Fix internal compiler error for map_get/2

Code such as that the following:

    Val = map_get(a, Map),
    Map#{a:=z}   %Could be any map update

would incorrectly cause an internal consistency check failure:

   Internal consistency check failed - please report this bug.
   Instruction: {put_map_exact,{f,0},{x,0},{x,0},1,{list,[{atom,a},{atom,z}]}}
   Error:       {bad_type,{needed,map},{actual,term}}:

Update beam_validator so that it understands that the second
argument for map_get/2 is a map.

2 files changed, 12 insertions, 0 deletions

diff --git a/lib/compiler/src/beam_validator.erl b/lib/compiler/src/beam_validator.erl
index b03fadb197..a1b71251b7 100644
--- a/lib/compiler/src/beam_validator.erl
+++ b/lib/compiler/src/beam_validator.erl
@@ -546,6 +546,12 @@ valfun_4({bif,raise,{f,0},Src,_Dst}, Vst) ->
     kill_state(Vst);
 valfun_4(raw_raise=I, Vst) ->
     call(I, 3, Vst);
+valfun_4({bif,map_get,{f,Fail},[_Key,Map]=Src,Dst}, Vst0) ->
+    validate_src(Src, Vst0),
+    Vst1 = branch_state(Fail, Vst0),
+    Vst = set_type(map, Map, Vst1),
+    Type = propagate_fragility(term, Src, Vst),
+    set_type_reg(Type, Dst, Vst);
 valfun_4({bif,Op,{f,Fail},Src,Dst}, Vst0) ->
     validate_src(Src, Vst0),
     Vst = branch_state(Fail, Vst0),
diff --git a/lib/compiler/test/map_SUITE.erl b/lib/compiler/test/map_SUITE.erl
index 6badc7a8b8..494faf299b 100644
--- a/lib/compiler/test/map_SUITE.erl
+++ b/lib/compiler/test/map_SUITE.erl
@@ -706,6 +706,12 @@ t_map_get(Config) when is_list(Config) ->
     {'EXIT',{{badmap,[]},_}} = (catch map_get(a, [])),
     {'EXIT',{{badmap,<<1,2,3>>},_}} = (catch map_get(a, <<1,2,3>>)),
     {'EXIT',{{badmap,1},_}} = (catch map_get(a, 1)),
+
+    %% Test that beam_validator understands that NewMap is
+    %% a map after seeing map_get(a, NewMap).
+    NewMap = id(#{a=>b}),
+    b = map_get(a, NewMap),
+    #{a:=z} = NewMap#{a:=z},
     ok.
 
 check_map_value(Map, Key, Value) when map_get(Key, Map) =:= Value -> true;