Thanks for the report.

Unfortunately, I don't have access to Gentoo to investigate a solution myself, but as a workaround you can create a VERSION file with go1.16.3 in it (no new line), assuming that's the Go version you have. If you are interested in contributing a version check that would also work on Gentoo, it would be very welcome.

I think that this might be better, just assume that they installed GopherJS using the same version of Go as the one they are trying to run:

Subject: [PATCH] compiler: fix version check on gentoo

---
 compiler/version_check.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/compiler/version_check.go b/compiler/version_check.go
index cc6e3f3..b9830d8 100644
--- a/compiler/version_check.go
+++ b/compiler/version_check.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"path/filepath"
+	"runtime"
 )
 
 // Version is the GopherJS compiler version string.
@@ -21,7 +22,8 @@ const GoVersion = 16
 func CheckGoVersion(goroot string) error {
 	v, err := ioutil.ReadFile(filepath.Join(goroot, "VERSION"))
 	if err != nil {
-		return fmt.Errorf("GopherJS %s requires a Go 1.16.x distribution, but failed to read its VERSION file: %v", Version, err)
+		v = []byte(runtime.Version())
+		// return fmt.Errorf("GopherJS %s requires a Go 1.16.x distribution, but failed to read its VERSION file: %v", Version, err)
 	}
 	if !bytes.HasPrefix(v, []byte("go1.16")) {
 		return fmt.Errorf("GopherJS %s requires a Go 1.16.x distribution, but found version %s", Version, v)
-- 
2.31.1

I wonder if this solution would break GOPHERJS_GOROOT usage. Maybe a safer solution would be to provide some sort of command-line flag to ignore the version check?

Yeah, because we now explicitly support building GopherJS tool with a different Go version than the one to be used by GopherJS build itself, this isn't a very safe fallback.

So in my private fork I have this change, which allows to bypass the check by setting an environment variable. This is convenient for GopherJS development, but I think this is not very good for users, who'd have to set yet another variable even if they didn't do anything wrong per se (using your distribution's Go version should be supported).

Probably the safest option would be to to execute $goroot/bin/go version and parse that. This is marginally more code, and is a bit slower than reading a single text file, but such fallback should work with any valid Go install.