Recursively check for Evaluate() in MOO

HISTORY.md

@@ -16,6 +16,9 @@
  * Add clarifying comments in problems/ implementations
    ([#276](https://github.com/mlpack/ensmallen/pull/276)).
 
+ * CheckArbitraryFunctionTypeAPI extended for MOO support
+   ([#283](https://github.com/mlpack/ensmallen/pull/283)).
+
 ### ensmallen 2.16.1: "Severely Dented Can Of Polyurethane"
 ###### 2021-03-02
  * Fix test compilation issue when `ENS_USE_OPENMP` is set

include/ensmallen_bits/function/static_checks.hpp

@@ -375,7 +375,7 @@ inline void CheckSparseFunctionTypeAPI()
 /**
  * Perform checks for the ArbitraryFunctionType API.
  */
-template<typename FunctionType, typename MatType>
+template<typename FunctionType, typename MatType, std::size_t I = 0U>
 inline void CheckArbitraryFunctionTypeAPI()
 {
 #ifndef ENS_DISABLE_TYPE_CHECKS
@@ -387,6 +387,26 @@ inline void CheckArbitraryFunctionTypeAPI()
 #endif
 }
 
+template<typename... FunctionAndMatTypes, std::size_t I = 0U>
+typename std::enable_if<I < sizeof...(FunctionAndMatTypes) - 2U, void>::type
+CheckArbitraryFunctionTypeAPI()
+{
+#ifndef ENS_DISABLE_TYPE_CHECKS
+  constexpr size_t size = sizeof...(FunctionAndMatTypes);
+  using TupleType = typename std::tuple<FunctionAndMatTypes...>;
+  using FunctionType = typename std::tuple_element<I, TupleType>::type;
+  using MatType = typename std::tuple_element<size - 1, TupleType>::type;
+
+  static_assert(CheckEvaluate<FunctionType, MatType, MatType>::value,
+      "One of the provided FunctionType does not have a correct definition of Evaluate(). "
+      "Please check that the corresponding FunctionType fully satisfies the requirements"
+      "of the ArbitraryFunctionType API; see the optimizer tutorial for "
+      "more details.");
+
+  CheckArbitraryFunctionTypeAPI<FunctionAndMatTypes..., I + 1>();
+#endif
+}
+
 /**
  * Perform checks for the ResolvableFunctionType API.
  */