VERIFY_*_HEADER_SETS: Create verification targets even when empty

Always create `<target>_verify_*_header_sets` targets and the aggregate
`all_verify_*_header_sets` utility targets when the corresponding
property is enabled, even when no matching headers exist.

When headers exist, an OBJECT library is created as before. When no
headers are found, a UTILITY target is created instead to avoid
warnings from tools like Xcode's libtool about empty static libraries.

Update documentation with `.. versionchanged:: 4.4` directives, update
tests, and add a release note.

Fixes: #27685

Help/prop_tgt/VERIFY_INTERFACE_HEADER_SETS.rst

@@ -10,13 +10,20 @@ VERIFY_INTERFACE_HEADER_SETS
 Used to verify that all headers in a target's ``PUBLIC`` and ``INTERFACE``
 header sets can be included on their own.
 
+.. versionchanged:: 4.4
+  Previously, the verification target was only created when the target had
+  at least one matching header. Now it is always created when the property
+  is enabled.
+
 When this property is set to true, and the target is an object library, static
 library, shared library, interface library, or executable (subject to policy
-:policy:`CMP0209`) and the target has one or more ``PUBLIC`` or ``INTERFACE``
+:policy:`CMP0209`), an object library target named
-header sets, an object library target named
 ``<target_name>_verify_interface_header_sets`` is created. This verification
-target has one source file per header in the ``PUBLIC`` and ``INTERFACE``
+target has one source file per header in the target's ``PUBLIC`` and
-header sets. Each source file only includes its associated header file.
+``INTERFACE`` header sets. Each source file only includes its associated
+header file. If the target has no matching header sets, a utility target is
+created instead so that the target name always exists for build system
+dependencies.
 The verification target links against the original target to get all of its
 usage requirements.
 

Help/prop_tgt/VERIFY_PRIVATE_HEADER_SETS.rst

@@ -10,13 +10,19 @@ VERIFY_PRIVATE_HEADER_SETS
 Used to verify that all headers in a target's ``PUBLIC`` and ``PRIVATE``
 header sets can be included on their own.
 
+.. versionchanged:: 4.4
+  Previously, the verification target was only created when the target had
+  at least one matching header. Now it is always created when the property
+  is enabled.
+
 When this property is set to true, and the target is an object library, static
-library, shared library, module library, interface library, or executable, and
+library, shared library, module library, interface library, or executable, an
-the target has one or more ``PUBLIC`` or ``PRIVATE`` header sets, an object
+object library target named ``<target_name>_verify_private_header_sets`` is
-library target named ``<target_name>_verify_private_header_sets`` is created.
+created. This verification target has one source file per header in the
-This verification target has one source file per header in the ``PUBLIC`` and
+target's ``PUBLIC`` and ``PRIVATE`` header sets. Each source file only
-``PRIVATE`` header sets. Each source file only includes its associated header
+includes its associated header file. If the target has no matching header
-file.
+sets, a utility target is created instead so that the target name always
+exists for build system dependencies.
 
 Properties affecting compilation are copied from the original target to the
 verification target so that the headers will be interpreted the same way by

Help/prop_tgt/include/VERIFY_XXX_HEADER_SETS.rst

@@ -26,11 +26,10 @@ conditions.  The compiler flags used for private and interface contexts can be
 different, leading to the compiler interpreting the contents of the header
 differently.
 
-If any |xxx| file set verification targets are created, a top-level target
+A top-level target called |THIS_ALL_TARGET| is created which depends on all
-called |THIS_ALL_TARGET| is created which depends on all |xxx| verification
+|xxx| verification targets.  Another target called ``all_verify_header_sets``
-targets.  Another target called ``all_verify_header_sets`` is also created
+is also created which depends on |THIS_ALL_TARGET|, and on
-which depends on |THIS_ALL_TARGET|, and on |COMPLEMENTARY_ALL_TARGET| if it
+|COMPLEMENTARY_ALL_TARGET| if it exists (see |COMPLEMENTARY_PROPERTY|).
-exists (see |COMPLEMENTARY_PROPERTY|).
 
 This property is initialized by the value of the |INIT_VARIABLE| variable if
 it is set when a target is created.

Help/release/dev/always-create-all-verify-targets.rst

@@ -0,0 +1,9 @@
+always-create-all-verify-targets
+---------------------------------
+
+* The :prop_tgt:`VERIFY_INTERFACE_HEADER_SETS` and
+  :prop_tgt:`VERIFY_PRIVATE_HEADER_SETS` target properties now always
+  create the per-target verification target and the aggregate
+  ``all_verify_interface_header_sets``, ``all_verify_private_header_sets``,
+  and ``all_verify_header_sets`` targets when the property is enabled,
+  even if the target has no matching header sets.

