I'd be quite surprised if an optimizing compiler generated C++ code somewhere in its pipeline!
https://felix-lang.github.io/felix/
Ignore the 'scripting' language claim.
I do have to wonder though - do you know what proportion of the C++ compiler time is spent parsing your generated C++ code vs. optimizing it?
Felix is quite old at this point. It's a very interesting language, with many interesting ideas. It did not quite take off though.
It has many other interesting capabilities, for example, the ability to change its own grammar, that is rather too much, not for a pleb like me. It has unique (linear and affine) types too. It is really quite a handful.
Go did bring coroutines back into limelight but Felix predates Go by a margin.
Skaller, Felix's author, used Felix as a playground for novel language design ideas, so it was always in a state of flux.