tuple/value: allow calls to 'each' with empty tuples

tuple/value.hpp

@@ -18,9 +18,13 @@
 
 
 namespace cruft::tuple::value {
-    ///////////////////////////////////////////////////////////////////////////
+    namespace detail {
-    /// Call a provided functor of type FunctionT with each value in a
+        // Call the function for the value at the index `S`, then increment
-    /// provided tuple-like object TupleT
+        // the index and call ourselves again if we haven't reached the end.
+        //
+        // A detail function is used to simplify the case for empty tuples;
+        // ie, we'd like to keep the index assertion, but a constexpr-if won't
+        // elide the assertion.
         template<
             typename FunctionT,
             typename TupleT,
@@ -35,7 +39,29 @@ namespace cruft::tuple::value {
             std::invoke (func, std::get<S> (value));
 
             if constexpr (S + 1 < std::tuple_size_v<tuple_t>) {
-            each<FunctionT,TupleT,S+1> (std::forward<FunctionT> (func), std::forward<TupleT> (value));
+                each<FunctionT,TupleT,S+1> (
+                    std::forward<FunctionT> (func),
+                    std::forward<TupleT> (value)
+                );
+            }
+        }
+    }
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Call a provided functor of type FunctionT with each value in a
+    /// provided tuple-like object TupleT
+    template<
+        typename FunctionT,
+        typename TupleT
+    >
+    void
+    each (FunctionT &&func, TupleT &&value)
+    {
+        if constexpr (std::tuple_size_v<std::decay_t<TupleT>> > 0) {
+            return detail::each (
+                std::forward<FunctionT> (func),
+                std::forward<TupleT> (value)
+            );
         }
     }