return error if keywords used as alias in groupby (#3725)

gql/parser.go

@@ -1983,6 +1983,9 @@ func parseGroupby(it *lex.ItemIterator, gq *GraphQuery) error {
 				if alias != "" {
 					return item.Errorf("Expected predicate after %s:", alias)
 				}
+				if validKey(val) {
+					return item.Errorf("Can't use keyword %s as alias in groupby", val)
+				}
 				alias = val
 				it.Next() // Consume the itemColon
 				continue

gql/parser_test.go

@@ -3119,6 +3119,22 @@ func TestParseGroupbyWithAliasForKey(t *testing.T) {
 	require.Equal(t, "SchooL", res.Query[0].Children[0].GroupbyAttrs[1].Alias)
 }
 
+func TestParseGroupbyWithAliasForError(t *testing.T) {
+	query := `
+	query {
+		me(func: uid(0x1)) {
+			friends @groupby(first: 10, SchooL: school) {
+				count(uid)
+			}
+			hometown
+			age
+		}
+	}
+`
+	_, err := Parse(Request{Str: query})
+	require.Contains(t, err.Error(), "Can't use keyword first as alias in groupby")
+}
+
 func TestParseGroupbyError(t *testing.T) {
 	// predicates not allowed inside groupby.
 	query := `