diff --git a/BUILD.bazel b/BUILD.bazel
index bb5c22ff..d698190e 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -13,7 +13,7 @@ cc_library(
     srcs = ["indirect.cc"],
     hdrs = ["indirect.h"],
     copts = ["-Iexternal/value_types/"],
-    defines = ["XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION"],
+    defines = [],
     visibility = ["//visibility:public"],
     deps = ["feature_check"],
 )
@@ -92,8 +92,9 @@ cc_library(
     srcs = ["polymorphic.cc"],
     hdrs = ["polymorphic.h"],
     copts = ["-Iexternal/value_types/"],
-    defines = ["XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION"],
+    defines = [],
     visibility = ["//visibility:public"],
+    deps = ["feature_check"],
 )
 
 cc_test(
@@ -117,6 +118,7 @@ cc_library(
     copts = ["-Iexternal/value_types/"],
     defines = ["XYZ_POLYMORPHIC_CXX_14"],
     visibility = ["//visibility:public"],
+    deps = ["feature_check"],
 )
 
 cc_test(
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0e151aa8..9ea6f117 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -39,7 +39,6 @@ target_sources(xyz_value_types
 xyz_add_library(
     NAME indirect
     ALIAS xyz_value_types::indirect
-    DEFINITIONS XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
 )
 target_sources(indirect
     INTERFACE
@@ -76,7 +75,6 @@ target_sources(indirect_cxx17
 xyz_add_library(
     NAME polymorphic
     ALIAS xyz_value_types::polymorphic
-    DEFINITIONS XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
 )
 target_sources(polymorphic
     INTERFACE
diff --git a/indirect.h b/indirect.h
index 09b5eba2..f491faeb 100644
--- a/indirect.h
+++ b/indirect.h
@@ -31,6 +31,20 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 namespace xyz {
 
+#ifndef XYZ_TYPE_IDENTITY_DEFINED
+#define XYZ_TYPE_IDENTITY_DEFINED
+#ifdef XYZ_HAS_STD_TYPE_IDENTITY
+using std::type_identity_t;
+#else
+template <class T>
+struct type_identity {
+  using type = T;
+};
+template <class T>
+using type_identity_t = typename type_identity<T>::type;
+#endif  // XYZ_HAS_STD_TYPE_IDENTITY
+#endif  // XYZ_TYPE_IDENTITY_DEFINED
+
 #ifndef XYZ_UNREACHABLE_DEFINED
 #define XYZ_UNREACHABLE_DEFINED
 
@@ -182,7 +196,7 @@ class indirect {
     p_ = construct_from(alloc_, ilist, std::forward<Us>(us)...);
   }
 
-  constexpr indirect(std::allocator_arg_t, const std::type_identity_t<A>& alloc,
+  constexpr indirect(std::allocator_arg_t, const xyz::type_identity_t<A>& alloc,
                      const indirect& other)
       : alloc_(alloc) {
     static_assert(std::copy_constructible<T>);
@@ -195,7 +209,7 @@ class indirect {
   }
 
   constexpr indirect(
-      std::allocator_arg_t, const std::type_identity_t<A>& alloc,
+      std::allocator_arg_t, const xyz::type_identity_t<A>& alloc,
       indirect&& other) noexcept(allocator_traits::is_always_equal::value)
       : p_(nullptr), alloc_(alloc) {
     static_assert(std::move_constructible<T>);
@@ -465,16 +479,12 @@ concept is_hashable = requires(T t) { std::hash<T>{}(t); };
 template <typename Value>
 indirect(Value) -> indirect<Value>;
 
-template <typename Alloc, typename Value,
-          typename std::enable_if_t<!is_indirect_v<Value>, int> = 0>
-indirect(std::allocator_arg_t, Alloc, Value) -> indirect<
-    Value, typename std::allocator_traits<Alloc>::template rebind_alloc<Value>>;
-
-#ifdef XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
 template <typename Alloc, typename Value>
-indirect(std::allocator_arg_t, std::type_identity_t<Alloc>,
-         indirect<Value, Alloc>) -> indirect<Value, Alloc>;
-#endif  // XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
+indirect(std::allocator_arg_t, Alloc, Value) -> indirect<Value, Alloc>;
+
+template <typename Alloc, typename Alloc2, typename Value>
+indirect(std::allocator_arg_t, Alloc2, indirect<Value, Alloc>)
+    -> indirect<Value, Alloc>;
 
