cmd/compile: incorrect typecheck error for multiple assignment

[rsc]

% cat /tmp/x.go
package p

func f() string

func _() {
	x := f(), 1, 2
}
% go tool compile /tmp/x.go
/tmp/x.go:6:4: assignment mismatch: 1 variables but f returns 3 values
% 

Narrator: f does not return 3 values.

Been this way back to Go 1.12 at least (I don't have earlier versions on my Mac laptop, and honestly I'm aamzed Go 1.12 still runs).

/cc @griesemer @mdempsky

[ianlancetaylor]

Go 1.11 is better:

foo.go:6:4: assignment mismatch: 1 variables but 3 values

gccgo says

foo.go:6:9: error: wrong number of initializations
    6 |         x := f(), 1, 2
      |         ^

[griesemer]

The compiler with types2 reports:

$ go tool compile -G main.go
main.go:6:2: x declared but not used
main.go:6:7: cannot initialize 1 variables with 3 values

I think this is about as good as the old error message.
(I agree that the error about x not being used doesn't need to be there. That's a separate issue.)

[gopherbot]

Change https://golang.org/cl/280112 mentions this issue: [dev.regabi] cmd/compile: cleanup assignment typechecking