General ⇒ latex: -output-directory affects input paths for \open?

[fomin]

Hello all.

I have to check existence of file on given path.

Please, consider small example.

Two files:
dummy.tex (empty)
main.tex

main.tex:

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\documentclass{article}

\makeatletter
\def\IsFileExists#1{%   Simple macro to check file existence
  \openin\@inputcheck#1 %
  \ifeof\@inputcheck
     \message{File #1 -- not exists. 
     }
  \else
     \message{File #1 -- exists. 
     }
  \fi
  \closein\@inputcheck}
\makeatother

\begin{document}
\IsFileExists{dummy.tex}      
\IsFileExists{../dummy.tex}   
\IsFileExists{../../dummy.tex} 
\end{document}

If we run

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latex main.tex  

all OK, we have

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File dummy.tex -- exists.  File ../dummy.tex -- not exists. 
File ../../dummy.tex -- not exists.  (./main.aux) ) 

But if we run (for example, to hide all crap)

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latex -output-directory="subdir" main.tex

we have

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File dummy.tex -- exists.  ***** File ../dummy.tex -- exists. ***** WTF???? 
File ../../dummy.tex -- not exists.  (subdir/main.aux) ) 

Is it a bug or a feature?

How can I do robust check of file existence, not depending from "-output-directory" parameter?

Thanks all in advance,
Sincerely, Stas Fomin

P.S. I use Windows TexLive distrib. Sample files attached.