For aggregates you don’t need to write constructors. For double/other non-integrals it would be as simple as
struct radians{
   double value;

Also, I suppose a question would be how you intended to get the value out of the “new” types. If by static_cast, I could see the removal of the boilerplate to get that being a potential positive for your method (though maybe not enough to justify a core language extension, I don’t know). 
I currently get by with a suite of macros and hana introspection to add relational, iostream, and arithmetic operations as is appropriate to value wrappers and aggregates. I wrote the OP shortly after yet another frustrating debugging session with hana's EqualityComparable on Struct concepts. Simple things should be simple is my thought for the day :-)