Would be great if this was usable as a git-submodule or via CMakes ExternalProject or FetchContent.

This requires

• a proper target name, e.g. tcspan
• a namespaced alias for that target, e.g. tc::span
• being able to exclude tests

For the latter I'd suggest to remove the enable_testing and use:

if(PROJECT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
  include(CTest)
endif()
if(BUILD_TESTING)
  add_subdirectory(test)
endif()

This include adds an option BUILD_TESTING with default ON and enable_testing call if the current project is not the super project (aka cmake was called for another folder and this folder was added via add_subdirectory)

PS: CMAKE_CURRENT_SOURCE_DIR as used is not required and can be omitted.