automatic NaN propogation detection?

[Elronnd]

On Saturday, 25 September 2021 at 07:53:11 UTC, Chris Katko wrote:
> Is that some sort of "NP complete" can't-fix issue or something?

The general case is obviously unsolvable.  Trivial proof: float x 
= nan; if (undecidable) use x.  I'm sure your imagination can 
supply more realistic cases (but I promise they really do come 
up!)

However that doesn't mean we can't do flow analysis 
conservatively.  Similar caveats apply to @live, but that does 
not mean it is useless.  (Not saying @live isn't useless, just 
that this doesn't indicate that.)  The bigger problem is that it 
really is hard to tell if you 'meant' to use a nan somewhere or 
not.

And if you tried to apply such a semantic analysis pass to 
existing code, you would find it riddled with false positives, 
where a float was default-initialized to nan and the compiler was 
unable to convince itself the variable was overridden before 
being used in all paths.