From af617aebe254da636b7d9c2ac59e1109998b0b1c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Wed, 12 Jun 2019 09:35:03 +0200
Subject: beam_call_types: Improve type handling of lists:zip/2 and friends

---
 lib/compiler/src/beam_call_types.erl | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/lib/compiler/src/beam_call_types.erl b/lib/compiler/src/beam_call_types.erl
index 45fbead804..b1ff200fb6 100644
--- a/lib/compiler/src/beam_call_types.erl
+++ b/lib/compiler/src/beam_call_types.erl
@@ -340,16 +340,17 @@ types(lists, takewhile, [_,_]) ->
 types(lists, usort, [List]) ->
     sub_unsafe(same_length_type(List), [list]);
 types(lists, zip, [A,B]) ->
-    RetType = beam_types:join(same_length_type(A), same_length_type(B)),
-    sub_unsafe(RetType, [list, list]);
+    ZipType = lists_zip_type([A,B]),
+    sub_unsafe(ZipType, [ZipType, ZipType]);
+types(lists, zip3, [A,B,C]) ->
+    ZipType = lists_zip_type([A,B,C]),
+    sub_unsafe(ZipType, [ZipType, ZipType, ZipType]);
 types(lists, zipwith, [_,A,B]) ->
-    RetType = beam_types:join(same_length_type(A), same_length_type(B)),
-    sub_unsafe(RetType, [#t_fun{arity=2}, list, list]);
+    ZipType = lists_zip_type([A,B]),
+    sub_unsafe(ZipType, [#t_fun{arity=2}, ZipType, ZipType]);
 types(lists, zipwith3, [_,A,B,C]) ->
-    RetType = beam_types:join([same_length_type(A),
-                               same_length_type(B),
-                               same_length_type(C)]),
-    sub_unsafe(RetType, [#t_fun{arity=3}, list, list, list]);
+    ZipType = lists_zip_type([A,B,C]),
+    sub_unsafe(ZipType, [#t_fun{arity=3}, ZipType, ZipType, ZipType]);
 
 %% Functions with complex return values.
 types(lists, partition, [_,_]) ->
@@ -455,12 +456,22 @@ discard_tuple_element_info(Min, Max, Es) ->
              (_El, Acc) -> Acc
           end, Es, maps:keys(Es)).
 
-%% For a lists function that return a list of the same
-%% length as the input list, return the type of the list.
+%% For a lists function that return a list of the same length as the input
+%% list, return the type of the list.
 same_length_type(cons) -> cons;
 same_length_type(nil) -> nil;
 same_length_type(_) -> list.
 
+%% lists:zip/2 and friends only succeed when all arguments have the same
+%% length, so if one of them is cons, we can infer that all of them are cons
+%% on success.
+lists_zip_type(Types) ->
+    foldl(fun(cons, _) -> cons;
+             (_, cons) -> cons;
+             (nil, _) -> nil;
+             (_, T) -> T
+          end, list, Types).
+
 make_two_tuple(Type1, Type2) ->
     #t_tuple{size=2,exact=true,
              elements=#{1=>Type1,2=>Type2}}.
-- 
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