`defined?` as conditional expression in two rubies

gistfile1.rb

# ruby 1.8.7

if defined? THOUGHT           ; defined?(self) ; end  # returns nil
if defined? THOUGHT && THINK  ; defined?(self) ; end  # returns nil
if defined?(THOUGHT) && THINK ; defined?(self) ; end  # returns nil
if defined?(THOUGHT && THINK) ; defined?(self) ; end  # returns nil

defined?(THOUGHT && THINK)  # returns nil


# ruby 1.9.3

if defined? THOUGHT           ; defined?(self) ; end  # returns nil
if defined? THOUGHT && THINK  ; defined?(self) ; end  # returns "self"
if defined?(THOUGHT) && THINK ; defined?(self) ; end  # returns nil
if defined?(THOUGHT && THINK) ; defined?(self) ; end  # returns "self"

defined?(THOUGHT && THINK)  # returns "expression"


# hmm... it occurs to me that i'm being a bit too cute above.
# an example of where this might be a real issue follows.
# from the oauth-plugin lib at https://github.com/pelle/oauth-plugin:
if defined? OAUTH_10_SUPPORT && OAUTH_10_SUPPORT
  # do OAuth 1.0 stuff (maybe not in ruby 1.8, but **always** in 1.9)
end