go/parser: parser doesn't accept trailing comma where it should

[griesemer]

package p

type S[T1 any, T2 any] struct {
	v1 T1
	v2 T2
}

type S1 struct {
	S[byte, []byte] // ok
}

type S2 struct {
	S[byte, []byte,] // not ok: trailing comma not accepted even though it is legal
}

leads to a syntax error when parsed with go/parser. In the playground, the program cannot be formatted (but it can be compiled and run because the compiler uses a different parser).

[griesemer]

See CL 450175.

[gopherbot]

Change https://go.dev/cl/450175 mentions this issue: go/parser: allow trailing commas in embedded instantiated types