https://github.com/bitcoin/bitcoin/blob/68f3c7eb080e461cfeac37f8db7034fe507241d0/src/prevector.h#L152-L158

I'm opening an issue as its fix is too tiny to become a pull request.

Can be solved by renaming the struct to
```c
     struct indirect_contents { 
         size_type capacity; 
         char* indirect; 
     }; 
```
if (is_direct()) {
            return 0;
        } else {
            return ((size_t)(sizeof(T))) * _union.capacity;
        }
    }
    value_type* data() {
        return item_ptr(0);
    }
    const value_type* data() const {
        return item_ptr(0);
    }
};
#endif