cmd/compile: confusing error message when using an indexing expression as an array size

[go101]

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

$ go version
go version go1.20.5 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

package main

const S = "Go"

type T [S[0]]int // syntax error: type parameters must be named

func main() {}

What did you expect to see?

syntax error: array length must be a constant

or

syntax error: type constraint is incomplete

or

syntax error: either type constraint is incomplete or array length is not a constant

What did you see instead?

syntax error: type parameters must be named

[Nasfame]

By human reasoning and gofmt its pretty clear that the square brackets doesn't enclose constraints. Nearest reasoning is that there is typo in defining the slice!

So most plausible error should be syntax error: array length must be a constant

[go101]

It looks the current implementation tries to think the only item in [] as a constraint.
Go 1.17.13 outputs non-constant array bound S[N] for the following code.
I prefer a message combining the two messages.

const S = "Go"

const N = 0

type T [S[N]]int

[ianlancetaylor]

CC @griesemer