MRuby instance-eval is applied to the wrong object

[fundamental]

When upgrading from commit 36fc1f1 to 3703aed a regression is observed in the behavior of instance_eval. In this period I see 6 commits to eval.c:

fca17b4 Avoid trampoline when #eval is called from mrb_funcall(); fix #3522
f8b31a0 `instance_eval` method does not introduce C function boundary; fix #3508
73e5a19 Configure callinfo target_class as CRuby; ref #3429
097da23 Avoid executing OP_STOP in eval(); fix #3429
5e3c329 Adjust callinfo env and target_class; ref #3429
48e0bbb Make `eval` to use trampoline technique; fix #3415

Presumably one of these commits is setting the target class for instance eval incorrectly. The test case used to observe the behavior is shown below:

class A
    def method
        :a
    end
end

class B
    def initialize()
        @a = A.new
        @a.instance_eval "def method;:a_eval;end"
        nil
    end

    attr_reader :a

    def method
        :b
    end
end

a = A.new
b = B.new

puts "Expected unmodified method value ':a'.      Got '#{a.method}'"
puts "Expected unmodified method value ':b'.      Got '#{b.method}'"
puts "Expected   modified method value ':a_eval'. Got '#{b.a.method}'"

Behavior observed with mruby and CRuby (2.2.3p173) show the bug:

mark@darkstar:~/mytmp/mruby$ ruby instance_eval_test.rb
Expected unmodified method value ':a'.      Got 'a'
Expected unmodified method value ':b'.      Got 'b'
Expected   modified method value ':a_eval'. Got 'a_eval'
mark@darkstar:~/mytmp/mruby$ ./bin/mruby instance_eval_test.rb
Expected unmodified method value ':a'.      Got 'a'
Expected unmodified method value ':b'.      Got 'a_eval'
Expected   modified method value ':a_eval'. Got 'a'

[fundamental]

Some quick printf() based debugging latter it looks like the RProc's target_class field might be being ignored in the mrb_exec_irep() function in vm.c.