Circle is the "C++ Successor" language I am most excited about, after writing a fair amount of D, and trying the Carbon interpreter.

The issue with D is that while it's a great language, and it has C++ interop, it doesn't have _direct_ C++ interop. As in, "just import an existing header and it works." You have to write or try to auto-generate "extern C++" definitions.

The issue with Carbon is that it is slowly-progressing vaporware at this point with as-yet-untested C++ interop.

Circle is real, has a "Carbon" mode that makes it syntactically identical, and yet has "just import it" interop. It's not open-source yet but hey, you can't have everything at once I guess.

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(I want to add that I think D is a great language in it's own right, just not as "C++ successor")