FetchContent: Disable header set verification for dependencies

The CMAKE_VERIFY_INTERFACE_HEADER_SETS variable is intended to
be under the control of the user. It doesn't discriminate between
header sets defined in the main project and those defined by
dependencies brought into the build directly via FetchContent.
Developers will usually only be interested in verifying the main project's
header sets, not those from dependencies.

Make the variable effectively only enable header set verification of the
main project by turning it off during FetchContent_MakeAvailable() calls.
The user still has variables like CMAKE_PROJECT_INCLUDE and
CMAKE_PROJECT_<projectName>_INCLUDE available to them if they
want to enable verification of all or specific dependencies respectively.

Fixes: #23808

7 files changed, 81 insertions, 1 deletions

diff --git a/Help/variable/CMAKE_VERIFY_INTERFACE_HEADER_SETS.rst b/Help/variable/CMAKE_VERIFY_INTERFACE_HEADER_SETS.rst
index 6f14e6fa4f..3fb8817f4e 100644
--- a/Help/variable/CMAKE_VERIFY_INTERFACE_HEADER_SETS.rst
+++ b/Help/variable/CMAKE_VERIFY_INTERFACE_HEADER_SETS.rst
@@ -7,11 +7,33 @@ This variable is used to initialize the
 :prop_tgt:`VERIFY_INTERFACE_HEADER_SETS` property of targets when they are
 created.  Setting it to true enables header set verification.
 
-Projects should not set this variable, it is intended as a developer
+Projects should not normally set this variable, it is intended as a developer
 control to be set on the :manual:`cmake(1)` command line or other
 equivalent methods.  The developer must have the ability to enable or
 disable header set verification according to the capabilities of their own
 machine and compiler.
 
+Verification of a dependency's header sets is not typically of interest
+to developers.  Therefore, :command:`FetchContent_MakeAvailable` explicitly
+sets ``CMAKE_VERIFY_INTERFACE_HEADER_SETS`` to false for the duration of its
+call, but restores its original value before returning.  If a project brings
+a dependency directly into the main build (e.g. calling
+:command:`add_subdirectory` on a vendored project from a git submodule), it
+should also do likewise.  For example:
+
+.. code:: cmake
+
+  # Save original setting so we can restore it later
+  set(want_header_set_verification ${CMAKE_VERIFY_INTERFACE_HEADER_SETS})
+
+  # Include the vendored dependency with header set verification disabled
+  set(CMAKE_VERIFY_INTERFACE_HEADER_SETS OFF)
+  add_subdirectory(...)   # Vendored sources, e.g. from git submodules
+
+  # Add the project's own sources. Restore the developer's original choice
+  # for whether to enable header set verification.
+  set(CMAKE_VERIFY_INTERFACE_HEADER_SETS ${want_header_set_verification})
+  add_subdirectory(src)
+
 By default, this variable is not set, which will result in header set
 verification being disabled.
diff --git a/Modules/FetchContent.cmake b/Modules/FetchContent.cmake
index 27070c05a4..d016fb5aed 100644
--- a/Modules/FetchContent.cmake
+++ b/Modules/FetchContent.cmake
@@ -364,6 +364,18 @@ Commands
     FetchContent_Declare(other ...)
     FetchContent_MakeAvailable(uses_other other)
 
+  Note that :variable:`CMAKE_VERIFY_INTERFACE_HEADER_SETS` is explicitly set
+  to false upon entry to ``FetchContent_MakeAvailable()``, and is restored to
+  its original value before the command returns.  Developers typically only
+  want to verify header sets from the main project, not those from any
+  dependencies.  This local manipulation of the
+  :variable:`CMAKE_VERIFY_INTERFACE_HEADER_SETS` variable provides that
+  intuitive behavior.  You can use variables like
+  :variable:`CMAKE_PROJECT_INCLUDE` or
+  :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE` to turn verification back
+  on for all or some dependencies.  You can also set the
+  :prop_tgt:`VERIFY_INTERFACE_HEADER_SETS` property of individual targets.
+
 .. command:: FetchContent_Populate
 
   .. note::
@@ -1814,6 +1826,13 @@ endfunction()
 # calls will be available to the caller.
 macro(FetchContent_MakeAvailable)
 
