[libc++] Remove the packaged_task::result_type extension (#147671)

No escape hatch added, as there doesn't seem anyone critically relying
on this.

Author: frederick-vs-ja

libcxx/docs/ReleaseNotes/21.rst

@@ -100,7 +100,7 @@ Deprecations and Removals
 - libc++ no longer adds ``constexpr`` to ``std::hash<std::vector<bool, A>>::operator()``, as the ``constexpr`` addition
   since C++20 was an unintended extension.
 
-- TODO: The non-conforming extension ``packaged_task::result_type`` has been removed in LLVM 21.
+- The non-conforming extension ``packaged_task::result_type`` has been removed in LLVM 21.
 
 Potentially breaking changes
 ----------------------------

libcxx/include/future

@@ -322,8 +322,6 @@ template <class R, class... ArgTypes>
 class packaged_task<R(ArgTypes...)>
 {
 public:
-    typedef R result_type; // extension
-
     // construction and destruction
     packaged_task() noexcept;
     template <class F>
@@ -1605,9 +1603,6 @@ inline _Rp __packaged_task_function<_Rp(_ArgTypes...)>::operator()(_ArgTypes...
 
 template <class _Rp, class... _ArgTypes>
 class packaged_task<_Rp(_ArgTypes...)> {
-public:
-  using result_type _LIBCPP_DEPRECATED = _Rp; // extension
-
 private:
   __packaged_task_function<_Rp(_ArgTypes...)> __f_;
   promise<_Rp> __p_;
@@ -1699,9 +1694,6 @@ void packaged_task<_Rp(_ArgTypes...)>::reset() {
 
 template <class... _ArgTypes>
 class packaged_task<void(_ArgTypes...)> {
-public:
-  using result_type _LIBCPP_DEPRECATED = void; // extension
-
 private:
   __packaged_task_function<void(_ArgTypes...)> __f_;
   promise<void> __p_;

libcxx/test/libcxx/thread/futures/futures.task/types.pass.cpp

@@ -11,18 +11,12 @@
 
 // <future>
 
-// template<class R, class... ArgTypes>
-//     class packaged_task<R(ArgTypes...)>
-// {
-// public:
-//     typedef R result_type; // extension
-
-// This libc++ extension is deprecated. See https://github.com/llvm/llvm-project/issues/112856.
+// Verify that the non-conforming extension packaged_task::result_type is removed.
+// See https://github.com/llvm/llvm-project/issues/112856.
 
 #include <future>
-#include <type_traits>
 
 struct A {};
 
-using RA = std::packaged_task<A(int, char)>::result_type;    // expected-warning {{'result_type' is deprecated}}
-using RV = std::packaged_task<void(int, char)>::result_type; // expected-warning {{'result_type' is deprecated}}
+using RA = std::packaged_task<A(int, char)>::result_type;    // expected-error {{no type named 'result_type'}}
+using RV = std::packaged_task<void(int, char)>::result_type; // expected-error {{no type named 'result_type'}}