`math-mode=fast` is too aggressive

[Keno]

I know it implies that computations aren't supposed to have NaNs in them. However, NaN is frequently used as a placeholder value in storage. I feel like just looking at them with isnan should be safe, as long as we're not computing on them. However:

julia> NaN
2.696539702293474e308

julia> isnan(NaN)
true

julia> x = NaN
2.696539702293474e308

julia> isnan(x)
true

julia> f(x) = !isnan(x)
f (generic function with 1 method)

julia> f(x)
true

[vtjnash]

All of those examples are doing substantial computation on them.

[Keno]

You mean, in showing them?

[vtjnash]

Asking isnan correctly is computationally expensive and slow

[oscardssmith]

We've killed math-mode=fast