Is instantiabilty of templated types decidable?

[evansl]

On 11/11/12 06:49, Peter Alexander wrote:
> On Sunday, 11 November 2012 at 12:33:25 UTC, Manfred Nowak wrote:
>> a) Instantiability is decidable
>> Why does the compiler stop with the evaluation at that randomly
>> choosen and apparently hard coded value of 500 recursive
>> expansions?
> 
> It's not decidable. Consider use of static if. It's Turing complete. I
> can give an example if you like.
> 

I'd like.  An example might help me better understand why it's
undecidable.
[snip]