Add a test case (currently disabled, since we don't pass it)

Estimated hours taken: 0.5
Branches: main

tests/valid/Mmakefile:
tests/valid/mode_selection.m:
	Add a test case (currently disabled, since we don't pass it)
	which tests that we do flattening bottom-up rather than top-down.

tests/valid/Mmakefile

@@ -163,11 +163,14 @@ OTHER_SOURCES= \
 	vn_float.m \
 	zero_arity.m
 
-# The mode system can't handle the following test cases yet:
+# XXX The mode system can't handle the following test cases yet:
 #	assoc_list.m
 #	determinism.m
 #	mode_merge_insts.m
 #	inst_perf_bug_2.m
+#
+# XXX We also don't pass this one (see the comments in it for details):
+#	mode_selection.m
 
 # There used to be problems with compiling typeclass
 # and AGC stuff in grades jump.* and fast.*, but they should

tests/valid/mode_selection.m

@@ -0,0 +1,45 @@
+:- module mode_selection.
+:- interface.
+:- import_module io.
+
+:- pred main(io__state::di, io__state::uo) is det.
+
+:- implementation.
+:- import_module require.
+
+% Currently (May 2001) we don't pass this test case,
+% because the compiler's expression flattening puts
+% the sub-goals in top-down order, rather than
+% (as the language reference manual requires)
+% ordering them bottom-up.  This means that the call to
+% func2 gets flattened as
+%	{ V_1 = func2(In, V_2) },
+%	{ V_2 = In },
+%	{ print(V_1) }
+% rather than as
+%	{ V_1 = func2(In, V_2) },
+%	{ V_2 = In },
+%	{ print(V_1) }
+% which causes mode analysis to select the wrong mode in the
+% call to func2, which in turn causes a determinism error.
+%
+% The rationale for keeping the current behaviour is that
+% the naive fix of just flattening to bottom-up order
+% causes performance problems in type checking, in particular
+% when compiling compiler/options.m.
+% Eventually we ought to change the type checker to use
+% a different algorithm that doesn't have this performance
+% problem, then we can fix flattening, and this test case
+% will then pass.
+
+main -->
+	{ In = 42 },
+	print(func2(In, In)), nl.
+
+:- func func2(int, int) = string.
+:- mode func2(in, in) = out is det.
+:- mode func2(in, out) = out is det.
+:- mode func2(out, in) = out is det.
+:- mode func2(out, out) = out is det.
+func2(_, _) = _ :-
+	error("called func2/2").