[CMake] Problem piping 'execute_process' output

[J.S. van Bethlehem]

Hello,

Since I'm only just starting to learn how to use CMake I'm not sure 
whether I found a bug, or whether I'm misunderstanding something. So I 
figured I could just as well see what others have to say about it.
I managed to create a minimalistic example using CMake 2.8.4:

Suppose I have the following simple code (say source.cc):

#include <iostream>
using namespace std;

int main() {
  cout << "Some info to std::cout\n";
  cerr << "Some info to std::cerr\n";

  return 0;
}

Somewhere along the line of building my project I want to run this 
source and pipe the output to 'source.out'. For this example I'll 
manually build the executable source.cc -> source.out and write the 
names of all files explicitly in the following one-line CMake-file (say 
RunCommand.cmake):

# - run 'source' and pipe output to source.out
execute_process(COMMAND ./source OUTPUT_FILE source.out ERROR_FILE 
source.out)

In reality I'd call this script through '${CMAKE_COMMAND} -P 
${CMAKE_MODULE_PATH}/RunCommand.cmake' from some CMakeLists.txt file 
with variables storing the binary and output through a set of -D 
options. But now I do:
cmake -P RunCommand.cmake

I would expect to see both lines in 'source.out'. But in reality only 
the second line, that went to the error stream is in the outputfile.
Interestingly, if I change the ordering of the two lines in source.cc, 
again only the second line appears (now the 'cout' line) Only if I send 
both lines to the same stream, either cout or cerr, I get both lines in 
the outputfile.

Am I trying do something that is not possible (I think not, given that 
the manual explicitly states that OUTPUT_* and ERROR_* may be the same 
and that results will be merged), do I need to add some more syntax, or 
did I stumble across a genuine bug?

Yours sincerely,
Jakob van Bethlehem