Prevent inlining of CamlinternalLazy.force

stdlib/camlinternalLazy.ml

@@ -45,11 +45,12 @@ let force_val_lazy_block (blk : 'arg lazy_t) =
   result
 
 
-(* [force] is not used, since [Lazy.force] is declared as a primitive
-   whose code inlines the tag tests of its argument.  This function is
-   here for the sake of completeness, and for debugging purpose. *)
+(* [force] is not used except when afl instrumentation is on.
+   In this case, it must not be inlined in flambda mode as it can modify
+   its argument in some cases, and is allowed to be called
+   with immutable arguments, which will generate warning 59. *)
 
-let force (lzv : 'arg lazy_t) =
+let[@inline never] force (lzv : 'arg lazy_t) =
   let x = Obj.repr lzv in
   let t = Obj.tag x in
   if t = Obj.forward_tag then (Obj.obj (Obj.field x 0) : 'arg) else

testsuite/tests/flambda/afl_lazy.ml

@@ -0,0 +1,11 @@
+(* TEST
+   * flambda
+   ** native
+   ocamlopt_flags = "-O3 -afl-instrument"
+*)
+
+let f l =
+  Lazy.force l
+
+let _ =
+  Sys.opaque_identity (f (lazy "Hello"))