Pointer to dlang spec for this alias construct?

[Andy Valencia via Digitalmars-d-learn]

In the alias:

    alias Unshared(T) = T;
    alias Unshared(T: shared U, U) = U;
as used in:

    cast(Unshared!mytype)value

turns a mytype with shared attribute into one without shared.

I deduce the alias is using some sort of type matching and 
decomposition?

I've read through the language spec, and didn't spot this 
mechanism.  Can somebody tell me what section of the spec covers 
this very interesting mechanism?

Thanks in advance,
Andy