" format. Additionally, serialize would not emit the new format, and I don't think it would be worth the risk there.

diff --git a/src/flexbuffer_json-inl.h b/src/flexbuffer_json-inl.h
index 3e161a7c3c..c9ac2fd6c0 100644
--- a/src/flexbuffer_json-inl.h
+++ b/src/flexbuffer_json-inl.h
@@ -353,6 +353,21 @@ bool JsonValue::read( T &val, bool throw_on_error ) const
 template<typename T, typename U>
 bool JsonValue::read( std::pair<T, U> &p, bool throw_on_error ) const
 {
+    if( test_object() ) {
+        if constexpr( key_from_json_string<T>::valid ) {
+            JsonObject jo = get_object();
+            if( jo.size() != 1 ) {
+                return error_or_false( throw_on_error, "json object encoding pair can only have one element" );
+            }
+            // technically a loop, but only to unwrap the object into the member it consists of
+            for( JsonMember jm : jo ) {
+                p.first = key_from_json_string<T>()( dynamic_cast<const JsonMember *>( this )->name() );
+                return read( p.second, throw_on_error );
+            }
+        } else {
+            return error_or_false( throw_on_error, "Expected json array encoding pair" );
+        }
+    }
     if( !test_array() ) {
         return error_or_false( throw_on_error, "Expected json array encoding pair" );
     }

[Maleclypse]

Conflicts

[RenechCDDA]

Thanks for doing this, and your comments are a much better explanation than what I wrote!