is type checking in D undecidable?

[Paul Backus]

On Friday, 23 October 2020 at 00:53:19 UTC, Bruce Carneal wrote:
> When you write functions, the compiler helps you out with fully 
> automated constraint checking.  When you write templates you 
> can write them so that they look like simple functions, in 
> which case you're on pretty solid ground.  Your manual 
> constraints will probably work.  Hard to screw up a four line 
> eponymous template with constraints.  Hard to screw up a "leaf" 
> template with a small number of template args.  Possible but 
> hard.  Not so hard to screw up 
> "wanna-be-as-general-as-possible-but-special-case-performant-and-sometimes-wierdly-recursive-cuz-otherwise-the-compiler-blows-up" templates.

This is true, but it has nothing at all to do with 
decidability--which is a term with a precise technical definition 
in computer science.

The reason writing correct generic code using templates (or any 
macro system) is so difficult is that templates (and macros in 
general) are, effectively, dynamically typed. Unlike regular 
functions, templates are not type checked when they are declared, 
but when they are "executed" (that is, instantiated). In that 
sense, writing templates in D is very similar to writing code in 
a dynamically-typed language like Python or Javascript.