+  # We must append an item, even if the variable is unset, so prefix its value.
+  # We will strip that prefix when we pop the value at the end of the macro.
+  list(APPEND __cmake_fcCurrentVarsStack
+    "__fcprefix__${CMAKE_VERIFY_INTERFACE_HEADER_SETS}"
+  )
+  set(CMAKE_VERIFY_INTERFACE_HEADER_SETS FALSE)
+
   get_property(__cmake_providerCommand GLOBAL PROPERTY
     __FETCHCONTENT_MAKEAVAILABLE_SERIAL_PROVIDER
   )
@@ -1965,10 +1984,18 @@ macro(FetchContent_MakeAvailable)
     endif()
   endforeach()
 
+  # Prefix will be "__fcprefix__"
+  list(POP_BACK __cmake_fcCurrentVarsStack __cmake_original_verify_setting)
+  string(SUBSTRING "${__cmake_original_verify_setting}"
+    12 -1 __cmake_original_verify_setting
+  )
+  set(CMAKE_VERIFY_INTERFACE_HEADER_SETS ${__cmake_original_verify_setting})
+
   # clear local variables to prevent leaking into the caller's scope
   unset(__cmake_contentName)
   unset(__cmake_contentNameLower)
   unset(__cmake_contentNameUpper)
   unset(__cmake_providerCommand)
+  unset(__cmake_original_verify_setting)
 
 endmacro()
diff --git a/Tests/RunCMake/FetchContent/RunCMakeTest.cmake b/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
index e83c45e703..a7ccf83557 100644
--- a/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
+++ b/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
@@ -15,6 +15,7 @@ run_cmake(UsesTerminalOverride)
 run_cmake(MakeAvailable)
 run_cmake(MakeAvailableTwice)
 run_cmake(MakeAvailableUndeclared)
+run_cmake(VerifyHeaderSet)
 
 run_cmake_with_options(ManualSourceDirectory
   -D "FETCHCONTENT_SOURCE_DIR_WITHPROJECT=${CMAKE_CURRENT_LIST_DIR}/WithProject"
diff --git a/Tests/RunCMake/FetchContent/VerifyHeaderSet-stdout.txt b/Tests/RunCMake/FetchContent/VerifyHeaderSet-stdout.txt
new file mode 100644
index 0000000000..354529da6a
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/VerifyHeaderSet-stdout.txt
@@ -0,0 +1,4 @@
+-- Before subproject, var = 'TRUE'
+-- Inside subproject, var = 'FALSE'
+-- After subproject, var = 'TRUE'
+-- Subproject target property VERIFY_INTERFACE_HEADER_SETS='FALSE'
diff --git a/Tests/RunCMake/FetchContent/VerifyHeaderSet.cmake b/Tests/RunCMake/FetchContent/VerifyHeaderSet.cmake
new file mode 100644
index 0000000000..8adfceaa25
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/VerifyHeaderSet.cmake
@@ -0,0 +1,16 @@
+enable_language(C)
+
+set(CMAKE_VERIFY_INTERFACE_HEADER_SETS TRUE)
+
+include(FetchContent)
+FetchContent_Declare(verify_subproj
+  SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/VerifyHeaderSet
+)
+message(STATUS "Before subproject, var = '${CMAKE_VERIFY_INTERFACE_HEADER_SETS}'")
+FetchContent_MakeAvailable(verify_subproj)
+
+# Provide a way to verify the variable was reset back to its original value
+message(STATUS "After subproject, var = '${CMAKE_VERIFY_INTERFACE_HEADER_SETS}'")
+
+get_property(verify TARGET Blah PROPERTY VERIFY_INTERFACE_HEADER_SETS)
+message(STATUS "Subproject target property VERIFY_INTERFACE_HEADER_SETS='${verify}'")
diff --git a/Tests/RunCMake/FetchContent/VerifyHeaderSet/CMakeLists.txt b/Tests/RunCMake/FetchContent/VerifyHeaderSet/CMakeLists.txt
new file mode 100644
index 0000000000..c6ba37e78c
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/VerifyHeaderSet/CMakeLists.txt
@@ -0,0 +1,9 @@
+cmake_minimum_required(VERSION 3.24)
+project(VerifyHeaderSet LANGUAGES C)
+
+message(STATUS "Inside subproject, var = '${CMAKE_VERIFY_INTERFACE_HEADER_SETS}'")
+
+add_library(Blah INTERFACE)
+target_sources(Blah
+  INTERFACE FILE_SET HEADERS FILES blah.h
+)
diff --git a/Tests/RunCMake/FetchContent/VerifyHeaderSet/blah.h b/Tests/RunCMake/FetchContent/VerifyHeaderSet/blah.h
new file mode 100644
index 0000000000..5b47e14e21
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/VerifyHeaderSet/blah.h
@@ -0,0 +1 @@
+#error Header was used