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

2 files changed, 17 insertions, 9 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;