For some dynamic applications, it's important to be able to know e.g. which PG type is mapped to a given CLR type, or vice-versa. Our answer for this has always been to use NpgsqlParameter:

var p = new NpgsqlParameter();
p.Value = /* some CLR type */;
Console.WriteLine(p.DataTypeName);

However, this mechanism is problematic in various ways... Since the parameter isn't necessarily bound to an open connection, it can only take into account global type mappings. With the shift to data source mappings (#4494), this becomes more problematic.

We could have some sort of API on the data source itself that exposes the various mapping information.

Note: if we actually remove parameter inference, we can remove lots of messy support code (see everything on TypeHandlerResolverFactory). Though that would be a potentially bad breaking change.