CTFE calling a template: Error: expression ... is not a valid template value argument

[Timon Gehr]

On 09/21/2012 12:29 AM, Jonathan M Davis wrote:
> ...
> In order for your foo function to be called, it must be fully compiled first
> (including its entire body, since CTFE needs the full definition of the
> function, not just its signature). The body cannot be fully compiled until the
> template that it's using is instantiated. But that template can't be compiled
> until foo has been compiled, because you're passing a call to foo to it as a
> template argument. So, you have a circular dependency.
>
> Normal recursion avoids this, because it only depends on the function's
> signature, but what you're doing requires that the function be _run_ as part
> of the process of defining it. That's an unbreakable circular dependency and
> will never work. ...

It is not exactly unbreakable. This can work. Just delay spitting out
an error to the point the execution actually depends upon its own
result.