Issue #1250 has been updated by tadayoshi funaba.
It's not a bug.
The parser accepts float form as numerator.
However, denominator is not.
main@191-20090130> Rational('1.5')
#=> (3/2)
main@191-20090130> Rational('1.5/1')
#=> (3/2)
main@191-20090130> Rational('1.5/2')
#=> (3/4)
main@191-20090130> Rational('1.5/2.5')
ArgumentError: invalid value for Rational: "1.5/2.5"
from (irb):4:in `convert'
from (irb):4:in `Rational'
from (irb):4
from /usr/local/bin/irb191:12:in `<main>'
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http://redmine.ruby-lang.org/issues/show/1250
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http://redmine.ruby-lang.org