FILE_SET: Make INTERFACE libraries with HEADER_SETS participate in buildsystem

If an INTERFACE library has HEADER_SETS, and its header sets contain
files generated by a custom command, the library needs to participate in
the buildsystem so that the files will be generated.

Fixes: #23422

Help/command/add_library.rst

@@ -151,6 +151,7 @@ itself and is not included as a target in the generated buildsystem.
   ``PUBLIC`` keywords.
 
   If an interface library has source files (i.e. the :prop_tgt:`SOURCES`
+  target property is set), or header sets (i.e. the :prop_tgt:`HEADER_SETS`
   target property is set), it will appear in the generated buildsystem
   as a build target much like a target defined by the
   :command:`add_custom_target` command.  It does not compile any sources,

Source/cmGeneratorTarget.cxx

@@ -1208,8 +1208,10 @@ bool cmGeneratorTarget::IsInBuildSystem() const
     case cmStateEnums::GLOBAL_TARGET:
       return true;
     case cmStateEnums::INTERFACE_LIBRARY:
-      // An INTERFACE library is in the build system if it has SOURCES.
-      if (!this->SourceEntries.empty()) {
+      // An INTERFACE library is in the build system if it has SOURCES or
+      // HEADER_SETS.
+      if (!this->SourceEntries.empty() ||
+          !this->Target->GetHeaderSetsEntries().empty()) {
         return true;
       }
       break;

Tests/RunCMake/target_sources/FileSetGeneratedDependency.cmake

@@ -0,0 +1,12 @@
+enable_language(C)
+
+add_library(lib INTERFACE)
+add_custom_command(
+  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/dependency.h
+  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/FileSetGeneratedDependency.h.in ${CMAKE_CURRENT_BINARY_DIR}/dependency.h
+  VERBATIM
+  )
+target_sources(lib PUBLIC FILE_SET HEADERS BASE_DIRS ${CMAKE_CURRENT_BINARY_DIR} FILES ${CMAKE_CURRENT_BINARY_DIR}/dependency.h)
+
+add_executable(exe dependency.c)
+target_link_libraries(exe PRIVATE lib)

Tests/RunCMake/target_sources/FileSetGeneratedDependency.h.in

@@ -0,0 +1 @@
+/* empty */

Tests/RunCMake/target_sources/RunCMakeTest.cmake

@@ -43,6 +43,15 @@ if(APPLE)
   run_cmake(FileSetFramework)
 endif()
 
+set(RunCMake_TEST_NO_CLEAN 1)
+set(RunCMake_TEST_BINARY_DIR "${RunCMake_BINARY_DIR}/FileSetGeneratedDependency-build")
+file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
+file(MAKE_DIRECTORY "${RunCMake_TEST_BINARY_DIR}")
+run_cmake(FileSetGeneratedDependency)
+run_cmake_command(FileSetGeneratedDependency-build ${CMAKE_COMMAND} --build . --config Debug)
+unset(RunCMake_TEST_BINARY_DIR)
+unset(RunCMake_TEST_NO_CLEAN)
+
 set(RunCMake_TEST_OPTIONS -DCMAKE_POLICY_DEFAULT_CMP0115=NEW)
 run_cmake(FileSetFileNoExist)
 unset(RunCMake_TEST_OPTIONS)

Tests/RunCMake/target_sources/dependency.c

@@ -0,0 +1,6 @@
+#include <dependency.h>
+
+int main(void)
+{
+  return 0;
+}