The magic of function_ref. For more details see https://github.com/acgetchell/CDT-plusplus/commit/c2c59bb971064e1f1c0efc5815030b3b631f803b

Function_ref_example.cpp

#include <utility>
#include "Function_ref.h"
#include "S3ErgodicMoves.h"
#include "SimplicialManifold.h"

auto increment_lambda = [](int a) { return ++a; };
function_ref<int(int)> lambda_ref(increment_lambda);

increment_lambda(0); // = 1
lambda_ref(5); // = 6

SimplicialManifold test_universe(6400, 13);
Move_tuple moves(std::make_tuple(0, 0, 0, 0, 0));

auto move_23_lambda = [](SimplicialManifold manifold,
                           Move_tuple& attempted_moves) -> SimplicialManifold {
    return make_23_move(std::move(manifold), attempted_moves);
  };
  
function_ref<SimplicialManifold(SimplicialManifold, Move_tuple&)> complex_ref(
      move_23_lambda);

test_universe = move_23_lambda(test_universe, moves); // compiles, fails at runtime
test_universe2 = complex_ref(test_universe, moves); // works perfectly!