libstdc++: Consolidate duplicate metaprogramming utilities

Currently std::variant uses __index_of<T, Types...> to find the first
occurence of a type in a pack, and __exactly_once<T, Types...> to check
that there is no other occurrence.

We can reuse the __find_uniq_type_in_pack<T, Types...>() function for
both tasks, and remove the recursive templates used to implement
__index_of and __exactly_once.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (__find_uniq_type_in_pack): Move
	definition to here, ...
	* include/std/tuple (__find_uniq_type_in_pack): ... from here.
	* include/std/variant (__detail__variant::__index_of): Remove.
	(__detail::__variant::__exactly_once): Define using
	__find_uniq_type_in_pack instead of __index_of.
	(get<T>, get_if<T>, variant::__index_of): Likewise.

3 files changed, 47 insertions, 66 deletions

diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index c9ffa00..ec5ed04 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -102,6 +102,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     };
 
 #if __cplusplus >= 201402L
+
+  // Return the index of _Tp in _Types, if it occurs exactly once.
+  // Otherwise, return sizeof...(_Types).
+  template<typename _Tp, typename... _Types>
+    constexpr size_t
+    __find_uniq_type_in_pack()
+    {
+      constexpr size_t __sz = sizeof...(_Types);
+      constexpr bool __found[__sz] = { __is_same(_Tp, _Types) ... };
+      size_t __n = __sz;
+      for (size_t __i = 0; __i < __sz; ++__i)
+	{
+	  if (__found[__i])
+	    {
+	      if (__n < __sz) // more than one _Tp found
+		return __sz;
+	      __n = __i;
+	    }
+	}
+      return __n;
+    }
+
 // The standard says this macro and alias template should be in <tuple> but we
 // we define them here, to be available in <array>, <utility> and <ranges> too.
 // _GLIBCXX_RESOLVE_LIB_DEFECTS
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index b82cdf1..4617393 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1419,28 +1419,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #define __cpp_lib_tuples_by_type 201304L
 
-  // Return the index of _Tp in _Types, if it occurs exactly once.
-  // Otherwise, return sizeof...(_Types).
-  // TODO reuse this for __detail::__variant::__exactly_once.
-  template<typename _Tp, typename... _Types>
-    constexpr size_t
-    __find_uniq_type_in_pack()
-    {
-      constexpr size_t __sz = sizeof...(_Types);
-      constexpr bool __found[__sz] = { __is_same(_Tp, _Types) ... };
-      size_t __n = __sz;
-      for (size_t __i = 0; __i < __sz; ++__i)
-	{
-	  if (__found[__i])
-	    {
-	      if (__n < __sz) // more than one _Tp found
-		return __sz;
-	      __n = __i;
-	    }
-	}
-      return __n;
-    }
-
   /// Return a reference to the unique element of type _Tp of a tuple.
   template <typename _Tp, typename... _Types>
     constexpr _Tp&
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 993ce3d..ab4503b 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -161,19 +161,6 @@ namespace __detail
 {
 namespace __variant
 {
-  // Returns the first appearance of _Tp in _Types.
-  // Returns sizeof...(_Types) if _Tp is not in _Types.
-  template<typename _Tp, typename... _Types>
-    struct __index_of : std::integral_constant<size_t, 0> {};
-
-  template<typename _Tp, typename... _Types>
-    inline constexpr size_t __index_of_v = __index_of<_Tp, _Types...>::value;
-
-  template<typename _Tp, typename _First, typename... _Rest>
-    struct __index_of<_Tp, _First, _Rest...> :
-      std::integral_constant<size_t, is_same_v<_Tp, _First>
-	? 0 : __index_of_v<_Tp, _Rest...> + 1> {};
-
   // used for raw visitation
   struct __variant_cookie {};
   // used for raw visitation with indices passed in
@@ -766,21 +753,9 @@ namespace __variant
       _Variant_base& operator=(_Variant_base&&) = default;
     };
 
