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

[zigo101]

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

[zigo101]

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

[gopherbot]

Change https://go.dev/cl/538856 mentions this issue: cmd/compile/internal/syntax: better error messages for incorrect type parameter list