`<future>`: `promise` default constructs the value type

Describe the bug
The value type T of a promise<T> is default constructed when the promise is constructed, which is surprising. I am not sure what the requirements of the standard are. The current implementation requires T to be default constructible and assignable. It is possible to implement promise with none of these two requirements.

Command-line test case

C:\Temp>type repro.cpp
#include <future>
#include <iostream>

struct test
{
  test()
    : result_{-1}
  {
    std::cout << "Default constructor" << std::endl;
  }

  explicit test(int i)
    : result_{i}
  {
  }

  int result() const
  {
    return result_;
  }

private:
  int result_;
};

int main()
{
  std::promise<test> prm;
  std::future<test> fut = prm.get_future();

  prm.set_value(test{0});

  return fut.get().result();
}

C:\Temp>cl /EHsc /W4 /WX .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30139 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
Microsoft (R) Incremental Linker Version 14.29.30139.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:repro.exe
repro.obj

C:\Temp>.\repro.exe
Default constructor

Expected behavior
The line "Default constructor" should not be printed. Moreover, replacing the default constructor test() by

test() = delete;
test(const test&) = default;
test& operator=(const test&) = delete;

should still compile.

STL version
Microsoft Visual Studio Professional 2019
Version 16.11.9

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with the command-line reproduction and the expected behavior described in the issue. Review the vNext and binary-compatibility constraint, along with the history of merged PR #2568. Done means promise no longer default-constructs T and supports the shown non-default-constructible, non-assignable type.