Tweaks to avoid incomplete typenames for non-tuples in new traits

Author: WardBrian

stan/math/prim/meta.hpp

@@ -130,7 +130,8 @@
 #include <stan/math/prim/meta/scalar_type.hpp>
 #include <stan/math/prim/meta/seq_view.hpp>
 #include <stan/math/prim/meta/static_select.hpp>
-#include <stan/math/prim/meta/tuple_elt.hpp>
+#include <stan/math/prim/meta/tuple_element.hpp>
+#include <stan/math/prim/meta/tuple_size.hpp>
 #include <stan/math/prim/meta/value_type.hpp>
 #include <stan/math/prim/meta/void_t.hpp>
 #include <stan/math/prim/meta/StdVectorBuilder.hpp>

stan/math/prim/meta/is_tuple.hpp

@@ -62,10 +62,12 @@ inline constexpr bool is_tuple_v = math::is_tuple_v<T>;
  * @tparam N expected size
  * @ingroup type_trait
  */
+template <typename T, std::size_t N, typename = void>
+struct is_tuple_of_size : std::false_type {};
+
 template <typename T, std::size_t N>
-struct is_tuple_of_size
-    : std::bool_constant<
-          is_tuple_v<T> && std::tuple_size_v<std::decay_t<T>> == N> {};
+struct is_tuple_of_size<T, N, std::enable_if_t<is_tuple_v<T>>>
+    : std::bool_constant<std::tuple_size_v<std::decay_t<T>> == N> {};
 
 template <typename T, std::size_t N>
 inline constexpr bool is_tuple_of_size_v = is_tuple_of_size<T, N>::value;

stan/math/prim/meta/tuple_elt.hpp stan/math/prim/meta/tuple_element.hppstan/math/prim/meta/tuple_elt.hpp renamed to stan/math/prim/meta/tuple_element.hpp

@@ -1,5 +1,8 @@
-#ifndef STAN_MATH_PRIM_META_IS_TUPLE_ELT_HPP
-#define STAN_MATH_PRIM_META_IS_TUPLE_ELT_HPP
+#ifndef STAN_MATH_PRIM_META_TUPLE_ELEMENT_HPP
+#define STAN_MATH_PRIM_META_TUPLE_ELEMENT_HPP
+
+#include <stan/math/prim/meta/is_tuple.hpp>
+#include <stan/math/prim/meta/tuple_size.hpp>
 
 #include <type_traits>
 #include <cstddef>
@@ -20,8 +23,8 @@ struct tuple_element {
 };
 
 template <std::size_t N, typename T>
-struct tuple_element<
-    N, T, std::enable_if_t<(N < std::tuple_size_v<std::decay_t<T>>)>> {
+struct tuple_element<N, T,
+                     std::enable_if_t<is_tuple_v<T> && (N < tuple_size_v<T>)>> {
   using type = std::tuple_element_t<N, std::decay_t<T>>;
 };
 

stan/math/prim/meta/tuple_size.hpp

@@ -0,0 +1,28 @@
+#ifndef STAN_MATH_PRIM_META_TUPLE_SIZE_HPP
+#define STAN_MATH_PRIM_META_TUPLE_SIZE_HPP
+
+#include <stan/math/prim/meta/is_tuple.hpp>
+#include <type_traits>
+#include <cstddef>
+#include <tuple>
+
+namespace stan {
+
+/**
+ * Equivalent to std::tuple_size but returns 0 T is not a tuple
+ * @tparam T type to get tuple size of
+ * @ingroup type_trait
+ */
+template <typename T, typename = void>
+struct tuple_size : std::integral_constant<std::size_t, 0> {};
+
+template <typename T>
+struct tuple_size<T, std::enable_if_t<is_tuple_v<T>>>
+    : std::integral_constant<std::size_t, std::tuple_size_v<std::decay_t<T>>> {
+};
+
+template <typename T>
+constexpr std::size_t tuple_size_v = tuple_size<T>::value;
+}  // namespace stan
+
+#endif