Comment author: bouaziz

Of course, the same applies for <, <=, >, >=

Comment author: @damiendoligez

I'm not sure I understand what you mean: the compiler cannot know in general whether a given float expression is going to be NaN.

As for emitting a warning at run-time, that cannot be done because equality comparison must not do side-effects.

Comment author: bouaziz

I do not want a warning at runtime but it would be great to have it at compile time when comparing with Pervasives.nan. E.g.
if x = nan then "nan" else "a number" -> warning

Of course it would not work with any other value being equal to nan (static analysis, e.g. optimization, could catch some of them though). E.g.
let y = 0. in
if x = y /. 0. then "nan" else "a number"

Another way of seeing it is to optimize ... = nan (and I mean any value known at compile time to always be equal to nan) as false and then report dead branches like the "then" branch in the previous examples.

Comment author: @alainfrisch

Careful with optimizations: you probably don't want to trigger a warning after inlining a function which results in comparing a float with NaN, as in:

let f lim x =
.... if x < lim then ....

let g = f nan x

Comment author: @xavierleroy

The use case is uncommon, and there are limits to the number of special cases the compiler can reasonably warn about. Suspending this PR.