objlib: Allow OBJECT libraries to link to OBJECT libraries.

Note: This still does not link the object-files of the `OBJECT` library
from the right-hand side of `target_link_libraries` to the target on the
left-hand side. (In this particular case of another `OBJECT` library on
the left-hand side this would not make any sense anyway. The target on
the left-hand side has no link-step.)

Issue: #14778

Source/cmGeneratorTarget.cxx

@@ -1054,9 +1054,6 @@ void cmGeneratorTarget::ComputeKindedSources(KindedSources& files,
       kind = SourceKindHeader;
     } else if (sf->GetPropertyAsBool("EXTERNAL_OBJECT")) {
       kind = SourceKindExternalObject;
-      if (this->GetType() == cmStateEnums::OBJECT_LIBRARY) {
-        badObjLib.push_back(sf);
-      }
     } else if (!sf->GetLanguage().empty()) {
       kind = SourceKindObjectSource;
     } else if (ext == "def") {

Tests/RunCMake/ObjectLibrary/BadObjSource2-result.txt

@@ -1 +0,0 @@
-1

Tests/RunCMake/ObjectLibrary/BadObjSource2-stderr.txt

@@ -1,9 +0,0 @@
-CMake Error at BadObjSource2.cmake:1 \(add_library\):
-  OBJECT library "A" contains:
-
-    bad.obj
-
-  but may contain only sources that compile, header files, and other files
-  that would not affect linking of a normal library.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:3 \(include\)

Tests/RunCMake/ObjectLibrary/LinkObjRHSObject-build-result.txt

@@ -0,0 +1 @@
+[1-9][0-9]*

Tests/RunCMake/ObjectLibrary/LinkObjRHSObject-build-stdout.txt

@@ -0,0 +1 @@
+REQUIRED needs to be defined

Tests/RunCMake/ObjectLibrary/LinkObjRHSObject.cmake

@@ -0,0 +1,12 @@
+cmake_policy(SET CMP0022 NEW)
+
+enable_language(C)
+
+add_library(AnObjLib OBJECT a.c)
+target_compile_definitions(AnObjLib INTERFACE REQUIRED)
+
+add_library(AnotherObjLib OBJECT b.c)
+target_link_libraries(AnotherObjLib PRIVATE AnObjLib)
+
+add_executable(exe exe.c)
+target_link_libraries(exe AnotherObjLib)

Tests/RunCMake/ObjectLibrary/LinkObjRHSObject2-build-result.txt

@@ -0,0 +1 @@
+[1-9][0-9]*

Tests/RunCMake/ObjectLibrary/LinkObjRHSObject2.cmake

@@ -0,0 +1,12 @@
+cmake_policy(SET CMP0022 NEW)
+
+enable_language(C)
+
+add_library(AnObjLib OBJECT a.c)
+target_compile_definitions(AnObjLib INTERFACE REQUIRED)
+
+add_library(AnotherObjLib OBJECT b.c)
+target_link_libraries(AnotherObjLib PUBLIC AnObjLib)
+
+add_executable(exe exe.c)
+target_link_libraries(exe AnotherObjLib)

Tests/RunCMake/ObjectLibrary/ObjWithObj-result.txt

@@ -1 +0,0 @@
-1

Tests/RunCMake/ObjectLibrary/ObjWithObj-stderr.txt

@@ -1,10 +0,0 @@
-^CMake Error at ObjWithObj.cmake:[0-9]+ \(add_library\):
-  OBJECT library \"B\" contains:
-
-    [^
-]*a(\.c)?\.o(bj)?
-
-  but may contain only sources that compile, header files, and other files
-  that would not affect linking of a normal library.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:[0-9]+ \(include\)$

Tests/RunCMake/ObjectLibrary/RunCMakeTest.cmake

@@ -24,10 +24,23 @@ function (run_object_lib_build name)
   run_cmake_command(${name}-build ${CMAKE_COMMAND} --build .)
 endfunction ()
 
+function (run_object_lib_build2 name)
+  # Use a single build tree for a few tests without cleaning.
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/${name}-build)
+  set(RunCMake_TEST_NO_CLEAN 1)
+  file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
+  file(MAKE_DIRECTORY "${RunCMake_TEST_BINARY_DIR}")
+  run_cmake(${name})
+  set(RunCMake_TEST_OUTPUT_MERGE 1)
+  run_cmake_command(${name}-build ${CMAKE_COMMAND} --build .)
+endfunction ()
+
 run_object_lib_build(LinkObjLHSShared)
 run_object_lib_build(LinkObjLHSStatic)
 run_object_lib_build(LinkObjRHSShared)
 run_object_lib_build(LinkObjRHSStatic)
+run_object_lib_build2(LinkObjRHSObject)
+run_object_lib_build2(LinkObjRHSObject2)
 
 run_cmake(MissingSource)
 run_cmake(ObjWithObj)

Tests/RunCMake/add_library/OBJECTwithOnlyObjectSources-result.txt

@@ -1 +0,0 @@
-1

Tests/RunCMake/add_library/OBJECTwithOnlyObjectSources-stderr.txt

@@ -1,10 +0,0 @@
-^CMake Error at OBJECTwithOnlyObjectSources.cmake:[0-9]+ \(add_library\):
-  OBJECT library \"TestObjectLibWithoutSources\" contains:
-
-    [^
-]*test(\.cpp)?\.o(bj)?
-
-  but may contain only sources that compile, header files, and other files
-  that would not affect linking of a normal library.
-Call Stack \(most recent call first\):
-  CMakeLists.txt:[0-9]+ \(include\)$