cmd/compile: avoid multiple errors on the same line (reminder issue)

[griesemer]

This is a follow-up on #28450 which improved an error message but reports multiple errors on the same line for the added test case below ( https://go-review.googlesource.com/c/go/+/152417 ):

package p

func f(a, b, c, d ...int)       {} // ERROR "non-final parameter a" "non-final parameter b" "non-final parameter c"
func g(a ...int, b ...int)      {} // ERROR "non-final parameter a"
func h(...int, ...int, float32) {} // ERROR "non-final parameter"

type a func(...float32, ...interface{}) // ERROR "non-final parameter"
type b interface {
	f(...int, ...int)                // ERROR "non-final parameter"
	g(a ...int, b ...int, c float32) // ERROR "non-final parameter a" "non-final parameter b"
	valid(...int)
}

Investigate if there's something off in gc/subr.go (error handling).

[fraenkel]

@griesemer The sameline check compares messages which won't work in this case given the parameter name is contained within.

[gopherbot]

Change https://golang.org/cl/152758 mentions this issue: cmd/compile: avoid multiple errors regarding misuse of ... in signatures

[griesemer]

@fraenkel Thanks. Still, it turns out there's an easy way to make this specific case work better. See the CL.