enzyme error when differentiating stateful lambda, but no problems with explicitly defined functor

[samuelpmishLLNL]

Hi, I'm trying to figure how to differentiate functors/lambdas with Enzyme.

Functors seem to work after introducing a level of indirection to turn the operator() into a free function:

struct Functor {
  auto operator() (const std::array<double,2> & x) const {
    return x[0] * a + x[1] * x[1] * b * b;
  }
  double a, b;
};

template < typename T, typename ... arg_types >
auto functor_wrapper(const T & f, arg_types && ... args) {
  return f(args ...);
}

...

Functor f{1.0, 2.0};
std::array<double, 2> x{3.0, 4.0};
std::array<double, 2> dx{5.0, 6.0};

// works!
double dy = __enzyme_fwddiff<double>((void*)functor_wrapper<decltype(f), std::array<double,2> >, enzyme_const, &f, enzyme_dup, &x, &dx);

but when following the same pattern with a stateful lambda we get an error message:

auto f = [a = 1.0, b = 2.0](std::array<double,2> x) {
  return x[0] * a + x[1] * x[1] * b * b;
};
std::array<double, 2> x{3.0, 4.0};
std::array<double, 2> dx{5.0, 6.0};

// error: function '__enzyme_fwddiff<
//    double, 
//    int, 
//    (lambda at <source>:29:12) *, 
//    int, 
//    std::array<double, 2> *, 
//    std::array<double, 2> *
//  >' is used but not defined in this translation unit, 
//     and cannot be defined in any other translation unit because its type does not have linkage
double dy = __enzyme_fwddiff<double>((void*)functor_wrapper<decltype(f), std::array<double,2> >, enzyme_const, &f, enzyme_dup, &x, &dx);

Is there a limitation in enzyme that prevents differentiation of stateful lambdas? The error message seems misleading, it doesn't seem like a linkage issue, since all of this code exists in a single translation unit.

From a C++ point of view, I think of these two examples as practically equivalent, since the compiler implements lambda functions by turning them into functor objects behind the scenes (using cppinsights.io):

---

Here's a complete example of the issue: https://fwd.gymni.ch/PlRQpl

[wsmoses]

change it to (void*)&f like below and its fine:

  double dy = __enzyme_fwddiff<double>((void*)functor_wrapper<decltype(f), std::array<double,2> >, enzyme_const, (void*)&f, enzyme_dup, &x, &dx);

[wsmoses]

This is a purely C++ error, outside of Enzyme. Basically its saying that no possible code could definite the definition of enzyme_fwdddiff, since it uses a local variable, and the template is marked extern. However, the compiler actually can definite (as we do here). Marking the type as a non-local type (such as void* in my fix, or your functor in above) means that it's fine.

[samuelpmishLLNL]

Thanks for the quick response-- is there any way to emit a descriptive error message here to help users diagnose this sort of problem and implement your fix?