[libcxx-commits] [libcxx] [libc++] Simplify std::function implementation further (PR #145153)

[via libcxx-commits]

================
@@ -206,12 +204,13 @@ class __value_func<_Rp(_ArgTypes...)> {
   _LIBCPP_HIDE_FROM_ABI explicit __value_func(_Fp&& __f) : __f_(nullptr) {
     typedef __function::__func<_Fp, _Rp(_ArgTypes...)> _Fun;
 
-    if (__function::__not_null(__f)) {
-      if (sizeof(_Fun) <= sizeof(__buf_) && is_nothrow_copy_constructible<_Fp>::value) {
-        __f_ = ::new (std::addressof(__buf_)) _Fun(std::move(__f));
-      } else {
-        __f_ = new _Fun(std::move(__f));
-      }
+    if (__function::__is_null(__f))
+      return;
+
+    if (sizeof(_Fun) <= sizeof(__buf_) && is_nothrow_copy_constructible<_Fp>::value) {
+      __f_ = ::new (std::addressof(__buf_)) _Fun(std::move(__f));
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EricWF wrote:

I think  this needs to be `::new ((void*)std::addressof(__buf_)) _Fun(std::move(__f));` otherwise users  can hijack the call to placement new using an overload like 

```
template <class T>
void* operator new(size_t, T*) = delete;
```

See https://godbolt.org/z/Wfhes8Mff 

https://github.com/llvm/llvm-project/pull/145153