Opaque type (struct) with a static immutable fails to compile without constructor - why?

[frankp]

Hi all,

I want to make an opaque type that simply contains an integer 
with some immutable constants and toString pretty printing. Like 
this:

struct Foo_t
{
    private long foo;
    alias foo this;
    static immutable long Inf = long.max; //1)

    void toString(...){}
}

On dmd 2.092.1 this fails with:
   1) Error: cannot implicitly convert expression 9223...L of type 
immutable(long) to Foo_t

I simply want to initialize an immutable long with long.max.
Why the conversion to Foo_t ?

If I add a constructor:
private this(long f)
{
    foo = f;
}

It compiles but according to code coverage this constructor is 
never called.
What's going on?