Template Usage with Eponymous Trick

[ShadoLight]

On Sunday, 2 February 2020 at 16:23:42 UTC, Paul Backus wrote:

[..]
>
> No, it would still be ambiguous:
>
> struct S(T) {}
>
> alias a = S!(int);
>
> // Should this assertion pass or fail?
> static assert(is(a));

Sorry, I don't get it. AFAICS 'is(a)' should return true (since a 
is an alias for a full type here) - and braces being compulsory 
or optional does not affect this.

AFAICS:

struct S(T) {}

alias a = S!(int);
alias b = S;

// Should this assertion pass or fail?
static assert(is(a));  //PASS
static assert(is(b));  //FAIL

But I don't see how braces will affect this. Can you explain?