Template overload causing an error even when a better non-template match is found

[Andrej Mitrovic]

You're going to need DMD git-head to run this reduced example:

-----
struct S
{
    void opAssign(T)(T t)
        if (Constraint!T)
    {
    }

    void opAssign(typeof(null))
    {
    }

    template Constraint(T)
        if (is(T == int))
    {
        enum bool Constraint = false;
    }
}

void main()
{
    S s;
    s = null;
}
-----

This fails with:

test.d(6): Error: template instance Constraint!(typeof(null)) does not
match template declaration Constraint(T) if (is(T == int))

However I don't think the compiler should even attempt to instantiate
the template when there's a better non-template overload match.

The code example is contrived, but in the real code there's several
overloads of "Constraint" which do more sophisticated checking on
their type (some work on functions, others on delegates, structs,
classes, etc).

Hence why the constraint templates themselves need to have
constraints, but this is then causing the above error.

I'm not looking for a workaround (there's plenty of ways to work
around this), but I think this should be filed as a bug?