Pointer to dlang spec for this alias construct?

[evilrat]

On Monday, 17 June 2024 at 04:32:50 UTC, Andy Valencia wrote:
> 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

not sure if it works the same with template arguments but here is 
the rules
https://dlang.org/spec/expression.html#IsExpression