Source/cmGeneratorTarget.cxx

@@ -5925,7 +5925,15 @@ bool cmGeneratorTarget::AddHeaderSetVerification()
       }
     }
 
-    if (!stubSources.empty()) {
+    if (stubSources.empty()) {
+      // No headers to verify. Create a utility target so the target
+      // name always exists (e.g. for build system dependencies) without
+      // needing a placeholder source. This avoids warnings from tools
+      // like Xcode's libtool about empty static libraries.
+      verifyTarget =
+        this->Makefile->AddNewUtilityTarget(verifyTargetName, true);
+    } else {
+      // Create an OBJECT library to compile the verification stubs.
       {
         cmMakefile::PolicyPushPop polScope(this->Makefile);
         this->Makefile->SetPolicy(cmPolicies::CMP0119, cmPolicies::NEW);

Tests/RunCMake/VerifyHeaderSets/RunCMakeTest.cmake

@@ -125,3 +125,21 @@ run_cmake_build(AllVerifyPrivateHeaderSets private all)
 set(RunCMake_TEST_OPTIONS -DCMAKE_VERIFY_PRIVATE_HEADER_SETS=ON)
 run_cmake(VerifyPrivateHeaderSetsNonexistent)
 unset(RunCMake_TEST_OPTIONS)
+
+# Test that verify targets are always created even with no headers
+if(NOT RunCMake_GENERATOR_IS_MULTI_CONFIG)
+  set(RunCMake_TEST_OPTIONS -DCMAKE_BUILD_TYPE=Debug)
+endif()
+run_cmake(VerifyEmptyHeaderSets)
+unset(RunCMake_TEST_OPTIONS)
+
+run_cmake_build(VerifyEmptyHeaderSets interface empty_iface)
+run_cmake_build(VerifyEmptyHeaderSets private empty_priv)
+run_cmake_build(VerifyEmptyHeaderSets interface all)
+run_cmake_build(VerifyEmptyHeaderSets private all)
+
+set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/VerifyEmptyHeaderSets-build)
+set(RunCMake_TEST_NO_CLEAN 1)
+run_cmake_command(VerifyEmptyHeaderSets-all_verify_header_sets-Debug-build
+  ${CMAKE_COMMAND} --build . --config Debug --target all_verify_header_sets
+)

Tests/RunCMake/VerifyHeaderSets/VerifyEmptyHeaderSets.cmake

@@ -0,0 +1,9 @@
+enable_language(C)
+
+# Target with VERIFY_INTERFACE_HEADER_SETS ON but no interface file sets
+add_library(empty_iface STATIC lib.c)
+set_property(TARGET empty_iface PROPERTY VERIFY_INTERFACE_HEADER_SETS ON)
+
+# Target with VERIFY_PRIVATE_HEADER_SETS ON but no private file sets
+add_library(empty_priv STATIC lib.c)
+set_property(TARGET empty_priv PROPERTY VERIFY_PRIVATE_HEADER_SETS ON)

Tests/RunCMake/VerifyHeaderSets/VerifyInterfaceHeaderSets-none-Debug-build-result.txt

@@ -1 +0,0 @@
-[^0]

Tests/RunCMake/VerifyHeaderSets/VerifyInterfaceHeaderSets-none-Debug-build-stderr.txt

@@ -1 +0,0 @@
-.*

Tests/RunCMake/VerifyHeaderSets/VerifyInterfaceHeaderSets-private-Debug-build-result.txt

@@ -1 +0,0 @@
-[^0]

Tests/RunCMake/VerifyHeaderSets/VerifyInterfaceHeaderSets-private-Debug-build-stderr.txt

@@ -1 +0,0 @@
-.*

Tests/RunCMake/VerifyHeaderSets/VerifyPrivateHeaderSets-iface_lang_cxx-Debug-build-result.txt

@@ -1 +0,0 @@
-[^0]

Tests/RunCMake/VerifyHeaderSets/VerifyPrivateHeaderSets-iface_lang_cxx-Debug-build-stderr.txt

@@ -1 +0,0 @@
-.*

Tests/RunCMake/VerifyHeaderSets/VerifyPrivateHeaderSets-interface-Debug-build-result.txt

@@ -1 +0,0 @@
-[^0]

Tests/RunCMake/VerifyHeaderSets/VerifyPrivateHeaderSets-interface-Debug-build-stderr.txt

@@ -1 +0,0 @@
-.*

Tests/RunCMake/VerifyHeaderSets/VerifyPrivateHeaderSets-none-Debug-build-result.txt

@@ -1 +0,0 @@
-[^0]

Tests/RunCMake/VerifyHeaderSets/VerifyPrivateHeaderSets-none-Debug-build-stderr.txt

@@ -1 +0,0 @@
-.*