Why does this work?

[VonGrass via Digitalmars-d-learn]

On Monday, 23 June 2014 at 08:30:44 UTC, h_zet wrote:
> import std.typecons;
>
> auto foo2(R)(R foopara){
>     return tuple(foopara, is(R==int));
> }
>
> void main(){
>     auto tuple(a,b) = foo2(1);
> }
>
>
> I'm expecting some error such as can not act as left value but 
> when I compiled this, no error occured. DMD version is DMD64 
> v2.065.(ldc2 exited with error function declaration without 
> return type)
>
> Why does this work? Or it is a bug?

Looks grammaticallyu correct: R type is guessed/infered from the 
parameter