ifdef is always eval'ed

[gaoyong pan]

I don't define variable B for below makefile, but it always gets
printed and eval'ed with below outputs, why I got this expected
behavior?

B is defined
template demo

===============
A := helloA
define change
  ifdef B
    $(info B is defined)
  endif
endef
$(eval $(change))
default:
  $(info template demo)
-- 
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