Movement against float.init being nan

[Paul Backus]

On Thursday, 25 August 2022 at 14:34:52 UTC, Paul Backus wrote:
> Looks like D allows you to enable floating-point exceptions 
> using the [`std.math.hardware.FloatingPointControl`][1] 
> interface.
>
> Of course, these are hardware exceptions, so you will get 
> SIGFPE rather than a thrown Error with a backtrace. To turn 
> them into D Errors you'd need to set up a signal handler, like 
> how `etc.linux.memoryerror` does it (or whatever the equivalent 
> is on Windows).
>
> [1]: 
> https://phobos.dpldocs.info/std.math.hardware.FloatingPointControl.html

Unfortunately, it looks like this will not work in practice, 
because the "invalid" floating-point exception is raised only 
when a NaN is *created*, not when it's *propagated*. Since the 
initial NaN values of default-initialized `float`/`double` 
variables are created at compile time, not runtime, no exception 
is ever raised for them.

Example program:

```d
import std.math.hardware;

float div(float x, float y)
{
     return x / y;
}

void main()
{
     FloatingPointControl fpctrl;
     
fpctrl.enableExceptions(FloatingPointControl.severeExceptions);

     float f = float.nan; // ok - no exception
     float g = div(0.0f, 0.0f); // crashes with SIGFPE
}
```