Example code:
--------
import std.variant;

class Bob {}

void main() {
	immutable(Bob) bob = new immutable(Bob)();
	Variant v = bob;
	
	immutable(Bob) bob2 = v.get!(immutable(Bob))();
}
--------
Runtime Error:
core.exception.AssertError@C:\D\dmd2\windows\bin\..\..\src\phobos\std\variant.d(286): immutable(Bob)

This comes from here in std.variant:
--------
static if (is(typeof(*cast(T*) target = *src)))
{
    auto zat = cast(T*) target;
    if (src)
    {
        assert(target, "target must be non-null");
        *zat = *src;
    }
}
else
{
    // type is not assignable
    if (src) assert(false, A.stringof); // <-- line 286
}
--------

In this case, T is of type immutable(Bob). The check for assignability fails for any immutable type (or any type that contains immutable values) even though technically this code is initializing an immutable value and should be legal.

One way to make this work is to rewrite the above code to this:
--------
static if (is(typeof(*cast(T*) target = *src)))
{
    auto zat = cast(T*) target;
    if (src)
    {
        assert(target, "target must be non-null");
        *zat = *src;
    }
}
else static if (is(T V == const(U), U) || is(T V == immutable(U), U))
{
	auto zat = cast(U*) target;
	if (src)
	{
		assert(target, "target must be non-null");
		*zat = *(cast(U*) (src));
	}
}
else
{
    // type is not assignable
    if (src) assert(false, A.stringof);
}
--------

Which is basically casting away immutability to copy the reference.
This sort of situation makes a compelling argument for the idea of a tail const reference, or a mutable reference to const or immutable data.