Why does this cmake initial cache file result in s

I have a file /home/me/.config/Kitware/CMakeCache.txt that contains some initial cache settings I'd like to use everywhere. I run cmake -C /home/me/.config/Kitware/CMakeCache.txt and it gives me a parse error on line 3 with this file:

set(CMAKE_CXX_FLAGS "-march=native -mtune=native" CACHE STRING "Flags used by the compiler during all build types.")

set(CMAKE_CXX_FLAGS_DEBUG "-Og -ggdb" CACHE STRING "Flags used by the compiler during debug builds.")

This is the error I get:

CMake Error: Parse error in cache file /home/me/.config/Kitware/CMakeCache.txt on line 3. Offending entry: set(CMAKE_CXX_FLAGS_DEBUG "-Og -ggdb" CACHE STRING "Flags used by the compiler during debug builds.")

But, if I change the line (adding =) to:

set(CMAKE_CXX_FLAGS_DEBUG "=-Og -ggdb" CACHE STRING "Flags used by the compiler during debug builds.")

it works fine, but then the resulting cache file in my project has the option set to =-Og -ggdb. In fact, the = sign can appear anywhere in the entire set statement. But it results in different problems no matter where you put it.

If I rename the file to /home/me/.config/Kitware/default.cmake (or anything else besides CMakeCache.txt it also works fine and doesn't require an = sign in the value for every option.

This is very weird behavior, and I don't understand it. What's going on here?

Turning my comments into an answer

See cmake.cxx: Anything in the command line that has the name cmakelists.txt or cmakecache.txt is taken as the path to those files.

So CMake is ignoring your -C option here and tries to load your CMakeCache.txt as an actual variable cache file. And those files have a different formatting/syntax of NAME:TYPE=VALUE.

You can consider this behavior a bug in CMake. Or you just avoid using CMakeCache.txt for your initial cache file name.