[DRAFT] One idea for the deduction guides

BUILD.bazel

@@ -13,6 +13,7 @@ cc_library(
     srcs = ["indirect.cc"],
     hdrs = ["indirect.h"],
     copts = ["-Iexternal/value_types/"],
+    defines = [],
     visibility = ["//visibility:public"],
     deps = ["feature_check"],
 )
@@ -91,7 +92,9 @@ cc_library(
     srcs = ["polymorphic.cc"],
     hdrs = ["polymorphic.h"],
     copts = ["-Iexternal/value_types/"],
+    defines = [],
     visibility = ["//visibility:public"],
+    deps = ["feature_check"],
 )
 
 cc_test(
@@ -115,6 +118,7 @@ cc_library(
     copts = ["-Iexternal/value_types/"],
     defines = ["XYZ_POLYMORPHIC_CXX_14"],
     visibility = ["//visibility:public"],
+    deps = ["feature_check"],
 )
 
 cc_test(

feature_check.h

@@ -95,4 +95,18 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #endif  //(__cplusplus >= 202002L) || (defined(_MSVC_LANG) && _MSVC_LANG >=
         // 202002L)
 
+//
+// XYZ_HAS_STD_TYPE_IDENTITY
+// The macro is defined when std::type_identity_t<T> is available.
+//
+
+#ifdef XYZ_HAS_STD_TYPE_IDENTITY
+#error "XYZ_HAS_STD_TYPE_IDENTITY is already defined"
+#endif  // XYZ_HAS_STD_TYPE_IDENTITY
+
+#if (__cplusplus >= 202002L) || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
+#define XYZ_HAS_STD_TYPE_IDENTITY
+#endif  //(__cplusplus >= 202002L) || (defined(_MSVC_LANG) && _MSVC_LANG >=
+// 202002L)
+
 #endif  // XYZ_FEATURE_CHECK_H

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 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 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>);
@@ -466,8 +480,11 @@ 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>;
 
 }  // namespace xyz
 

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

indirect_test.cc

@@ -120,13 +120,41 @@ TEST(IndirectTest, AllocatorExtendedInitializerListConstructor) {
 
 #ifdef XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
 TEST(IndirectTest, TemplateArgumentDeduction) {
-  xyz::indirect a(42);
-  EXPECT_EQ(*a, 42);
+  xyz::indirect i(42);
+  EXPECT_EQ(*i, 42);
+}
+
+TEST(IndirectTest, TemplateArgumentDeductionCopy) {
+  xyz::indirect i(42);
+  xyz::indirect ii(i);
+
+  static_assert(std::is_same_v<decltype(i), decltype(ii)>);
+  EXPECT_EQ(*ii, 42);
 }
 
 TEST(IndirectTest, TemplateArgumentDeductionWithAllocator) {
-  xyz::indirect a(std::allocator_arg, std::allocator<int>{}, 42);
-  EXPECT_EQ(*a, 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);
+}
+
+TEST(IndirectTest, TemplateArgumentDeductionWithDeducedAllocatorAndCopy) {
+  xyz::indirect i(std::allocator_arg, xyz::TaggedAllocator<int>(1), 42);
+  xyz::indirect ii(std::allocator_arg, 2, i);
+
+  static_assert(std::is_same_v<decltype(i.get_allocator()),
+                               decltype(ii.get_allocator())>);
+  EXPECT_EQ(*ii, 42);
+  EXPECT_EQ(ii.get_allocator().tag, 2);
+}
+
+TEST(IndirectTest, TemplateArgumentDeductionWithDeducedAllocatorAndMove) {
+  xyz::indirect i(std::allocator_arg, xyz::TaggedAllocator<int>(1), 42);
+  xyz::indirect ii(std::allocator_arg, 2, std::move(i));
+  EXPECT_EQ(*ii, 42);
+  EXPECT_EQ(ii.get_allocator().tag, 2);
 }
 #endif  // XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
 

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
 
@@ -116,6 +130,9 @@ class direct_control_block final : public control_block<T, A> {
 
 }  // namespace detail
 
+template <class T, class A>
+class polymorphic;
+
 template <class T, class A = std::allocator<T>>
 class polymorphic {
   using cblock_t = detail::control_block<T, A>;
@@ -241,7 +258,8 @@ class polymorphic {
     cb_ = create_control_block<U>(ilist, std::forward<Ts>(ts)...);
   }
 
-  constexpr polymorphic(std::allocator_arg_t, const A& alloc,
+  constexpr polymorphic(std::allocator_arg_t,
+                        const xyz::type_identity_t<A>& alloc,
                         const polymorphic& other)
       : alloc_(alloc) {
     if (!other.valueless_after_move()) {
@@ -252,7 +270,7 @@ class polymorphic {
   }
 
   constexpr 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_(alloc) {
     if constexpr (allocator_traits::is_always_equal::value) {
@@ -405,6 +423,16 @@ class polymorphic {
   }
 };
 
+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>;
+
 }  // namespace xyz
 
 #endif  // XYZ_POLYMORPHIC_H_

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_

polymorphic_test.cc

@@ -845,4 +845,48 @@ TEST(PolymorphicTest, TaggedAllocatorsNotEqualMoveConstructFromValueless) {
   EXPECT_TRUE(iii.valueless_after_move());
 }
 
+#ifdef XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
+TEST(PolymorphicTest, TemplateArgumentDeduction) {
+  xyz::polymorphic p(Derived(4));
+
+  EXPECT_EQ(p->value(), 4);
+}
+
+TEST(PolymorphicTest, TemplateArgumentDeductionCopy) {
+  xyz::polymorphic p(Derived(4));
+  xyz::polymorphic pp(p);
+
+  static_assert(std::is_same_v<decltype(p), decltype(pp)>);
+
+  EXPECT_EQ(pp->value(), 4);
+}
+
+TEST(PolymorphicTest, TemplateArgumentDeductionWithAllocator) {
+  xyz::TaggedAllocator<Derived> a(1);
+  xyz::polymorphic p(std::allocator_arg, a, Derived(4));
+
+  EXPECT_EQ(p->value(), 4);
+  EXPECT_EQ(p.get_allocator().tag, 1);
+}
+
+TEST(PolymorphicTest, TemplateArgumentDeductionWithDeducedAllocatorAndCopy) {
+  xyz::TaggedAllocator<Derived> a(1);
+  xyz::polymorphic p(std::allocator_arg, a, Derived(4));
+  xyz::polymorphic pp(std::allocator_arg, 2, p);
+
+  EXPECT_EQ(pp->value(), 4);
+  EXPECT_EQ(pp.get_allocator().tag, 2);
+}
+
+TEST(PolymorphicTest, TemplateArgumentDeductionWithDeducedAllocatorAndMove) {
+  xyz::TaggedAllocator<Derived> a(1);
+  xyz::polymorphic p(std::allocator_arg, a, Derived(4));
+  xyz::polymorphic pp(std::allocator_arg, 2, std::move(p));
+
+  EXPECT_EQ(pp->value(), 4);
+  EXPECT_EQ(pp.get_allocator().tag, 2);
+}
+
+#endif  // XYZ_HAS_TEMPLATE_ARGUMENT_DEDUCTION
+
 }  // namespace