Regarding function syntax in C++: GCC's C++ frontend does not support nested functions. But more importantly, even if it did, I do not think there would be any conflict at all. While lambdas are lowered to function objects with an unique anonymous type in C++, the semantics of a lambda that uses lvalue capture

  auto f = [&](int x) - > int { return x + z; };

is the same as GCC's nested function

  int f(int x) { return x + z; }

except that latter can be converted via a trampoline to a regular function pointer (and maybe the observable type). But you could do just the same with a lambda using a trampoline! In any case, there is no conflict, either this conversion is allowed and one needs some hack to make it work such as a trampoline or it is not.

So in C++ you could simply lower such nested functions to lambdas and it would cause no confusion with GCC's nested functions at all, because from a user's point of view they would work identically.