[libc++][hash] Applied [[nodiscard]]

`[[nodiscard]]`` should be applied to functions where discarding the return value is most likely a correctness issue.

- https://libcxx.llvm.org/CodingGuidelines.html

Author: H-G-Hristov

libcxx/include/__functional/hash.h

@@ -435,7 +435,7 @@ struct hash : public __hash_impl<_Tp> {};
 
 template <>
 struct hash<nullptr_t> : public __unary_function<nullptr_t, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(nullptr_t) const _NOEXCEPT { return 662607004ull; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_t operator()(nullptr_t) const _NOEXCEPT { return 662607004ull; }
 };
 
 #ifndef _LIBCPP_CXX03_LANG

libcxx/include/__memory/shared_ptr.h

@@ -1423,7 +1423,7 @@ struct hash<shared_ptr<_Tp> > {
   _LIBCPP_DEPRECATED_IN_CXX17 typedef size_t result_type;
 #endif
 
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(const shared_ptr<_Tp>& __ptr) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_t operator()(const shared_ptr<_Tp>& __ptr) const _NOEXCEPT {
     return hash<typename shared_ptr<_Tp>::element_type*>()(__ptr.get());
   }
 };

libcxx/include/__memory/unique_ptr.h

@@ -800,7 +800,7 @@ struct hash<__enable_hash_helper< unique_ptr<_Tp, _Dp>, typename unique_ptr<_Tp,
   _LIBCPP_DEPRECATED_IN_CXX17 typedef size_t result_type;
 #endif
 
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(const unique_ptr<_Tp, _Dp>& __ptr) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_t operator()(const unique_ptr<_Tp, _Dp>& __ptr) const {
     typedef typename unique_ptr<_Tp, _Dp>::pointer pointer;
     return hash<pointer>()(__ptr.get());
   }

libcxx/include/module.modulemap.in

@@ -1660,7 +1660,10 @@ module std [system] {
     }
     module raw_storage_iterator               { header "__memory/raw_storage_iterator.h" }
     module shared_count                       { header "__memory/shared_count.h" }
-    module shared_ptr                         { header "__memory/shared_ptr.h" }
+    module shared_ptr                         {
+      header "__memory/shared_ptr.h"
+      export std.functional.hash
+    }
     module swap_allocator                     { header "__memory/swap_allocator.h" }
     module temp_value                         { header "__memory/temp_value.h" }
     module temporary_buffer                   {
@@ -1673,6 +1676,7 @@ module std [system] {
     }
     module unique_ptr {
       header "__memory/unique_ptr.h"
+      export std.functional.hash
     }
     module unique_temporary_buffer {
       header "__memory/unique_temporary_buffer.h"
@@ -2391,6 +2395,7 @@ module std [system] {
 
     header "coroutine"
     export *
+    export std.functional.hash
   }
 } // module std
 

libcxx/test/libcxx/diagnostics/functional.nodiscard.verify.cpp

@@ -59,4 +59,9 @@ void test() {
 
   std::ref(i);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   std::cref(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  // Hash specializations
+
+  std::hash<std::nullptr_t> hash;
+  hash(nullptr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 }

libcxx/test/libcxx/language.support/nodiscard.verify.cpp

@@ -12,7 +12,6 @@
 
 #include <compare>
 #include <coroutine>
-#include <functional>
 #include <initializer_list>
 
 #include "test_macros.h"

libcxx/test/libcxx/utilities/smartptr/nodiscard.verify.cpp

@@ -41,6 +41,9 @@ void test() {
     // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
     std::make_unique_for_overwrite<int[]>(5);
 #endif
+
+    std::hash<std::unique_ptr<int>> hash;
+    hash(uPtr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   }
   { // [util.sharedptr]
     std::shared_ptr<int[]> sPtr;
@@ -118,6 +121,9 @@ void test() {
     // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
     std::get_deleter<int[]>(sPtr);
 #endif
+
+    std::hash<std::shared_ptr<int[]>> hash;
+    hash(sPtr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   }
   { // [util.smartptr.weak]
     std::weak_ptr<int> wPtr;