Cannot Instantiate SumType

[surlymoor]

I'm experiencing an error when instantiating `SumType` with an 
aggregate template that itself is instantiated with a void 
function. In other words...
```d
struct Foo(alias f) {}
// As the error implies, altering it to be non-void makes 
everything copacetic~
void bar() {}
alias S = SumType!(Foo!bar);
```
The error in question...
```
Error: variable 
`std.typecons.ReplaceTypeUnless!(isSumTypeInstance, This, 
SumType!(Foo!(bar)), Foo!(bar)).F!(bar).replaceTemplateArgs` type 
`void` is inferred from initializer `bar()`, and variables cannot 
be of type `void`
```
Perhaps I'm dong something stupid, or failing to realize, because 
all the glue sniffing I did when I was younger has finally 
manifested itself, but if it's not, is there a workaround?