cmd: disable *.go domains lookup in go get command

Using 'go get x.go' instead of 'go build x.go' or some other
go command is a common mistake. By that mistake, a user gets
a misleading error message about unsuccessful `x.go` domain lookup.
This improvement handles such cases, by validating, whether the
argument hasn't specified version, has .go suffix, and either has
no slashes or such file locally exists. Handled both GOPATH
and GOMOD modes.

Fixes #38478

Change-Id: I583a4ef7f7ca8901deb07ebc811e2b3c0e828fa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/229938
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/src/cmd/go/internal/get/get.go b/src/cmd/go/internal/get/get.go
index f7b2fa96e8..d38350c2a8 100644
--- a/src/cmd/go/internal/get/get.go
+++ b/src/cmd/go/internal/get/get.go
@@ -193,8 +193,24 @@ func downloadPaths(patterns []string) []string {
 	for _, arg := range patterns {
 		if strings.Contains(arg, "@") {
 			base.Fatalf("go: cannot use path@version syntax in GOPATH mode")
+			continue
+		}
+
+		// Guard against 'go get x.go', a common mistake.
+		// Note that package and module paths may end with '.go', so only print an error
+		// if the argument has no slash or refers to an existing file.
+		if strings.HasSuffix(arg, ".go") {
+			if !strings.Contains(arg, "/") {
+				base.Errorf("go get %s: arguments must be package or module paths", arg)
+				continue
+			}
+			if fi, err := os.Stat(arg); err == nil && !fi.IsDir() {
+				base.Errorf("go get: %s exists as a file, but 'go get' requires package arguments", arg)
+			}
 		}
 	}
+	base.ExitIfErrors()
+
 	var pkgs []string
 	for _, m := range search.ImportPathsQuiet(patterns) {
 		if len(m.Pkgs) == 0 && strings.Contains(m.Pattern(), "...") {