Fix ARGF#lineno to return Integer as documented

[Bug #18753]

1 files changed, 1 insertions, 1 deletions

diff --git a/io.c b/io.c
index 3fe1f6eb16..3b092e5dce 100644
--- a/io.c
+++ b/io.c
@@ -9348,7 +9348,7 @@ argf_set_lineno(VALUE argf, VALUE val)
 {
     ARGF.lineno = NUM2INT(val);
     ARGF.last_lineno = ARGF.lineno;
-    return Qnil;
+    return val;
 }
 
 /*