Support TfHashAppend leveraging of std::hash for unspecialized types

pxr/base/tf/hash.h

@@ -92,9 +92,12 @@ TfHashAppend(HashState &h, std::pair<T, U> const &p)
 
 // Support std::vector. std::vector<bool> specialized below.
 template <class HashState, class T>
-inline std::enable_if_t<!std::is_same<std::remove_const_t<T>, bool>::value>
+inline void
 TfHashAppend(HashState &h, std::vector<T> const &vec)
 {
+    static_assert(!std::is_same_v<std::remove_cv_t<T>, bool>,
+                  "Unexpected usage of vector of 'bool'."
+                  "Expected explicit overload.");
     h.AppendContiguous(vec.data(), vec.size());
 }
 
@@ -126,15 +129,6 @@ TfHashAppend(HashState& h, std::map<Key, Value, Compare> const &elements)
     h.AppendRange(std::begin(elements), std::end(elements));
 }
 
-// Support std::type_index.  When TfHash support for std::hash is enabled,
-// this explicit specialization will no longer be necessary.
-template <class HashState>
-inline void
-TfHashAppend(HashState& h, std::type_index const &type_index)
-{
-    return h.Append(type_index.hash_code());
-}
-
 // Support for hashing std::string.
 template <class HashState>
 inline void
@@ -151,22 +145,6 @@ TfHashAppend(HashState &h, const T* ptr) {
     return h.Append(reinterpret_cast<uintptr_t>(ptr));
 }
 
-// Support for hashing std::shared_ptr. When TfHash support for std::hash is
-// enabled, this explicit specialization will no longer be necessary.
-template <class HashState, class T>
-inline void
-TfHashAppend(HashState &h, const std::shared_ptr<T>& ptr) {
-    h.Append(std::hash<std::shared_ptr<T>>{}(ptr));
-}
-
-// Support for hashing std::unique_ptr. When TfHash support for std::hash is
-// enabled, this explicit specialization will no longer be necessary.
-template <class HashState, class T>
-inline void
-TfHashAppend(HashState &h, const std::unique_ptr<T>& ptr) {
-    h.Append(std::hash<std::unique_ptr<T>>{}(ptr));
-}
-
 // We refuse to hash [const] char *.  You're almost certainly trying to hash the
 // pointed-to string and this will not do that (it will hash the pointer
 // itself).  To hash a c-style null terminated string, you can use
@@ -206,29 +184,24 @@ inline void TfHashAppend(HashState &h, TfCStrHashWrapper hcstr)
 }
 
 // Implementation detail: dispatch based on hash capability: Try TfHashAppend
-// first, otherwise try hash_value.  We'd like to otherwise try std::hash<T>,
-// but std::hash<> is not SFINAE-friendly until c++17 and this code needs to
-// support c++14 currently.  We rely on a combination of expression SFINAE and
-// establishing preferred order by passing a 0 constant and having the overloads
-// take int (highest priority), long (next priority) and '...' (lowest
-// priority).
-
-// std::hash version, attempted last.  Consider adding when we move to
-// C++17 or newer.
-/*
+// first, otherwise try std::hash, followed by hash_value. We rely on a
+// combination of expression SFINAE and establishing preferred order by passing
+// a 0 constant and having the overloads take int (highest priority), long
+// (next priority) and '...' (lowest priority).
+
+// std::hash version, attempted second.
 template <class HashState, class T>
-inline auto Tf_HashImpl(HashState &h, T &&obj, ...)
+inline auto Tf_HashImpl(HashState &h, T &&obj, long)
     -> decltype(std::hash<typename std::decay<T>::type>()(
                     std::forward<T>(obj)), void())
 {
     TfHashAppend(
         h, std::hash<typename std::decay<T>::type>()(std::forward<T>(obj)));
 }
-*/
 
-// hash_value, attempted second.
+// hash_value, attempted last.
 template <class HashState, class T>
-inline auto Tf_HashImpl(HashState &h, T &&obj, long)
+inline auto Tf_HashImpl(HashState &h, T &&obj, ...)
     -> decltype(hash_value(std::forward<T>(obj)), void())
 {
     TfHashAppend(h, hash_value(std::forward<T>(obj)));
@@ -437,9 +410,8 @@ class Tf_HashState : public Tf_HashStateAPI<Tf_HashState>
 /// some STL types and types in Tf.  TfHash uses three methods to attempt to
 /// hash a passed object.  First, TfHash tries to call TfHashAppend() on its
 /// argument.  This is the primary customization point for TfHash.  If that is
-/// not viable, TfHash makes an unqualified call to hash_value().  We would like
-/// TfHash to try to use std::hash<T> next, but std::hash<T> is not
-/// SFINAE-friendly until c++17, and this code needs to support c++14.
+/// not viable, TfHash tries to call std::hash<T>{}(). Lastly, TfHash makes an
+/// unqualified call to hash_value.
 ///
 /// The best way to add TfHash support for user-defined types is to provide a
 /// function overload like the following.
@@ -487,6 +459,7 @@ class Tf_HashState : public Tf_HashStateAPI<Tf_HashState>
 ///   \li TfEnum
 ///   \li const void*
 ///   \li types that provide overloads for TfHashAppend
+///   \li types that provide overloads for std::hash
 ///   \li types that provide overloads for hash_value
 ///
 /// The \c TfHash class can be used to instantiate a \c TfHashMap with \c string

pxr/base/tf/testenv/hash.cpp

@@ -33,8 +33,10 @@
 // commented for testing purposes, for now (6/2020).
 //#include <boost/functional/hash.hpp>
 
+#include <optional>
 #include <set>
 #include <string>
+#include <variant>
 #include <vector>
 
 PXR_NAMESPACE_USING_DIRECTIVE
@@ -324,6 +326,17 @@ Test_TfHash()
     // Validate support for std::unique_ptr
     printf("hash(unique_ptr): %zu\n", h(std::make_unique<int>(7)));
 
+    // Validate support for std::optional
+    printf("hash(optional): %zu\n", h(std::make_optional<std::string>("xyz")));
+    TF_AXIOM(h(std::optional<std::string>("xyz")) ==
+             h(std::optional<std::string>("xyz")));
+
+    // Validate support for std::variant
+    printf("hash(variant): %zu\n",
+           h(std::variant<std::string, int, double>("abc")));
+    TF_AXIOM(h(std::variant<std::string, int, double>("abc")) ==
+             h(std::variant<std::string, int, double>("abc")));
+
     TfHasher tfh;
     //BoostHasher bh;