From b79cde3bb7ddf34b4c1f6411393feec9ef85340a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Mon, 18 Mar 2019 09:41:02 +0100
Subject: beam_validator: Infer types on both sides of '=:='

---
 lib/compiler/src/beam_validator.erl        | 14 +++++++++----
 lib/compiler/test/beam_validator_SUITE.erl | 32 ++++++++++++++++++++++++++++--
 2 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/lib/compiler/src/beam_validator.erl b/lib/compiler/src/beam_validator.erl
index ab8caa1a0d..4fba3fa1c6 100644
--- a/lib/compiler/src/beam_validator.erl
+++ b/lib/compiler/src/beam_validator.erl
@@ -1595,8 +1595,11 @@ infer_types(_, #vst{}) ->
 
 infer_types_1(#value{op={bif,'=:='},args=[LHS,RHS]}) ->
     fun({atom,true}, S) ->
-            Infer = infer_types(RHS, S),
-            Infer(LHS, S);
+            %% Either side might contain something worth inferring, so we need
+            %% to check them both.
+            Infer_L = infer_types(RHS, S),
+            Infer_R = infer_types(LHS, S),
+            Infer_R(RHS, Infer_L(LHS, S));
        (_, S) -> S
     end;
 infer_types_1(#value{op={bif,element},args=[{integer,Index}=Key,Tuple]}) ->
@@ -1772,8 +1775,11 @@ update_ne_types(LHS, RHS, Vst) ->
     end.
 
 update_eq_types(LHS, RHS, Vst0) ->
-    Infer = infer_types(LHS, Vst0),
-    Vst1 = Infer(RHS, Vst0),
+    %% Either side might contain something worth inferring, so we need
+    %% to check them both.
+    Infer_L = infer_types(RHS, Vst0),
+    Infer_R = infer_types(LHS, Vst0),
+    Vst1 = Infer_R(RHS, Infer_L(LHS, Vst0)),
 
     T1 = get_term_type(LHS, Vst1),
     T2 = get_term_type(RHS, Vst1),
diff --git a/lib/compiler/test/beam_validator_SUITE.erl b/lib/compiler/test/beam_validator_SUITE.erl
index 8b39fce479..de5a3c2873 100644
--- a/lib/compiler/test/beam_validator_SUITE.erl
+++ b/lib/compiler/test/beam_validator_SUITE.erl
@@ -34,7 +34,8 @@
 	 undef_label/1,illegal_instruction/1,failing_gc_guard_bif/1,
 	 map_field_lists/1,cover_bin_opt/1,
 	 val_dsetel/1,bad_tuples/1,bad_try_catch_nesting/1,
-         receive_stacked/1,aliased_types/1,type_conflict/1]).
+         receive_stacked/1,aliased_types/1,type_conflict/1,
+         infer_on_eq/1]).
 
 -include_lib("common_test/include/ct.hrl").
 
@@ -63,7 +64,8 @@ groups() ->
        undef_label,illegal_instruction,failing_gc_guard_bif,
        map_field_lists,cover_bin_opt,val_dsetel,
        bad_tuples,bad_try_catch_nesting,
-       receive_stacked,aliased_types,type_conflict]}].
+       receive_stacked,aliased_types,type_conflict,
+       infer_on_eq]}].
 
 init_per_suite(Config) ->
     test_lib:recompile(?MODULE),
@@ -651,6 +653,32 @@ type_conflict_1(C) ->
            end,
     {C#r.e1, TRes}.
 
+%% ERL-886; validation failed to infer types on both sides of '=:='
+
+infer_on_eq(Config) when is_list(Config) ->
+    {ok, gurka} = infer_on_eq_1(id({gurka})),
+    {ok, gaffel} = infer_on_eq_2(id({gaffel})),
+    {ok, elefant} = infer_on_eq_3(id({elefant})),
+    {ok, myra} = infer_on_eq_4(id({myra})),
+    ok.
+
+infer_on_eq_1(T) ->
+    1 = erlang:tuple_size(T),
+    {ok, erlang:element(1, T)}.
+
+infer_on_eq_2(T) ->
+    Size = erlang:tuple_size(T),
+    Size = 1,
+    {ok, erlang:element(1, T)}.
+
+infer_on_eq_3(T) ->
+    true = 1 =:= erlang:tuple_size(T),
+    {ok, erlang:element(1, T)}.
+
+infer_on_eq_4(T) ->
+    true = erlang:tuple_size(T) =:= 1,
+    {ok, erlang:element(1, T)}.
+
 %%%-------------------------------------------------------------------------
 
 transform_remove(Remove, Module) ->
-- 
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