prevent overriding gh commands

command/alias.go

@@ -11,7 +11,7 @@ import (
 
 // TODO
 // - [ ] DEBUG support
-// - [ ] prevent overriding existing gh command
+// - [x] prevent overriding existing gh command
 // - [x] allow overwriting alias
 // - [x] forward extra arguments
 // - [x] allow duplication of placeholder
@@ -68,6 +68,10 @@ func aliasSet(cmd *cobra.Command, args []string) error {
 	out := colorableOut(cmd)
 	fmt.Fprintf(out, "- Adding alias for %s: %s\n", utils.Bold(alias), utils.Bold(expansion))
 
+	if validCommand(alias) {
+		return fmt.Errorf("could not create alias: %q is already a gh command", alias)
+	}
+
 	if !validCommand(expansion) {
 		return fmt.Errorf("could not create alias: %s does not correspond to a gh command", utils.Bold(expansion))
 	}

command/alias_test.go

@@ -9,6 +9,17 @@ import (
 	"github.com/cli/cli/test"
 )
 
+func TestAliasSet_gh_command(t *testing.T) {
+	initBlankContext("", "OWNER/REPO", "trunk")
+
+	_, err := RunCommand("alias set pr pr status")
+	if err == nil {
+		t.Fatal("expected error")
+	}
+
+	eq(t, err.Error(), `could not create alias: "pr" is already a gh command`)
+}
+
 func TestAliasSet_existing_alias(t *testing.T) {
 	cfg := `---
 hosts: