Go test <function> returns undefined: <function> - go

Question

Trying to run "go test sum_test.go" returns an error:
./sum_test.go:18:13: undefined: SumInt8
FAIL command-line-arguments [build failed]
I'm taking an introductory course in golang. Our teacher distributed a code-file, sum.go, and a testing-file, sum_test.go. Trying to run "go test" on sum_test.go returns the error above. The code runs fine on our teachers mac, and he's having difficulties recreating the problem. Here's my go environment settings: https://pastebin.com/HcuNVcAF
sum.go
package sum
func SumInt8(a, b int8) int8 {
return a + b
}
func SumFloat64(a, b float64) float64 {
return a + b
}
sum_test.go
package sum
import "testing"
// Check https://golang.org/ref/spec#Numeric_types and stress the limits!
var sum_tests_int8 = []struct{
n1 int8
n2 int8
expected int8
}{
{1, 2, 3},
{4, 5, 9},
{120, 1, 121},
}
func TestSumInt8(t *testing.T) {
for _, v := range sum_tests_int8 {
if val := SumInt8(v.n1, v.n2); val != v.expected {
t.Errorf("Sum(%d, %d) returned %d, expected %d",
v.n1, v.n2, val, v.expected)
}
}
}
I see no particular errors, so I expected "go test sum_test.go" to run, and succeed. However it seems it can't find the method SumInt8 in sum.go.

Answer 1

$ go help packages
Many commands apply to a set of packages:
go action [packages]
Usually, [packages] is a list of import paths.
An import path that is a rooted path or that begins with a . or ..
element is interpreted as a file system path and denotes the package
in that directory.
Otherwise, the import path P denotes the package found in the
directory DIR/src/P for some DIR listed in the GOPATH environment
variable (For more details see: 'go help gopath').
If no import paths are given, the action applies to the package in the
current directory.
As a special case, if the package list is a list of .go files from a
single directory, the command is applied to a single synthesized
package made up of exactly those files, ignoring any build constraints
in those files and ignoring any other files in the directory.
List all the files in the current directory used in the test:
go test sum_test.go sum.go
or simply test the complete package in the current directory.
go test
For example,
$ go test -v sum_test.go sum.go
=== RUN TestSumInt8
--- PASS: TestSumInt8 (0.00s)
PASS
ok command-line-arguments 0.002s
$
or, for the complete package
$ go test -v
=== RUN TestSumInt8
--- PASS: TestSumInt8 (0.00s)
PASS
ok so/sum 0.002s
$
Option '-v' produces verbose output.
sum_test.go:
package sum
import "testing"
// Check https://golang.org/ref/spec#Numeric_types and stress the limits!
var sum_tests_int8 = []struct {
n1 int8
n2 int8
expected int8
}{
{1, 2, 3},
{4, 5, 9},
{120, 1, 121},
}
func TestSumInt8(t *testing.T) {
for _, v := range sum_tests_int8 {
if val := SumInt8(v.n1, v.n2); val != v.expected {
t.Errorf("Sum(%d, %d) returned %d, expected %d",
v.n1, v.n2, val, v.expected)
}
}
}
sum.go:
package sum
func SumInt8(a, b int8) int8 {
return a + b
}
func SumFloat64(a, b float64) float64 {
return a + b
}