beam_call_types: Fix incorrect typing for lists:mapfold[lr]/3

lib/compiler/src/beam_call_types.erl

@@ -1186,18 +1186,22 @@ lists_map_type_1(_, ElementType) ->
 
 lists_mapfold_type(Fun, Init, List) ->
     case {meet(Fun, #t_fun{type=#t_tuple{size=2}}), meet(List, #t_list{})} of
-        {_, nil} ->
-            make_two_tuple(nil, Init);
-        {#t_fun{type=#t_tuple{elements=Es}}, ListType} ->
+        {#t_fun{type=T}, ListType} when T =/= none ->
+            #t_tuple{elements=Es} = normalize(T),
             ElementType = beam_types:get_tuple_element(1, Es),
             AccType = beam_types:get_tuple_element(2, Es),
             lists_mapfold_type_1(ListType, ElementType, Init, AccType);
-        {#t_fun{type=none}, #t_list{}} ->
-            %% The fun never returns, so the only way we could return normally
-            %% is if the list is empty, in which case we'll return [] and the
-            %% initial value.
+        {_, #t_list{}} ->
+            %% The fun never returns, or is not a fun. The only way this can
+            %% succeed is if the given list is empty, in which case we'll
+            %% return [] and the initial value.
             make_two_tuple(nil, Init);
-        _ ->
+        {_, nil} ->
+            make_two_tuple(nil, Init);
+        {_, _} ->
+            %% The fun never returns, or is not a fun, and the list is either
+            %% not a list or is guaranteed not to be empty. This will never
+            %% return.
             none
     end.
 

lib/compiler/test/beam_type_SUITE.erl

@@ -25,7 +25,8 @@
          cons/1,tuple/1,
          record_float/1,binary_float/1,float_compare/1,float_overflow/1,
 	 arity_checks/1,elixir_binaries/1,find_best/1,
-         test_size/1,cover_lists_functions/1,list_append/1,bad_binary_unit/1,
+         test_size/1,cover_lists_functions/1,list_append/1,lists_mapfold/1,
+         bad_binary_unit/1,
          none_argument/1,success_type_oscillation/1,type_subtraction/1,
          container_subtraction/1,is_list_opt/1,connected_tuple_elements/1,
          switch_fail_inference/1,failures/1,
@@ -60,6 +61,7 @@ groups() ->
        test_size,
        cover_lists_functions,
        list_append,
+       lists_mapfold,
        bad_binary_unit,
        none_argument,
        success_type_oscillation,
@@ -978,6 +980,27 @@ list_append(_Config) ->
     hello = id([]) ++ id(hello),
     ok.
 
+%% GH-10354: Type inference broke when the fun passed to mapfoldl/mapfoldr
+%% returned a union of 2-tuples.
+lists_mapfold(_Config) ->
+    expected_result = id(lists_mapfold_1()),
+    ok.
+
+lists_mapfold_1() ->
+    List = [{key,[{number,1}]}],
+    {_, FinalAcc} =
+        lists:mapfoldl(
+            fun({_, PropListItem}, Acc) ->
+                Number = proplists:get_value(number, PropListItem),
+                case Number > 0 of
+                    true ->
+                        {false, Acc ++ [expected_result]};
+                    _ ->
+                        {true, Acc}
+                end
+            end, [], List),
+    hd(FinalAcc).
+
 %% OTP-15872: The compiler would treat the "Unit" of bs_init instructions as
 %% the unit of the result instead of the required unit of the input, causing
 %% is_binary checks to be wrongly optimized away.