I don't like this behaviour either (the same for the headers checks), but my understanding is that this is basically cloning the autotools behaviour, thus probably making the logic easier to maintain.

If the code breaks the build, then what's the proposed fix? The configure build seems to work so if so this was an incomplete "mimic"...

I doubt that trying to "clone behavior" is a very good idea long term, as the two systems are very different, using different paradigms and they are also often maintained and updated by a different set of people.

Well, it's like that since I can remember.
In fact, curl's CMake build even reads some Makefile.am files in to extract some information from there.

Maybe, just maybe, re-writing it from scratch would make sense (there are benefits and downsides of doing so).

It is not really "Makefile.am files" though. They are files we share between the build systems to avoid having to maintain two separate lists of the same things.

May very well be - I know nothing about autotools - and it makes sense to have single source of truth like that.

I understand that my first suggestion of removing the CURL_LIBS has an impact I don't understand entirely.

The real issue after all is that this list does not only contains libraries but also CMake targets. Would removing targets from the list before calling check_library_exists be a valid option to you ?

We have encountered this same issue. We don't use FetchContent but we use the same ZLIB::ZLIB CMake alias.

If you guys are looking to continue to emulate autotools, here is a possible solution that removes targets from the list before performing check_library_exists.

macro(check_library_exists_concat LIBRARY SYMBOL VARIABLE)
  # Enumerate CURL_LIBS and add each library to CMAKE_REQUIRED_LIBRARIES if it
  # is not a CMake target. System libraries should not depend on CMake targets.
  set(CMAKE_REQUIRED_LIBRARIES)
  foreach(LIB ${CURL_LIBS})
    if (NOT TARGET ${LIB})
      set(CMAKE_REQUIRED_LIBRARIES ${LIB} ${CMAKE_REQUIRED_LIBRARIES})
    endif()
  endforeach()
  # Using CMake targets in the call to check_library_exists() may cause it to fail
  # unless the target is IMPORTED.  
  check_library_exists("${LIBRARY}" ${SYMBOL} "${CMAKE_LIBRARY_PATH}"
    ${VARIABLE})
  set(CMAKE_REQUIRED_LIBRARIES)
  if(${VARIABLE})
    set(CURL_LIBS ${LIBRARY} ${CURL_LIBS})
  endif()
endmacro()

The comment for the check_library_exists_concat macro mentions:

# Some libraries depend on others to link correctly.

I don't doubt the claim, but I'm not aware of a specific instance where this is the case. However, if you don't want to continue to emulate autotools then this might be a simpler solution:

macro(check_library_exists_concat LIBRARY SYMBOL VARIABLE)
  check_library_exists("${LIBRARY}" ${SYMBOL} "${CMAKE_LIBRARY_PATH}"
    ${VARIABLE})
  if(${VARIABLE})
    set(CURL_LIBS ${LIBRARY} ${CURL_LIBS})
  endif()
endmacro()

@bagder I can submit a PR if you have a specific way you want to go.