C++ inplace destructor compile warning

I am using an inplace destructor in my code, similar to this stripped down piece of code:

#include <new>
#include <stdlib.h>

struct Node {
};

int main(int, char**) {
    Node* a = reinterpret_cast<Node*>(malloc(sizeof(Node)));
    new(a) Node;

    Node* b = a; 
    b->~Node(); 

    free(a);
}

Unfortunately this gives me a warning in Visual Studio 2015, both in Debug and Release:

warning C4189: 'b': local variable is initialized but not referenced

It compiles fine though in g++, even with -Wall. Any idea why I get the warning? Might this be a bug in the compiler? b is clearly used in the b->~Node() call.

It also seems to compile fine when I change the Node implementation to this:

struct Node {
    ~Node() {
    }
};

But as far as I can say this should not make a difference.

---

There are no standards for compiler warning in C++. Hence each compiler can warn you wherever he wants, it is a matter of choice .

In your case the warning does make sense, since the default destructor may not consider as a reference (for example: all local variable are defaultly destroyed at the end of their scope).

---

Trivial destructor

The destructor for class T is trivial if all of the following is true:

The destructor is not user-provided (meaning, it is either implicitly declared, or explicitly defined as defaulted on its first declaration)

The destructor is not virtual (that is, the base class destructor is not virtual)

All direct base classes have trivial destructors

All non-static data members of class type (or array of class type) have trivial destructors.

A trivial destructor is a destructor that performs no action. Objects with trivial destructors don't require a delete-expression and may be disposed of by simply deallocating their storage.

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