How can I have CMake compile the same input file in two different languages?

I have a file named foo.bar. I want to compile it once as a C++ file, into a mycpplib library target, and once as a C file, into a myclib target; and I want to do it in the same build, with the same CMakeLists.txt.

Now, I know I can arbitrarily set a source file's associated language, like so:

set_source_files_properties(foo.bar PROPERTIES LANGUAGE C)

but this doesn't look like it'll help in my case. Is there something I can do at the CMake level?

Notes:

• Related question: The single-arbitrary-language case.
• There are non-CMake solutions to this, e.g. duplicating the file; using a symlink with a different name; having a file with #include "otherfile" as its contents etc.

You could create library targets mycpplib and myclib in the different directories (in the different CMakeLists.txt). That way you may call set_source_files_properties in the directory where mycpplib library is created, and that call won't affect on myclib.

There are also DIRECTORY and TARGET_DIRECTORY options for command set_source_files_properties, which could affect on the directory where the property will be visible:

# In 'c/CMakeLists.txt`
# add_library(myclib ${CMAKE_SOURCE_DIR}/foo.bar)

# In 'cpp/CMakeLists.txt`
# add_library(mycpplib ${CMAKE_SOURCE_DIR}/foo.bar)

# In CMakeLists.txt
add_subdirectory(c)
add_subdirectory(cpp)
set_source_file_properties(foo.bar TARGET_DIRECTORY myclib
  PROPERTIES LANGUAGE C)
set_source_file_properties(foo.bar TARGET_DIRECTORY mycpplib
  PROPERTIES LANGUAGE CXX)