Do not convert map patterns to map expressions

In code such as:

  case {a,Map} of
    {a,#{}}=T ->
      T
  end

we must NOT rewrite a map pattern to a map expression like this:

  case Map of
    #{} ->
      {a,#{}}
  end

because the pattern '#{}' will match any map, but the expression
'#{}' will construct an empty map.

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/compiler/test/match_SUITE.erl b/lib/compiler/test/match_SUITE.erl
index e5aaf49d6f..1e778dca24 100644
--- a/lib/compiler/test/match_SUITE.erl
+++ b/lib/compiler/test/match_SUITE.erl
@@ -406,12 +406,19 @@ underscore(Config) when is_list(Config) ->
 match_map(Config) when is_list(Config) ->
     Map = #{key=>{x,y},ignore=>anything},
     #s{map=Map,t={x,y}} = do_match_map(#s{map=Map}),
+    {a,#{k:={a,b,c}}} = do_match_map_2(#{k=>{a,b,c}}),
     ok.
 
 do_match_map(#s{map=#{key:=Val}}=S) ->
     %% Would crash with a 'badarg' exception.
     S#s{t=Val}.
 
+do_match_map_2(Map) ->
+    case {a,Map} of
+	{a,#{k:=_}}=Tuple ->
+	    Tuple
+    end.
+
 coverage(Config) when is_list(Config) ->
     %% Cover beam_dead.
     ok = coverage_1(x, a),