This issue was solved with changeset r39918.
Tomoyuki, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
merge revision(s) 39677,39695: [Backport #8068]
* complex.c: described syntax of string form.
* rational.c: ditto.
* complex.c: edited rdoc.
* rational.c: ditto.
Updated by