Movement against float.init being nan

[jmh530]

On Monday, 22 August 2022 at 20:29:57 UTC, Bastiaan Veelo wrote:
> On Saturday, 20 August 2022 at 03:12:43 UTC, Steven 
> Schveighoffer wrote:
>> In other words, NaN is silent. You can't even `assert(x != 
>> double.init)`. You have to use an esoteric function `isNaN` 
>> for that.
>
> I had some fun with `isNaN` the other day. We used it to check 
> for initialisation in an access function to cache an expensive 
> computation. This worked brilliantly until we noticed a 
> malfunction in the release version. It took a while until I 
> realised that I had given the LDC `fastmath` option to the 
> release build, which assumes NaN does not occur, which makes 
> `isNaN` misbehave.
>

Is there a simple example of this behavior?