Why does the "go run" command fail to find a second file in the main package?

Question

I am experiencing an issue where go run main.go produces the error:

# command-line-arguments
./main.go:9: undefined: test

However the commands go build && ./goruntest compile and run the program just fine.

The output is:

Hi from test()

Hi from sameFileTest()

Hi from pkgtest.Test()

Hi from pkgtest.Test1()

I have the directory set-up like so:

go/src/github.com/username/goruntest/
    pkgtest/
        pkgtest.go
        pkgtest1.go
    main.go
    test2.go

Here is the code.

main.go

package main

import (
    "fmt"
    "github.com/username/goruntest/pkgtest"
)

func main() {
    fmt.Println(test())           // main.go:9
    fmt.Println(sameFileTest())
    fmt.Println(pkgtest.Test())
    fmt.Println(pkgtest.Test1())
}

func sameFileTest() string {
    return "Hi from sameFileTest()"
}

gotest1.go

package main

func test() string {
    return "Hi from test()"
}

pkgtest/pkgtest.go

package pkgtest

func Test() string {
    return "Hi from pkgtest.Test()"
}

pkgtest/pkgtest1.go

package pkgtest

func Test1() string {
    return "Hi from pkgtest.Test1()"
}

I understand that the problem is the second file as part of package main and I also understand that there is no real reason to have a second file in main.

My question is: Why is go run unable to handle this set-up but building and running the executable works just fine?

EDIT

Included a second file in pkgtest

I also understand that the command go run main.go gotest1.go works but why do I need to specify gotest1.go?

I originally omitted these details for the sake of brevity. But now I see they are important to the question.

Answer 1

Try providing all relevant files to go run

$ go help run
usage: go run [build flags] [-exec xprog] gofiles... [arguments...]

Run compiles and runs the main package comprising the named Go source files.
A Go source file is defined to be a file ending in a literal ".go" suffix.