fmt: %#v does not print “a Go-syntax representation” for NaN values #51486
Description
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.