 }  // namespace xyz
 
diff --git a/indirect_cxx14.h b/indirect_cxx14.h
index c45aeb7c..8883ab76 100644
--- a/indirect_cxx14.h
+++ b/indirect_cxx14.h
@@ -36,6 +36,20 @@ namespace xyz {
 struct in_place_t {};
 #endif  // XYZ_IN_PLACE_DEFINED
 
+#ifndef XYZ_TYPE_IDENTITY_DEFINED
+#define XYZ_TYPE_IDENTITY_DEFINED
+#ifdef XYZ_HAS_STD_TYPE_IDENTITY
+using std::type_identity_t;
+#else
+template <class T>
+struct type_identity {
+  using type = T;
+};
+template <class T>
+using type_identity_t = typename type_identity<T>::type;
+#endif  // XYZ_HAS_STD_TYPE_IDENTITY
+#endif  // XYZ_TYPE_IDENTITY_DEFINED
+
 #ifndef XYZ_UNREACHABLE_DEFINED
 #define XYZ_UNREACHABLE_DEFINED
 
@@ -575,8 +589,11 @@ template <typename Value>
 indirect(Value) -> indirect<Value>;
 
 template <typename Alloc, typename Value>
-indirect(std::allocator_arg_t, Alloc, Value) -> indirect<
-    Value, typename std::allocator_traits<Alloc>::template rebind_alloc<Value>>;
+indirect(std::allocator_arg_t, Alloc, Value) -> indirect<Value, Alloc>;
+
+template <typename Alloc, typename Alloc2, typename Value>
+indirect(std::allocator_arg_t, Alloc2, indirect<Value, Alloc>)
+    -> indirect<Value, Alloc>;
 #endif  // XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
 
 }  // namespace xyz
diff --git a/indirect_test.cc b/indirect_test.cc
index f80b9570..d99b1666 100644
--- a/indirect_test.cc
+++ b/indirect_test.cc
@@ -133,15 +133,13 @@ TEST(IndirectTest, TemplateArgumentDeductionCopy) {
 }
 
 TEST(IndirectTest, TemplateArgumentDeductionWithAllocator) {
-  xyz::indirect i(std::allocator_arg, xyz::TaggedAllocator<void>(1), 42);
+  xyz::indirect i(std::allocator_arg, xyz::TaggedAllocator<int>(1), 42);
 
   static_assert(
       std::is_same_v<decltype(i.get_allocator()), xyz::TaggedAllocator<int>>);
   EXPECT_EQ(*i, 42);
 }
 
-#ifdef XYZ_HAS_STD_TYPE_IDENTITY
-#ifdef XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
 TEST(IndirectTest, TemplateArgumentDeductionWithDeducedAllocatorAndCopy) {
   xyz::indirect i(std::allocator_arg, xyz::TaggedAllocator<int>(1), 42);
   xyz::indirect ii(std::allocator_arg, 2, i);
@@ -158,8 +156,6 @@ TEST(IndirectTest, TemplateArgumentDeductionWithDeducedAllocatorAndMove) {
   EXPECT_EQ(*ii, 42);
   EXPECT_EQ(ii.get_allocator().tag, 2);
 }
-#endif  // XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
-#endif  // XYZ_HAS_STD_TYPE_IDENTITY
 #endif  // XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
 
 template <typename Allocator = std::allocator<void>>
diff --git a/polymorphic.h b/polymorphic.h
index b27a6ca6..99e9d013 100644
--- a/polymorphic.h
+++ b/polymorphic.h
@@ -33,6 +33,20 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 namespace xyz {
 
+#ifndef XYZ_TYPE_IDENTITY_DEFINED
+#define XYZ_TYPE_IDENTITY_DEFINED
+#ifdef XYZ_HAS_STD_TYPE_IDENTITY
+using std::type_identity_t;
+#else
+template <class T>
+struct type_identity {
+  using type = T;
+};
+template <class T>
+using type_identity_t = typename type_identity<T>::type;
+#endif  // XYZ_HAS_STD_TYPE_IDENTITY
+#endif  // XYZ_TYPE_IDENTITY_DEFINED
+
 #ifndef XYZ_UNREACHABLE_DEFINED
 #define XYZ_UNREACHABLE_DEFINED
 
