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-rw-r--r--lib/compiler/src/beam_ssa_type.erl14
-rw-r--r--lib/compiler/src/beam_validator.erl12
-rw-r--r--lib/compiler/test/beam_type_SUITE.erl11
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.