Capturing perfectly-forwarded variable in lambda

Is it correct to capture the perfectly-forwarded mStuff variable with the &mStuff syntax?

Yes, assuming that you don't use this lambda outside doSomething. Your code captures mStuff per reference and will correctly forward it inside the lambda.

For mStuff being a parameter pack it suffices to use a simple-capture with a pack-expansion:

template <typename... T> void doSomething(T&&... mStuff)
{
    auto lambda = [&mStuff...]{ doStuff(std::forward<T>(mStuff)...); };
}

The lambda captures every element of mStuff per reference. The closure-object saves an lvalue reference for to each argument, regardless of its value category. Perfect forwarding still works; In fact, there isn't even a difference because named rvalue references would be lvalues anyway.

To make the lambda valid outside the scope where it's created, you need a wrapper class that handles lvalues and rvalues differently, i.e., keeps a reference to an lvalue, but makes a copy of (by moving) an rvalue.

Header file capture.h:

#pragma once

#include <type_traits>
#include <utility>

template < typename T >
class capture_wrapper
{
   static_assert(not std::is_rvalue_reference<T>{},"");
   std::remove_const_t<T> mutable val_;
public:
   constexpr explicit capture_wrapper(T&& v)
      noexcept(std::is_nothrow_move_constructible<std::remove_const_t<T>>{})
   :val_(std::move(v)){}
   constexpr T&& get() const noexcept { return std::move(val_); }
};

template < typename T >
class capture_wrapper<T&>
{
   T& ref_;
public:
   constexpr explicit capture_wrapper(T& r) noexcept : ref_(r){}
   constexpr T& get() const noexcept { return ref_; }
};

template < typename T >
constexpr typename std::enable_if<
   std::is_lvalue_reference<T>{},
   capture_wrapper<T>
>::type
capture(std::remove_reference_t<T>& t) noexcept
{
   return capture_wrapper<T>(t);
}

template < typename T >
constexpr typename std::enable_if<
   std::is_rvalue_reference<T&&>{},
   capture_wrapper<std::remove_reference_t<T>>
>::type
capture(std::remove_reference_t<T>&& t)
   noexcept(std::is_nothrow_constructible<capture_wrapper<std::remove_reference_t<T>>,T&&>{})
{
   return capture_wrapper<std::remove_reference_t<T>>(std::move(t));
}

template < typename T >
constexpr typename std::enable_if<
   std::is_rvalue_reference<T&&>{},
   capture_wrapper<std::remove_reference_t<T>>
>::type
capture(std::remove_reference_t<T>& t)
   noexcept(std::is_nothrow_constructible<capture_wrapper<std::remove_reference_t<T>>,T&&>{})
{
   return capture_wrapper<std::remove_reference_t<T>>(std::move(t));
}

Example/test code that shows it works. Note that the "bar" example shows how one can use std::tuple<...> to work around the lack of pack expansion in lambda capture initializer, useful for variadic capture.

#include <cassert>
#include <tuple>
#include "capture.h"

template < typename T >
auto foo(T&& t)
{
   return [t = capture<T>(t)]()->decltype(auto)
   {
      auto&& x = t.get();
      return std::forward<decltype(x)>(x);
      // or simply, return t.get();
   };
}

template < std::size_t... I, typename... T >
auto bar_impl(std::index_sequence<I...>, T&&... t)
{
   static_assert(std::is_same<std::index_sequence<I...>,std::index_sequence_for<T...>>{},"");
   return [t = std::make_tuple(capture<T>(t)...)]()
   {
      return std::forward_as_tuple(std::get<I>(t).get()...);
   };
}
template < typename... T >
auto bar(T&&... t)
{
   return bar_impl(std::index_sequence_for<T...>{}, std::forward<T>(t)...);
}

int main()
{
   static_assert(std::is_same<decltype(foo(0)()),int&&>{}, "");
   assert(foo(0)() == 0);

   auto i = 0;
   static_assert(std::is_same<decltype(foo(i)()),int&>{}, "");
   assert(&foo(i)() == &i);

   const auto j = 0;
   static_assert(std::is_same<decltype(foo(j)()),const int&>{}, "");
   assert(&foo(j)() == &j);

   const auto&& k = 0;
   static_assert(std::is_same<decltype(foo(std::move(k))()),const int&&>{}, "");
   assert(foo(std::move(k))() == k);

   auto t = bar(0,i,j,std::move(k))();
   static_assert(std::is_same<decltype(t),std::tuple<int&&,int&,const int&,const int&&>>{}, "");
   assert(std::get<0>(t) == 0);
   assert(&std::get<1>(t) == &i);
   assert(&std::get<2>(t) == &j);
   assert(std::get<3>(t) == k and &std::get<3>(t) != &k);

}