(Related: #31064, #14494)

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

1.12, but N/A

Does this issue reproduce with the latest release?

I believe so?

What operating system and processor architecture are you using (go env)?

N/A

What did you do?

Horrible monstrous things. Examples:

const iota = 0
type int struct{}
var nil = "haha"

Okay, fine, those are stupid. But you know what I've done unintentionally?

...
len := a -b
...
for i := 0; i < len(x); i++ {
...

and then get confused by the weird error message about len not being a function.

What did you expect to see?

I think redeclaring predeclared identifiers should be an error. Or, since that's probably impractical, it should be a thing in go vet.

What did you see instead?

Really weird and misleading error messages when I forget that something is a predeclared identifier. I've hit this most often with len, because that's a natural variable name and when I'm naming something that I'm usually not thinking about the function.