Edit: Following is in response to suggestion (now removed) by other user that provided this URL: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#tag_03_266

Interesting. It's inconsistent with how Linux and MacOS work though:

$ touch test
$ stat test
  File: ‘test’
  Size: 0         	Blocks: 0          IO Block: 4096   regular empty file
Device: ca01h/51713d	Inode: 262802      Links: 1
Access: (0664/-rw-rw-r--)  Uid: (  501/hadfielj)   Gid: (  501/hadfielj)
...
$ stat test/
stat: cannot stat ‘test/’: Not a directory

Unless the OS' stat is not comparable?

On GNU/Linux I get an error not a directory. I agree that I don't see this error on the playground, but the playground isn't using a real file system.

If you don't see this on macOS then I think macOS must work differently. Try writing the equivalent C program and see what happens.

Equivalent C program shows consistent result for both Linux and MacOS, i.e. if I add a trailing slash to the end of an existing file path when calling stat, it fails to stat the file.
Code used:

#include <stdio.h>
#include <sys/stat.h>

int main() {
    struct stat stats;
    char path[11] = "hello.txt/";
    if (stat(path, &stats) == 0)
    {
        printf("'%s' does exist\n", path);
    }
    else
    {
        printf("'%s' does not exist\n", path);
    }
    return 0;
}

On MacOS and Linux, when run in the same directory as a file called 'hello.txt', it returns:
'hello.txt/' does not exist

What version of MacOS are you using?

I can't reproduce the issue with go 1.12.9 and Mac OS 10.14.6:

⇢ $ sw_vers                                                                                                                                                                                                                                    2 ↵
go version

cat > t.go <<EOL
package main

import (
        "fmt"
        "io/ioutil"
        "os"
)

func main() {
        p := fmt.Sprintf("%s/hello.txt", os.TempDir())
        b := []byte("hello world")
        _ = ioutil.WriteFile(p, b, 0644)
        pWithSep := p + "/"
        _, err := os.Stat(pWithSep)
        fmt.Println(err)
}
EOL

go run t.go


ProductName:	Mac OS X
ProductVersion:	10.14.6
BuildVersion:	18G87
go version go1.12.9 darwin/amd64
stat /var/folders/mm/b3n2qsjx73lddzbw34x40w8w0000gn/T//hello.txt/: not a directory

/me can not reproduce with Mac OS 10.14.6, go 1.12.8, 1.12.9 and tip.

Original test carried out on MacOS 10.15 Beta with 1.12.7.
I cannot reproduce:

• since updating to 1.12.9
• downgrading to 1.12.7 using official installer, rather than homebrew version

Homebrew formula for 1.12.7 now missing, so unable to check if it was that specific build/env setup.
Will close for now and reopen if I manage to get homebrew 1.12.7 and am able to reproduce.

Try this:

brew unlink go
cd $( brew --prefix )/Homebrew/Library/Taps/homebrew/homebrew-core/Formula
git checkout fa46027fe
HOMEBREW_NO_AUTO_UPDATE=1 brew install go

Thanks for the instructions @gregory-m.
I'm able to test again with 1.12.7 but no longer able to reproduce the issue. I even ensured my 'go env' was identical to what I reported above, but no luck.
Can only assume something else has changed on my machine.

@jonhadfield Thank you for fast response.
Maybe this is just fluctuations of solar radiation.