std.functional.memoize : thread local or __gshared memoization?

[Ola Fosheim Grøstad via Digitalmars-d]

On Saturday, 27 May 2017 at 11:52:04 UTC, John Colvin wrote:
> Nope. __gshared isn't even part of the type so it can't do that.
>
> __gshared int a;
>
> void main()
> {
>     foo(a);

But this should fail since foo doesn't take shared?

> __gshared is a way of declaring a C-like global variable, but D 
> does not - in general - support accessing it in a C-like way. 
> My advice is to cast to shared before use, unless you're 
> totally sure you know what other threads will be doing.

Why isn't it always typed as shared?