add_library(): Allow imported object libraries with multi-arch

Fixes: #21276

Help/prop_tgt/IMPORTED_OBJECTS.rst

@@ -3,11 +3,91 @@ IMPORTED_OBJECTS
 
 .. versionadded:: 3.9
 
-A :ref:`semicolon-separated list <CMake Language Lists>` of absolute paths to the object
-files on disk for an :ref:`imported <Imported targets>`
+A :ref:`semicolon-separated list <CMake Language Lists>` of absolute paths
+to the object files on disk for an :ref:`imported <Imported targets>`
 :ref:`object library <object libraries>`.
 
 Ignored for non-imported targets.
 
 Projects may skip ``IMPORTED_OBJECTS`` if the configuration-specific
-property :prop_tgt:`IMPORTED_OBJECTS_<CONFIG>` is set instead.
+property :prop_tgt:`IMPORTED_OBJECTS_<CONFIG>` is set instead, except in
+situations as noted in the section below.
+
+
+Xcode Generator Considerations
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. versionadded:: 3.20
+
+For Apple platforms, a project may be built for more than one architecture.
+This is controlled by the :variable:`CMAKE_OSX_ARCHITECTURES` variable.
+For all but the :generator:`Xcode` generator, CMake invokes compilers once
+per source file and passes multiple ``-arch`` flags, leading to a single
+object file which will be a universal binary.  Such object files work well
+when listed in the ``IMPORTED_OBJECTS`` of a separate CMake build, even for
+the :generator:`Xcode` generator.  But producing such object files with the
+:generator:`Xcode` generator is more difficult, since it invokes the compiler
+once per architecture for each source file.  Unlike the other generators,
+it does not generate universal object file binaries.
+
+A further complication with the :generator:`Xcode` generator is that when
+targeting device platforms (iOS, tvOS or watchOS), the :generator:`Xcode`
+generator has the ability to use either the device or simulator SDK without
+needing CMake to be re-run.  The SDK can be selected at build time.
+But since some architectures can be supported by both the device and the
+simulator SDKs (e.g. ``arm64`` with Xcode 12 or later), not all combinations
+can be represented in a single universal binary.  The only solution in this
+case is to have multiple object files.
+
+``IMPORTED_OBJECTS`` doesn't support generator expressions, so every file
+it lists needs to be valid for every architecture and SDK.  If incorporating
+object files that are not universal binaries, the path and/or file name of
+each object file has to somehow encapsulate the different architectures and
+SDKs.  With the :generator:`Xcode` generator, Xcode variables of the form
+``$(...)`` can be used to represent these aspects and Xcode will substitute
+the appropriate values at build time.  CMake doesn't interpret these
+variables and embeds them unchanged in the Xcode project file.
+``$(CURRENT_ARCH)`` can be used to represent the architecture, while
+``$(EFFECTIVE_PLATFORM_NAME)`` can be used to differentiate between SDKs.
+
+The following shows one example of how these two variables can be used to
+refer to an object file whose location depends on both the SDK and the
+architecture:
+
+.. code-block:: cmake
+
+  add_library(someObjs OBJECT IMPORTED)
+
+  set_property(TARGET someObjs PROPERTY IMPORTED_OBJECTS
+    # Quotes are required because of the ()
+    "/path/to/somewhere/objects$(EFFECTIVE_PLATFORM_NAME)/$(CURRENT_ARCH)/func.o"
+  )
+
+  # Example paths:
+  #   /path/to/somewhere/objects-iphoneos/arm64/func.o
+  #   /path/to/somewhere/objects-iphonesimulator/x86_64/func.o
+
+In some cases, you may want to have configuration-specific object files
+as well.  The :variable:`CMAKE_CFG_INTDIR` variable can be a convenient
+way of capturing this in combination with the SDK:
+
+.. code-block:: cmake
+
+  add_library(someObjs OBJECT IMPORTED)
+  set_property(TARGET someObjs PROPERTY IMPORTED_OBJECTS
+    "/path/to/somewhere/${CMAKE_CFG_INTDIR}/$(CURRENT_ARCH)/func.o"
+  )
+
+  # Example paths:
+  #   /path/to/somewhere/Release-iphoneos/arm64/func.o
+  #   /path/to/somewhere/Debug-iphonesimulator/x86_64/func.o
+
+When any Xcode variable or :variable:`CMAKE_CFG_INTDIR` is used, CMake is
+not able to fully evaluate the path(s) at configure time.  One consequence
+of this is that the configuration-specific
+:prop_tgt:`IMPORTED_OBJECTS_<CONFIG>` properties cannot be used, since
+CMake cannot determine whether an object file exists at a particular
+``<CONFIG>`` location.  The ``IMPORTED_OBJECTS`` property must be used for
+these situations and the configuration-specific aspects of the path must be
+handled by using :variable:`CMAKE_CFG_INTDIR` or with another Xcode variable
+``$(CONFIGURATION)``.

Help/prop_tgt/IMPORTED_OBJECTS_CONFIG.rst

@@ -3,7 +3,18 @@ IMPORTED_OBJECTS_<CONFIG>
 
 .. versionadded:: 3.9
 
-<CONFIG>-specific version of :prop_tgt:`IMPORTED_OBJECTS` property.
+``<CONFIG>``-specific version of :prop_tgt:`IMPORTED_OBJECTS` property.
 
 Configuration names correspond to those provided by the project from
 which the target is imported.
+
+
+Xcode Generator Considerations
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Do not use this ``<CONFIG>``-specific property if you need to use Xcode
+variables like ``$(CURRENT_ARCH)`` or ``$(EFFECTIVE_PLATFORM_NAME)`` in
+the value.  The ``<CONFIG>``-specific properties will be ignored in such
+cases because CMake cannot determine whether a file exists at the
+configuration-specific path at configuration time.  For such cases, use
+:prop_tgt:`IMPORTED_OBJECTS` instead.

Help/release/dev/object-lib-multiarch.rst

@@ -0,0 +1,8 @@
+object-lib-multiarch
+--------------------
+
+* The :command:`add_library` command previously prohibited imported object
+  libraries when using potentially multi-architecture configurations.
+  This mostly affected the :generator:`Xcode` generator, e.g. when targeting
+  iOS or one of the other device platforms.  This restriction has now been
+  removed.