fmt: %#v does not print “a Go-syntax representation” for NaN values

[bcmills]

What version of Go are you using (go version)?

devel go1.19-c9b60632eb Fri Mar 4 14:10:38 2022 +0000

Does this issue reproduce with the latest release?

Yes

What did you do?

Format a floating-point NaN value with %#v (https://go.dev/play/p/zlxs4NzQ0bl?v=gotip):

	x := float32(math.NaN())
	fmt.Printf("x := %T(%#v)", x, x)

What did you expect to see?

Per https://pkg.go.dev/fmt#hdr-Printing, “a Go-syntax representation of the value” — that is, valid Go syntax for the value.

Perhaps a line like:

	x := float32(math.NaN())

or

	x := float32(math.Float32frombits(0x7fc00000))

What did you see instead?

The string NaN:

x := float32(NaN)

which is not “a Go-syntax representation”. Feeding it back into the program gives (https://go.dev/play/p/908uCl2KbVN?v=gotip):

./prog.go:13:15: undefined: NaN

Go build failed.

This problem is especially acute if the NaN value is not the same bits as math.NaN() (https://go.dev/play/p/Xl13miTI7pe?v=gotip), since there is no indication that there is anything unusual about it.

[robpike]

What if the documentation just said that it printed "NaN" for NaNs? For the purpose of debugging, that seems a lot more pleasant.

[gopherbot]

Change https://go.dev/cl/557235 mentions this issue: fmt: document how %#g handles infinities and NaNs