Evaluate element/2 at compile-time using type information

The erl_expand_records compiler pass translates the
following code:

  h(X) when X#r1.a =:= 1 -> ok.

to (essentially):

  h({r1,V1,V2}=X) when element(2, X) =:= 1 -> ok.

Since the guard can only be executed when the pattern matching
has succeeded, we know that the second element in the tuple X
must have been bound to V2. Thus we can eliminate the call
to element/2 like this:

  h({r1,V1,V2}=X) when V1 =:= 1 -> ok.

2 files changed, 19 insertions, 14 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 6202f07479..3e572d4b27 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -1194,19 +1194,22 @@ eval_element(Call, #c_literal{val=Pos}, #c_tuple{es=Es}, _Types) when is_integer
 	true ->
 	    eval_failure(Call, badarg)
     end;
-%% eval_element(Call, #c_literal{val=Pos}, #c_var{name=V}, Types)
-%%   when is_integer(Pos) ->
-%%     case orddict:find(V, Types#sub.t) of
-%% 	{ok,#c_tuple{es=Elements}} ->
-%% 	    if
-%% 		1 =< Pos, Pos =< length(Elements) ->
-%% 		    lists:nth(Pos, Elements);
-%% 		true ->
-%% 		    eval_failure(Call, badarg)
-%% 	    end;
-%% 	error ->
-%% 	    Call
-%%     end;
+eval_element(Call, #c_literal{val=Pos}, #c_var{name=V}, Types)
+  when is_integer(Pos) ->
+    case orddict:find(V, Types#sub.t) of
+	{ok,#c_tuple{es=Elements}} ->
+	    if
+		1 =< Pos, Pos =< length(Elements) ->
+		    case lists:nth(Pos, Elements) of
+			#c_alias{var=Alias} -> Alias;
+			Res -> Res
+		    end;
+		true ->
+		    eval_failure(Call, badarg)
+	    end;
+	error ->
+	    Call
+    end;
 eval_element(Call, Pos, Tuple, _Types) ->
     case is_not_integer(Pos) orelse is_not_tuple(Tuple) of
 	true ->
diff --git a/lib/stdlib/test/qlc_SUITE.erl b/lib/stdlib/test/qlc_SUITE.erl
index aa12ed57da..e21de8770a 100644
--- a/lib/stdlib/test/qlc_SUITE.erl
+++ b/lib/stdlib/test/qlc_SUITE.erl
@@ -3184,7 +3184,9 @@ lookup2(Config) when is_list(Config) ->
                  [] = qlc:e(Q),
                  false = lookup_keys(Q)
          end, [{1,b},{2,3}])">>,
-        {warnings,[{{3,48},qlc,nomatch_filter}]}},
+        {warnings,[{2,sys_core_fold,nomatch_guard},
+		   {3,qlc,nomatch_filter},
+		   {3,sys_core_fold,{eval_failure,badarg}}]}},
 
        <<"etsc(fun(E) ->
                 Q = qlc:q([X || {X} <- ets:table(E), element(1,{X}) =:= 1]),