The following code compiles:
```
alias T = shared(int)*;

shared T a;
shared T b;
shared T c;

void foo() {
    import core.atomic: cas;
    cas(&a, b, c);
}
```

The type of T has to be a pointer to a shared int (you get a template match error for `cas` if `T = int*`), which is annoying but I kind-of understand.
However, change b to null (`cas(&a, null, c);`) and things no longer work: "Error: template `core.atomic.cas` cannot deduce function from argument types"

I have not succeeded to make things work with function pointers (neither with nor without `null` as second argument). What am I doing wrong if `alias T = void function();` ?

Thanks,
  Johan

On Tuesday, 22 January 2019 at 14:13:23 UTC, Johan Engelen wrote:
> The following code compiles:
> ```
> alias T = shared(int)*;
>
> shared T a;
> shared T b;
> shared T c;
>
> void foo() {
>     import core.atomic: cas;
>     cas(&a, b, c);
> }
> ```
>
> The type of T has to be a pointer to a shared int (you get a template match error for `cas` if `T = int*`), which is annoying but I kind-of understand.
> However, change b to null (`cas(&a, null, c);`) and things no longer work: "Error: template `core.atomic.cas` cannot deduce function from argument types"
>
> I have not succeeded to make things work with function pointers (neither with nor without `null` as second argument). What am I doing wrong if `alias T = void function();` ?
>
> Thanks,
>   Johan

You need to cast stuff (as seems to be the case with everything to do with shared, at least until/if Manu's proposal goes through):

alias T = void function();
alias S = shared(size_t*);
shared T a;
shared T b;
shared T c;

void foo() {
    import core.atomic: cas;
    cas(cast(S*)&a, cast(S)b, cast(S)c);
}

I believe this is historical. It will fail because atomics are combinatorially parameterized because the compiler couldn't properly infer template arguments until recently (this also resulted in memory corruption in ldc fork of druntime). Now that the compiler was fixed, atomics can be fixed too and have single template parameter.