[CMake] On Darwin the LIBCXX_INSTALL_HEADERS and LIBCXX_INSTALL_LIBRARY options should default off

[beanz]

Summary

The current implementation doesn't work as intended because of CMake's caching behavior. The CMake set command doesn't globally override cached options.

This change removes LIBCXX_OVERRIDE_DARWIN_INSTALL, and instead defaults the options OFF on Darwin. The end result is the same, if you want to install the libraries or headers on Darwin you need to explicitly set the options.

[beanz]

My patch dropped one of the gating checks. This fixes that. It only needs to apply to Darwin when installing to /usr.

[EricWF]

The current implementation doesn't work as intended because of CMake's caching behavior. The CMake set command doesn't globally override cached options.

Sort of. It doesn't override the value in the cache but it should mean that the cache value is overridden by the newly set non-cache value. If I am correct the following code will print "VAR = OFF".

option(VAR "description" ON)
set(VAR OFF)
message(STATUS "VAR = ${VAR}")

Could you give me an example of where this breaks? It won't override the values in the cache but it should work correctly.

[beanz]

Short answer. I was doing something terrible and wrong in out-of-tree code, and this was getting tripped up. I was reading the LIBCXX_INSTALL_* variables from a higher-level CMake file. I've stopped doing that (because it was stupid), so I'll abandon this patch.

For context the CMake behavior I was hitting was this:

CMakeLists.txt:

add_subdirectory(foo)
message(STATUS "VAR = ${VAR}") # prints On

foo/CMakeLists.txt:

option(VAR "description" ON)
set(VAR OFF)
message(STATUS "VAR = ${VAR}") # prints OFF

Overriding a cached variable in CMake only overrides it in the current scope and lower.

CMakeLists.txt

 # Basic options ---------------------------------------------------------------
 option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 
 option(LIBCXX_INCLUDE_TESTS "Build the libc++ tests." ${LLVM_INCLUDE_TESTS})
 option(LIBCXX_INCLUDE_DOCS "Build the libc++ documentation." ${LLVM_INCLUDE_DOCS})
 set(LIBCXX_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")
-option(LIBCXX_INSTALL_HEADERS "Install the libc++ headers." ON)
-option(LIBCXX_INSTALL_LIBRARY "Install the libc++ library." ON)
+
+set(INSTALL_HEADERS_DEFAULT ON)
+set(INSTALL_LIBRARY_DEFAULT ON)
+if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin"
+  AND "${CMAKE_INSTALL_PREFIX}" STREQUAL "/usr")
+  set(INSTALL_HEADERS_DEFAULT OFF)
+  set(INSTALL_LIBRARY_DEFAULT OFF)
+endif()
+option(LIBCXX_INSTALL_HEADERS "Install the libc++ headers." ${INSTALL_HEADERS_DEFAULT})
+option(LIBCXX_INSTALL_LIBRARY "Install the libc++ library." ${INSTALL_LIBRARY_DEFAULT})
+
 option(LIBCXX_INSTALL_SUPPORT_HEADERS "Install libc++ support headers." ON)
 set(LIBCXX_ABI_VERSION 1 CACHE STRING "ABI version of libc++.")
 option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
 
 # ABI Library options ---------------------------------------------------------
 set(LIBCXX_CXX_ABI "${LIBCXX_CXX_ABI}" CACHE STRING
     "Specify C++ ABI library to use." FORCE)
 set(CXXABIS none libcxxabi libcxxrt libstdc++ libsupc++)

 # about #include_next which is used everywhere.
 option(LIBCXX_ENABLE_PEDANTIC "Compile with pedantic enabled." OFF)
 option(LIBCXX_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 
 option(LIBCXX_GENERATE_COVERAGE "Enable generating code coverage." OFF)
 set(LIBCXX_COVERAGE_LIBRARY "" CACHE STRING
     "The Profile-rt library used to build with code coverage")
 
-# Don't allow a user to accidentally overwrite the system libc++ installation on Darwin.
-# If the user specifies -DCMAKE_INSTALL_PREFIX=/usr the install rules for libc++
-# will not be generated and a warning will be issued.
-option(LIBCXX_OVERRIDE_DARWIN_INSTALL "Enable overwriting darwins libc++ installation." OFF)
-mark_as_advanced(LIBCXX_OVERRIDE_DARWIN_INSTALL) # Don't show this option by default.
-
-if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin" AND NOT LIBCXX_OVERRIDE_DARWIN_INSTALL)
-  if ("${CMAKE_INSTALL_PREFIX}" STREQUAL "/usr")
-    message(WARNING "Disabling libc++ install rules because installation would "
-                    "overwrite the systems installation. Configure with "
-                    "-DLIBCXX_OVERRIDE_DARWIN_INSTALL=ON to suppress this behaviour.")
-    mark_as_advanced(CLEAR LIBCXX_OVERRIDE_DARWIN_INSTALL) # Show the override option.
-    set(LIBCXX_INSTALL_HEADERS OFF)
-    set(LIBCXX_INSTALL_LIBRARY OFF)
-  endif()
-endif()
-
 #===============================================================================
 # Check option configurations
 #===============================================================================
 
 # Ensure LIBCXX_ENABLE_MONOTONIC_CLOCK is set to ON only when
 # LIBCXX_ENABLE_THREADS is on.
 if(LIBCXX_ENABLE_THREADS AND NOT LIBCXX_ENABLE_MONOTONIC_CLOCK)
   message(FATAL_ERROR "LIBCXX_ENABLE_MONOTONIC_CLOCK can only be set to OFF"