The following code currently does not compile:

import core.lifetime;
extern(C++) struct S
{
    @disable this();
    this(int)
    {
    }
    this(ref const(S) other)
    {
    }
}
void main()
{
    S s1 = S(1);
    S s2 = void;
    copyEmplace(s1, s2);
}

DMD prints the following error messages:

/usr/include/dlang/dmd/core/internal/lifetime.d(18): Error: static assert:  "Cannot emplace a S because S.this() is annotated with @disable."
/usr/include/dlang/dmd/core/lifetime.d(17):        instantiated from here: `emplaceRef!(S, S)`
/usr/include/dlang/dmd/core/lifetime.d(1251):        instantiated from here: `emplace!(S)`
testcopyctor.d(16):        instantiated from here: `copyEmplace!(S, S)`

The default constructor should not be necessary to use the copy constructor.