It is implemented as a library, without needing compiler support [*].
You can write C++ in TemplateHaskell (Haskell's strongly typed AST-generating macro system), which allows you to use Haskell variables in scope inside your C++ functions.
The way it works: It invokes the C++ compiler for you to generate a wrapper function that closes over the Haskell variables used in your code snippet, and on the Haskell side generates a corresponding type-safe function call for you.
This approach allows you to use all C++ features that a C++ compiler supports.
But it also carries the drawback of ... invoking a C++ compiler, which gives you the slow build speed of C++.
[*] Some compiler support was added to be able to specify compiler flags that are specific to the C++ compiler but not the C compiler.
Nim does as well. You can use the C++ backend and wrap a decent bit of C++. The imports aren’t automatic so you need to define the FFI, but you can call templates, implement virtual methods, call constructors, etc.
i've come across carbon (google's c++-next language), clasp (common lisp with an llvm backend), felix (compiler generates c++), and maybe a couple of others i'm forgetting.
