4 files changed, 158 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(), "...") {
diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index b1193dbfc4..8d740471b0 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -310,6 +310,20 @@ func runGet(cmd *base.Command, args []string) {
 			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 version and either has no slash or refers to an existing file.
+		if strings.HasSuffix(arg, ".go") && vers == "" {
+			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)
+				continue
+			}
+		}
+
 		// If no version suffix is specified, assume @upgrade.
 		// If -u=patch was specified, assume @patch instead.
 		if vers == "" {
diff --git a/src/cmd/go/testdata/script/get_go_file.txt b/src/cmd/go/testdata/script/get_go_file.txt
new file mode 100644
index 0000000000..97e0f1ac92
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_go_file.txt
@@ -0,0 +1,60 @@
+# Tests Issue #38478
+# Tests that go get in GOPATH mode returns a specific error if the argument
+# ends with '.go', and either has no slash or refers to an existing file.
+
+env GO111MODULE=off
+
+# argument doesn't have .go suffix
+go get test
+
+# argument has .go suffix, is a file and exists
+! go get test.go
+stderr 'go get test.go: arguments must be package or module paths'
+
+# argument has .go suffix, doesn't exist and has no slashes
+! go get test_missing.go
+stderr 'go get test_missing.go: arguments must be package or module paths'
+
+# argument has .go suffix, is a file and exists in sub-directory
+! go get test/test.go
+stderr 'go get: test/test.go exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, doesn't exist and has slashes
+! go get test/test_missing.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a symlink and exists
+[symlink] symlink test_sym.go -> test.go
+[symlink] ! go get test_sym.go
+[symlink] stderr 'go get test_sym.go: arguments must be package or module paths'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a symlink and exists in sub-directory
+[symlink] symlink test/test_sym.go -> test.go
+[symlink] ! go get test/test_sym.go
+[symlink] stderr 'go get: test/test_sym.go exists as a file, but ''go get'' requires package arguments'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a directory and exists
+mkdir test_dir.go
+! go get test_dir.go
+stderr 'go get test_dir.go: arguments must be package or module paths'
+rm test_dir.go
+
+# argument has .go suffix, is a directory and exists in sub-directory
+mkdir test/test_dir.go
+! go get test/test_dir.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+rm test/test_dir.go
+
+
+-- test.go --
+package main
+func main() {println("test")}
+
+
+-- test/test.go --
+package main
+func main() {println("test")}
diff --git a/src/cmd/go/testdata/script/mod_get_go_file.txt b/src/cmd/go/testdata/script/mod_get_go_file.txt
new file mode 100644
index 0000000000..0c7b5dc11c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_go_file.txt
@@ -0,0 +1,68 @@
+# Tests Issue #38478
+# Tests that go get in GOMOD mode returns a specific error if the argument
+# ends with '.go', has no version, and either has no slash or refers to an
+# existing file.
+
+env GO111MODULE=on
+
+# argument doesn't have .go suffix and has no version
+! go get test
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix and has version
+! go get test.go@v1.0.0
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a file and exists
+! go get test.go
+stderr 'go get test.go: arguments must be package or module paths'
+
+# argument has .go suffix, doesn't exist and has no slashes
+! go get test_missing.go
+stderr 'arguments must be package or module paths'
+
+# argument has .go suffix, is a file and exists in sub-directory
+! go get test/test.go
+stderr 'go get: test/test.go exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, doesn't exist and has slashes
+! go get test/test_missing.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a symlink and exists
+[symlink] symlink test_sym.go -> test.go
+[symlink] ! go get test_sym.go
+[symlink] stderr 'go get test_sym.go: arguments must be package or module paths'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a symlink and exists in sub-directory
+[symlink] symlink test/test_sym.go -> test.go
+[symlink] ! go get test/test_sym.go
+[symlink] stderr 'go get: test/test_sym.go exists as a file, but ''go get'' requires package arguments'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a directory and exists
+mkdir test_dir.go
+! go get test_dir.go
+stderr 'go get test_dir.go: arguments must be package or module paths'
+rm test_dir.go
+
+# argument has .go suffix, is a directory and exists in sub-directory
+mkdir test/test_dir.go
+! go get test/test_dir.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+rm test/test_dir.go
+
+
+-- test.go --
+package main
+func main() {println("test")}
+
+
+-- test/test.go --
+package main
+func main() {println("test")}