[Issue 21044] [CTFE] Infinite loop in ForStatement::interpret

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21044

Bruce Carneal <bcarneal11 at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bcarneal11 at gmail.com

--- Comment #2 from Bruce Carneal <bcarneal11 at gmail.com> ---
(In reply to Iain Buclaw from comment #0)
> Found on one iteration of mechanically reduced code (still dozens of files,
> thousands of lines of code). Will copy it and reduce it further, but I
> suspect that the final product will be code that looks like:
> 
> ---
> int foo()
> {
>     for (;;) { }
>     return 0;
> }
> 
> static assert(foo() == 0);
> ---
> 
> If that is the case, then the compiler really should have proper detection
> for this, to bail and error out if it finds a loop that never exits during
> CTFE.

As you noted during beerconf, dlang is Turing complete at compile time, which
leaves us with the halting problem. (yes, indeed, we're working with power
tools)

Per Stefan's comments at beerconf the current approach to this includes a limit
on recursion at compile time.  From your experience, and anticipating recursion
rewrites to iterative forms in the future, something more is needed.

Is there some notion of "progress" at compile time that is readily available? 
A count of the source level symbols resolved perhaps?  There's no "solving" the
halting problem of course, just looking for a practical bound.

Seems like current CTFE practice and implementation generate a number of
intermediate structures that ultimately collapse to something roughly the size
of the source input.

--