sys_core_fold: Fix non-tail-recursive list comprehensions

649d6e73 simplified opt_simple_let_2/6 a little bit too much,
so that some list comprehensions in effect context were not
properly tail-recursive.

2 files changed, 22 insertions, 4 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 7d1819ea15..2618f7adba 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -2288,10 +2288,17 @@ opt_simple_let_2(Let0, Vs0, Arg0, Body, Ctxt, Sub) ->
 	    %% No variables left.
 	    Body;
 	{_,Arg,#c_literal{}} ->
-	    %% Since the variable is not used in the body, we can rewrite the
-	    %% let to a sequence:
-	    %%   let <Var> = Arg in Literal  ==>  seq Arg Literal
-	    expr(#c_seq{arg=Arg,body=Body}, Ctxt, sub_new_preserve_types(Sub));
+	    E = case Ctxt of
+		    effect ->
+			%% Throw away the literal body.
+			Arg;
+		    value ->
+			%% Since the variable is not used in the body, we
+			%% can rewrite the let to a sequence.
+			%%  let <Var> = Arg in Literal ==> seq Arg Literal
+			#c_seq{arg=Arg,body=Body}
+		end,
+	    expr(E, Ctxt, sub_new_preserve_types(Sub));
 	{Vs,Arg,Body} ->
 	    %% If none of the variables are used in the body, we can
 	    %% rewrite the let to a sequence:
diff --git a/lib/compiler/test/lc_SUITE.erl b/lib/compiler/test/lc_SUITE.erl
index 6c5b34498b..62bada1407 100644
--- a/lib/compiler/test/lc_SUITE.erl
+++ b/lib/compiler/test/lc_SUITE.erl
@@ -208,6 +208,17 @@ effect(Config) when is_list(Config) ->
 			      #{<<1:500>>:=V1,<<2:301>>:=V2} <- L],
 			  ok
 		  end, id([#{},x,#{<<1:500>>=>42,<<2:301>>=>{a,b,c}}])),
+
+    %% Will trigger the time-trap timeout if not tail-recursive.
+    case ?MODULE of
+	lc_SUITE ->
+	    _ = [{'EXIT',{badarg,_}} =
+		     (catch binary_to_atom(<<C/utf8>>, utf8)) ||
+		    C <- lists:seq(16#10000, 16#FFFFF)];
+	_ ->
+	    ok
+    end,
+
     ok.
 
 do_effect(Lc, L) ->