compiler: Fix broken type for erlang:'++'/2

3 files changed, 26 insertions, 11 deletions

diff --git a/lib/compiler/src/beam_ssa_type.erl b/lib/compiler/src/beam_ssa_type.erl
index 06b42f1928..fe229d25df 100644
--- a/lib/compiler/src/beam_ssa_type.erl
+++ b/lib/compiler/src/beam_ssa_type.erl
@@ -896,11 +896,15 @@ type(call, [#b_remote{mod=#b_literal{val=Mod},
                 {_,_} ->
                     #t_tuple{}
             end;
-        {erlang,'++',[List1,List2]} ->
-            case get_type(List1, Ts) =:= cons orelse
-                get_type(List2, Ts) =:= cons of
-                true -> cons;
-                false -> list
+        {erlang,'++',[LHS,RHS]} ->
+            LType = get_type(LHS, Ts),
+            RType = get_type(RHS, Ts),
+            case LType =:= cons orelse RType =:= cons of
+                true ->
+                    cons;
+                false ->
+                    %% `[] ++ RHS` yields RHS, even if RHS is not a list.
+                    join(list, RType)
             end;
         {erlang,'--',[_,_]} ->
             list;
diff --git a/lib/compiler/src/beam_validator.erl b/lib/compiler/src/beam_validator.erl
index 09a5a6c104..ebe9631e09 100644
--- a/lib/compiler/src/beam_validator.erl
+++ b/lib/compiler/src/beam_validator.erl
@@ -2844,10 +2844,14 @@ call_return_type_1(erlang, setelement, 3, Vst) ->
             setelement(3, TupleType, #{})
     end;
 call_return_type_1(erlang, '++', 2, Vst) ->
-    case get_term_type({x,0}, Vst) =:= cons orelse
-        get_term_type({x,1}, Vst) =:= cons of
-        true -> cons;
-        false -> list
+    LType = get_term_type({x,0}, Vst),
+    RType = get_term_type({x,1}, Vst),
+    case LType =:= cons orelse RType =:= cons of
+        true ->
+            cons;
+        false ->
+            %% `[] ++ RHS` yields RHS, even if RHS is not a list
+            join(list, RType)
     end;
 call_return_type_1(erlang, '--', 2, _Vst) ->
     list;
diff --git a/lib/compiler/test/beam_type_SUITE.erl b/lib/compiler/test/beam_type_SUITE.erl
index 882e281a44..0ddd91498e 100644
--- a/lib/compiler/test/beam_type_SUITE.erl
+++ b/lib/compiler/test/beam_type_SUITE.erl
@@ -24,7 +24,7 @@
 	 integers/1,numbers/1,coverage/1,booleans/1,setelement/1,
 	 cons/1,tuple/1,record_float/1,binary_float/1,float_compare/1,
 	 arity_checks/1,elixir_binaries/1,find_best/1,
-         test_size/1,cover_lists_functions/1]).
+         test_size/1,cover_lists_functions/1,list_append/1]).
 
 suite() -> [{ct_hooks,[ts_install_cth]}].
 
@@ -47,7 +47,8 @@ groups() ->
        elixir_binaries,
        find_best,
        test_size,
-       cover_lists_functions
+       cover_lists_functions,
+       list_append
       ]}].
 
 init_per_suite(Config) ->
@@ -487,5 +488,11 @@ cover_lists_functions(Config) ->
     true = is_list(Zipped),
     ok.
 
+list_append(_Config) ->
+    %% '++'/2 has a quirk where it returns the right-hand argument as-is when
+    %% the left-hand is [].
+    hello = id([]) ++ id(hello),
+    ok.
+
 id(I) ->
     I.