-  // How many times does _Tp appear in _Types?
-  template<typename _Tp, typename... _Types>
-    inline constexpr size_t __count = 0;
-
-  template<typename _Tp, typename _Up, typename... _Types>
-    inline constexpr size_t __count<_Tp, _Up, _Types...>
-      = __count<_Tp, _Types...>;
-
-  template<typename _Tp, typename... _Types>
-    inline constexpr size_t __count<_Tp, _Tp, _Types...>
-      = 1 + __count<_Tp, _Types...>;
-
-  // TODO: Reuse this in <tuple> ?
   template<typename _Tp, typename... _Types>
-    inline constexpr bool __exactly_once = __count<_Tp, _Types...> == 1;
+    inline constexpr bool __exactly_once
+      = std::__find_uniq_type_in_pack<_Tp, _Types...>() < sizeof...(_Types);
 
   // Helper used to check for valid conversions that don't involve narrowing.
   template<typename _Ti> struct _Arr { _Ti _M_x[1]; };
@@ -1139,45 +1114,51 @@ namespace __variant
     {
       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 		    "T must occur exactly once in alternatives");
-      return __v.index() == __detail::__variant::__index_of_v<_Tp, _Types...>;
+      return __v.index() == std::__find_uniq_type_in_pack<_Tp, _Types...>();
     }
 
   template<typename _Tp, typename... _Types>
-    constexpr _Tp& get(variant<_Types...>& __v)
+    constexpr _Tp&
+    get(variant<_Types...>& __v)
     {
       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 		    "T must occur exactly once in alternatives");
       static_assert(!is_void_v<_Tp>, "_Tp must not be void");
-      return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
+      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
+      return std::get<__n>(__v);
     }
 
   template<typename _Tp, typename... _Types>
-    constexpr _Tp&& get(variant<_Types...>&& __v)
+    constexpr _Tp&&
+    get(variant<_Types...>&& __v)
     {
       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 		    "T must occur exactly once in alternatives");
       static_assert(!is_void_v<_Tp>, "_Tp must not be void");
-      return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
-	std::move(__v));
+      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
+      return std::get<__n>(std::move(__v));
     }
 
   template<typename _Tp, typename... _Types>
-    constexpr const _Tp& get(const variant<_Types...>& __v)
+    constexpr const _Tp&
+    get(const variant<_Types...>& __v)
     {
       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 		    "T must occur exactly once in alternatives");
       static_assert(!is_void_v<_Tp>, "_Tp must not be void");
-      return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
+      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
+      return std::get<__n>(__v);
     }
 
   template<typename _Tp, typename... _Types>
-    constexpr const _Tp&& get(const variant<_Types...>&& __v)
+    constexpr const _Tp&&
+    get(const variant<_Types...>&& __v)
     {
       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 		    "T must occur exactly once in alternatives");
       static_assert(!is_void_v<_Tp>, "_Tp must not be void");
-      return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
-	std::move(__v));
+      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
+      return std::get<__n>(std::move(__v));
     }
 
   template<size_t _Np, typename... _Types>
@@ -1214,8 +1195,8 @@ namespace __variant
       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 		    "T must occur exactly once in alternatives");
       static_assert(!is_void_v<_Tp>, "_Tp must not be void");
-      return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
-	  __ptr);
+      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
+      return std::get_if<__n>(__ptr);
     }
 
   template<typename _Tp, typename... _Types>
@@ -1225,8 +1206,8 @@ namespace __variant
       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 		    "T must occur exactly once in alternatives");
       static_assert(!is_void_v<_Tp>, "_Tp must not be void");
-      return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
-	  __ptr);
+      constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
+      return std::get_if<__n>(__ptr);
     }
 
   struct monostate { };
@@ -1402,8 +1383,8 @@ namespace __variant
 	using __accepted_type = __to_type<__accepted_index<_Tp>>;
 
       template<typename _Tp>
-	static constexpr size_t __index_of =
-	  __detail::__variant::__index_of_v<_Tp, _Types...>;
+	static constexpr size_t __index_of
+	  = std::__find_uniq_type_in_pack<_Tp, _Types...>();
 
       using _Traits = __detail::__variant::_Traits<_Types...>;