@@ -119,12 +133,6 @@ class direct_control_block final : public control_block<T, A> {
 template <class T, class A>
 class polymorphic;
 
-template <class>
-inline constexpr bool is_polymorphic_v = false;
-
-template <class T, class A>
-inline constexpr bool is_polymorphic_v<polymorphic<T, A>> = true;
-
 template <class T, class A = std::allocator<T>>
 class polymorphic {
   using cblock_t = detail::control_block<T, A>;
@@ -251,7 +259,7 @@ class polymorphic {
   }
 
   constexpr polymorphic(std::allocator_arg_t,
-                        const std::type_identity_t<A>& alloc,
+                        const xyz::type_identity_t<A>& alloc,
                         const polymorphic& other)
       : alloc_(alloc) {
     if (!other.valueless_after_move()) {
@@ -262,7 +270,7 @@ class polymorphic {
   }
 
   constexpr polymorphic(
-      std::allocator_arg_t, const std::type_identity_t<A>& alloc,
+      std::allocator_arg_t, const xyz::type_identity_t<A>& alloc,
       polymorphic&& other) noexcept(allocator_traits::is_always_equal::value)
       : alloc_(alloc) {
     if constexpr (allocator_traits::is_always_equal::value) {
@@ -414,19 +422,17 @@ class polymorphic {
     }
   }
 };
-#ifdef XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
+
 template <typename Value>
 polymorphic(Value) -> polymorphic<Value>;
 
-template <typename Alloc, typename Value,
-          typename std::enable_if_t<!is_polymorphic_v<Value>, int> = 0>
-polymorphic(std::allocator_arg_t, Alloc, Value) -> polymorphic<
-    Value, typename std::allocator_traits<Alloc>::template rebind_alloc<Value>>;
-
 template <typename Alloc, typename Value>
-polymorphic(std::allocator_arg_t, std::type_identity_t<Alloc>,
-            polymorphic<Value, Alloc>) -> polymorphic<Value, Alloc>;
-#endif  // XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
+polymorphic(std::allocator_arg_t, Alloc, Value) -> polymorphic<Value, Alloc>;
+
+template <typename Alloc, typename Alloc2, typename Value>
+polymorphic(std::allocator_arg_t, Alloc2, polymorphic<Value, Alloc>)
+    -> polymorphic<Value, Alloc>;
+
 }  // namespace xyz
 
 #endif  // XYZ_POLYMORPHIC_H_
diff --git a/polymorphic_cxx14.h b/polymorphic_cxx14.h
index 5b611b26..d75c07ca 100644
--- a/polymorphic_cxx14.h
+++ b/polymorphic_cxx14.h
@@ -27,6 +27,8 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #include <type_traits>
 #include <utility>
 
+#include "feature_check.h"
+
 #ifndef XYZ_POLYMORPHIC_HAS_EXTENDED_CONSTRUCTORS
 #define XYZ_POLYMORPHIC_HAS_EXTENDED_CONSTRUCTORS 1
 #endif  // XYZ_POLYMORPHIC_HAS_EXTENDED_CONSTRUCTORS
@@ -40,6 +42,22 @@ struct in_place_type_t {};
 }  // namespace xyz
 #endif  // XYZ_IN_PLACE_TYPE_DEFINED
 
+#ifndef XYZ_TYPE_IDENTITY_DEFINED
+#define XYZ_TYPE_IDENTITY_DEFINED
+namespace xyz {
+#ifdef XYZ_HAS_STD_TYPE_IDENTITY
+using std::type_identity_t;
+#else
+template <class T>
+struct type_identity {
+  using type = T;
+};
+template <class T>
+using type_identity_t = typename type_identity<T>::type;
+#endif  // XYZ_HAS_STD_TYPE_IDENTITY
+}  // namespace xyz
+#endif  // XYZ_TYPE_IDENTITY_DEFINED
+
 #ifndef XYZ_UNREACHABLE_DEFINED
 #define XYZ_UNREACHABLE_DEFINED
 
@@ -319,7 +337,8 @@ class polymorphic : private detail::empty_base_optimization<A> {
                         typename std::remove_reference<U>::type>::type>{},
                     std::forward<U>(u)) {}
 
