% cat x.go
package p

func f() (int, int, int)

func g() {
	x, y := f()
	var a, b int
	a, b = f()
	_ = x
	_ = y
	_ = a
	_ = b
}
% go tool compile x.go
x.go:6:10: cannot initialize 2 variables with 3 values
x.go:8:9: cannot assign 3 values to 2 variables
% go1.17 tool compile x.go
x.go:6:7: assignment mismatch: 2 variables but f returns 3 values
x.go:8:7: assignment mismatch: 2 variables but f returns 3 values
% 

% go test net/url
# net/url [net/url.test]
net/url/url.go:924:16: cannot assign 3 values to 2 variables
net/url/url.go:932:17: cannot initialize 2 variables with 3 values
net/url/url.go:1005:21: cannot assign 3 values to 2 variables
net/url/url_test.go:2068:16: cannot assign 3 values to 2 variables
FAIL	net/url [build failed]
FAIL
% go test -gcflags=-G=0 net/url
# net/url [net/url.test]
net/url/url.go:924:14: assignment mismatch: 2 variables but strings.Cut returns 3 values
net/url/url.go:932:14: assignment mismatch: 2 variables but strings.Cut returns 3 values
net/url/url.go:1005:19: assignment mismatch: 2 variables but strings.Cut returns 3 values
net/url/url_test.go:2068:14: assignment mismatch: 2 variables but strings.Cut returns 3 values
FAIL	net/url [build failed]
FAIL
%