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, 25 insertions, 0 deletions

diff --git a/lib/compiler/test/core_SUITE_data/fun_letrec_effect.core b/lib/compiler/test/core_SUITE_data/fun_letrec_effect.core
new file mode 100644
index 0000000000..ab6f5b7940
--- /dev/null
+++ b/lib/compiler/test/core_SUITE_data/fun_letrec_effect.core
@@ -0,0 +1,25 @@
+module 'fun_letrec_effect' ['fun_letrec_effect'/0, 'ok'/0, 'wat'/0]
+attributes []
+
+'fun_letrec_effect'/0 =
+  fun () ->
+    do apply 'wat'/0()
+       receive
+         <'bar'> when 'true' -> 'ok'
+         <_0> when 'true' -> 'failed'
+       after 'infinity' ->
+         'true'
+
+%% The return value (bar) of the fun was optimized away because the result of
+%% the `letrec ... in` was unused, despite the fun's return value being
+%% relevant for the side-effect of the expression.
+'wat'/0 =
+  fun () ->
+    let <Self> = call 'erlang':'self'() in
+      do letrec 'f'/0 = fun () ->
+                                do  call 'maps':'put'('foo', 'bar', ~{}~)
+                                    'bar'
+          in call 'erlang':'send'(Self, apply 'f'/0())
+          'undefined'
+
+end