-  polymorphic(std::allocator_arg_t, const A& alloc, const polymorphic& other)
+  polymorphic(std::allocator_arg_t, const xyz::type_identity_t<A>& alloc,
+              const polymorphic& other)
       : alloc_base(alloc) {
     if (!other.valueless_after_move()) {
       cb_ = other.cb_->clone(alloc_base::get());
@@ -335,7 +354,7 @@ class polymorphic : private detail::empty_base_optimization<A> {
                     other) {}
 
   polymorphic(
-      std::allocator_arg_t, const A& alloc,
+      std::allocator_arg_t, const xyz::type_identity_t<A>& alloc,
       polymorphic&& other) noexcept(allocator_traits::is_always_equal::value)
       : alloc_base(alloc) {
     if (allocator_traits::propagate_on_container_copy_assignment::value) {
@@ -476,6 +495,18 @@ class polymorphic : private detail::empty_base_optimization<A> {
   }
 };
 
+#ifdef XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
+template <typename Value>
+polymorphic(Value) -> polymorphic<Value>;
+
+template <typename Alloc, typename Value>
+polymorphic(std::allocator_arg_t, Alloc, Value) -> polymorphic<Value, Alloc>;
+
+template <typename Alloc, typename Alloc2, typename Value>
+polymorphic(std::allocator_arg_t, Alloc2, polymorphic<Value, Alloc>)
+    -> polymorphic<Value, Alloc>;
+#endif  // XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
+
 }  // namespace xyz
 
 #endif  // XYZ_POLYMORPHIC_H_
diff --git a/polymorphic_test.cc b/polymorphic_test.cc
index e0b8c007..725399d2 100644
--- a/polymorphic_test.cc
+++ b/polymorphic_test.cc
@@ -846,8 +846,6 @@ TEST(PolymorphicTest, TaggedAllocatorsNotEqualMoveConstructFromValueless) {
 }
 
 #ifdef XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
-#ifdef XYZ_HAS_STD_TYPE_IDENTITY
-#ifdef XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
 TEST(PolymorphicTest, TemplateArgumentDeduction) {
   xyz::polymorphic p(Derived(4));
 
@@ -864,7 +862,7 @@ TEST(PolymorphicTest, TemplateArgumentDeductionCopy) {
 }
 
 TEST(PolymorphicTest, TemplateArgumentDeductionWithAllocator) {
-  xyz::TaggedAllocator<int> a(1);
+  xyz::TaggedAllocator<Derived> a(1);
   xyz::polymorphic p(std::allocator_arg, a, Derived(4));
 
   EXPECT_EQ(p->value(), 4);
@@ -872,7 +870,7 @@ TEST(PolymorphicTest, TemplateArgumentDeductionWithAllocator) {
 }
 
 TEST(PolymorphicTest, TemplateArgumentDeductionWithDeducedAllocatorAndCopy) {
-  xyz::TaggedAllocator<int> a(1);
+  xyz::TaggedAllocator<Derived> a(1);
   xyz::polymorphic p(std::allocator_arg, a, Derived(4));
   xyz::polymorphic pp(std::allocator_arg, 2, p);
 
@@ -881,7 +879,7 @@ TEST(PolymorphicTest, TemplateArgumentDeductionWithDeducedAllocatorAndCopy) {
 }
 
 TEST(PolymorphicTest, TemplateArgumentDeductionWithDeducedAllocatorAndMove) {
-  xyz::TaggedAllocator<int> a(1);
+  xyz::TaggedAllocator<Derived> a(1);
   xyz::polymorphic p(std::allocator_arg, a, Derived(4));
   xyz::polymorphic pp(std::allocator_arg, 2, std::move(p));
 
@@ -889,8 +887,6 @@ TEST(PolymorphicTest, TemplateArgumentDeductionWithDeducedAllocatorAndMove) {
   EXPECT_EQ(pp.get_allocator().tag, 2);
 }
 
-#endif  // XYZ_HAS_EXTENDED_CONSTRUCTOR_TEMPLATE_ARGUMENT_DEDUCTION
-#endif  // XYZ_HAS_STD_TYPE_IDENTITY
 #endif  // XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
 
 }  // namespace