Fix side-effect optimization when compiling from Core Erlang

When an expression is only used for its side effects, we try to
remove everything that doesn't tie into a side-effect, but we
went a bit too far when we applied the optimization to funs
defined in such a context. Consider the following:

 do letrec 'f'/0 = fun () -> ... whatever ...
     in call 'side':'effect'(apply 'f'/0())
   'ok'

When f/0 is optimized under the assumption that its return value
is unused, side:effect/1 will be fed the result of the last
side-effecting expression in f/0 instead of its actual result.

https://bugs.erlang.org/browse/ERL-658

Co-authored-by: Björn Gustavsson <[email protected]>

1 files changed, 22 insertions, 2 deletions

diff --git a/lib/compiler/test/core_fold_SUITE.erl b/lib/compiler/test/core_fold_SUITE.erl
index 2a2369fff9..47606014c3 100644
--- a/lib/compiler/test/core_fold_SUITE.erl
+++ b/lib/compiler/test/core_fold_SUITE.erl
@@ -28,7 +28,7 @@
 	 mixed_matching_clauses/1,unnecessary_building/1,
 	 no_no_file/1,configuration/1,supplies/1,
          redundant_stack_frame/1,export_from_case/1,
-         empty_values/1]).
+         empty_values/1,cover_letrec_effect/1]).
 
 -export([foo/0,foo/1,foo/2,foo/3]).
 
@@ -48,7 +48,7 @@ groups() ->
        mixed_matching_clauses,unnecessary_building,
        no_no_file,configuration,supplies,
        redundant_stack_frame,export_from_case,
-       empty_values]}].
+       empty_values,cover_letrec_effect]}].
 
 
 init_per_suite(Config) ->
@@ -598,5 +598,25 @@ empty_values(_Config) ->
 do_empty_values() when (#{})#{} ->
     c.
 
+cover_letrec_effect(_Config) ->
+    self() ! {tag,42},
+    _ = try
+            try
+                ignore
+            after
+                receive
+                    {tag,Int}=Term ->
+                        Res = #{k => {Term,<<Int:16>>}},
+                        self() ! Res
+                end
+            end
+        after
+            ok
+        end,
+    receive
+        Any ->
+            #{k := {{tag,42},<<42:16>>}} = Any
+    end,
+    ok.
 
 id(I) -> I.