Test corner cases of stream aliasing

src/tests/stream-aliasing.pl

@@ -0,0 +1,19 @@
+% NOTE: the tests in this file will need to be changed once
+% `open(stream(S1), read, _, NewOptions)` creates a new stream handle `S2` instead of updating
+% the options in `S1`. In that case, that specific query will need to be updated
+% to instead change the options of `S1`.
+
+alias_dropped_stream :-
+    open("README.md", read, S, [alias(readme)]),
+    open(stream(S), read, _, [alias(not_readme)]),
+    close(S),
+    stream_property(readme, file_name(_)). % Should throw an existence_error
+
+realias_user_output :-
+    current_output(S),
+    open(stream(S), read, _, [alias(not_user_output)]),
+    stream_property(S, alias(user_output)). % Should succeed
+
+set_output_alias :-
+    set_output(user_error),
+    write(user_output, hello). % Should write into stderr, not stdout