More identities for hash<trivial>s

Author: rsaddatimov

libcxx/include/__chrono/month.h

@@ -115,7 +115,7 @@ inline constexpr month December{12};
 template <>
 struct hash<chrono::month> {
   _LIBCPP_HIDE_FROM_ABI static size_t operator()(const chrono::month& __m) noexcept {
-    return hash<unsigned>{}(static_cast<unsigned>(__m));
+    return static_cast<unsigned>(__m);
   }
 };
 

libcxx/include/__chrono/weekday.h

@@ -166,15 +166,13 @@ inline constexpr weekday Saturday{6};
 
 template <>
 struct hash<chrono::weekday> {
-  _LIBCPP_HIDE_FROM_ABI static size_t operator()(const chrono::weekday& __w) noexcept {
-    return hash<unsigned>{}(__w.c_encoding());
-  }
+  _LIBCPP_HIDE_FROM_ABI static size_t operator()(const chrono::weekday& __w) noexcept { return __w.c_encoding(); }
 };
 
 template <>
 struct hash<chrono::weekday_indexed> {
   _LIBCPP_HIDE_FROM_ABI static size_t operator()(const chrono::weekday_indexed& __wi) noexcept {
-    return std::__hash_combine(hash<chrono::weekday>{}(__wi.weekday()), hash<unsigned>{}(__wi.index()));
+    return std::__hash_combine(hash<chrono::weekday>{}(__wi.weekday()), __wi.index());
   }
 };
 

libcxx/include/__chrono/year.h

@@ -115,9 +115,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool year::ok() const noexcept {
 
 template <>
 struct hash<chrono::year> {
-  _LIBCPP_HIDE_FROM_ABI static size_t operator()(const chrono::year& __y) noexcept {
-    return hash<int>{}(static_cast<int>(__y));
-  }
+  _LIBCPP_HIDE_FROM_ABI static size_t operator()(const chrono::year& __y) noexcept { return static_cast<int>(__y); }
 };
 
 #  endif // _LIBCPP_STD_VER >= 26