[cmake-developers] add_custom_command() OUTPUT does not accept generator expressions, why?

[Dan Liew]

Hi,

I tried writing a add_custom_command statement that will output the
generated file into the same directory as another target


```
set(OUTPUT_NAME "$<TARGET_FILE_DIR:foo>/MyThing.dll")
add_custom_command(OUTPUT "${OUTPUT_NAME}"
  COMMAND "csc.exe" "/output:$<SHELL_PATH:${OUTPUT_NAME}>" "MySource.cs"
)
```

This doesn't work. CMake emits this error message

```
add_custom_command called with OUTPUT containing a "<".  This
character is not allowed
```

If I can't use generator expressions with ``OUTPUT``, how am I
supposed to output a file into the build directory of a target? It
doesn't look like reading the ``LIBRARY_OUTPUT_DIRECTORY`` property of
a target is really an option as according to the documentation that
itself may contain generator expressions.

Any thoughts would be appreciated.

Thanks,
Dan.