Autogen: Turn off moc path prefix generation by default

Change the default value of `CMAKE_AUTOMOC_PATH_PREFIX` to `OFF` to
restore compatibility with behavior of CMake 3.15 and below.

C++ source files that are generated by Qt's meta object compiler (moc)
include the header file that was passed as input argument to moc. This
is usually a path relative to the source directory, for example

    #include "../../source/dir/myobject.h"

That is problematic for reproducible builds as described in #18815.
To cope with that, the target property AUTOMOC_PATH_PREFIX was
introduced in CMake 3.16 by commit d018d27c10 (Autogen: Add moc path
prefix generation (AUTOMOC_PATH_PREFIX), 2019-09-13, v3.16.0-rc1~94^2~4).
The property is default-initialized from the variable
`CMAKE_AUTOMOC_PATH_PREFIX`, which defaults to `ON`.

If this property is ON, and myobject.h is located in an include
directory of the target, moc-generated C++ files include the file
without the "path prefix":

    #include "myobject.h"

This behavior, however, can break projects that have equally named
header files in different include directories.  As "not breaking
existing projects" trumps "have reproducible builds by default" we
change the default of `CMAKE_AUTOMOC_PATH_PREFIX` to `OFF`.

Also, it is now possible to pass `-DCMAKE_AUTOMOC_PATH_PREFIX=ON` on the
CMake command line.  Before, it was overridden in `CMakeGenericSystem`.

Fixes: #20598
Issue: #18815

4 files changed, 7 insertions, 5 deletions

diff --git a/Help/prop_tgt/AUTOMOC_PATH_PREFIX.rst b/Help/prop_tgt/AUTOMOC_PATH_PREFIX.rst
index e2ebb3f..f789724 100644
--- a/Help/prop_tgt/AUTOMOC_PATH_PREFIX.rst
+++ b/Help/prop_tgt/AUTOMOC_PATH_PREFIX.rst
@@ -13,7 +13,7 @@ the ``-p`` path prefix option.  ``moc`` usually generates a
 relative include path in that case.
 
 :prop_tgt:`AUTOMOC_PATH_PREFIX` is initialized from the variable
-:variable:`CMAKE_AUTOMOC_PATH_PREFIX`, which is ``ON`` by default.
+:variable:`CMAKE_AUTOMOC_PATH_PREFIX`, which is ``OFF`` by default.
 
 See the :manual:`cmake-qt(7)` manual for more information on using CMake
 with Qt.
@@ -24,8 +24,7 @@ Reproducible builds
 For reproducible builds is is recommended to keep headers that are ``moc``
 compiled in one of the target
 :command:`include directories <target_include_directories>` and set
-:prop_tgt:`AUTOMOC_PATH_PREFIX` to ``ON`` (which is the default).  This ensures
-that
+:prop_tgt:`AUTOMOC_PATH_PREFIX` to ``ON``.  This ensures that:
 
 - ``moc`` output files are identical on different build setups,
 - ``moc`` output files will compile correctly when the source and/or
diff --git a/Help/variable/CMAKE_AUTOMOC_PATH_PREFIX.rst b/Help/variable/CMAKE_AUTOMOC_PATH_PREFIX.rst
index dca0b06..1e9790f 100644
--- a/Help/variable/CMAKE_AUTOMOC_PATH_PREFIX.rst
+++ b/Help/variable/CMAKE_AUTOMOC_PATH_PREFIX.rst
@@ -8,4 +8,4 @@ This variable is used to initialize the :prop_tgt:`AUTOMOC_PATH_PREFIX`
 property on all the targets.  See that target property for additional
 information.
 
-The default value is ``ON``.
+The default value is ``OFF``.
diff --git a/Modules/CMakeGenericSystem.cmake b/Modules/CMakeGenericSystem.cmake
index 77d8cfd..4b9e80d 100644
--- a/Modules/CMakeGenericSystem.cmake
+++ b/Modules/CMakeGenericSystem.cmake
@@ -26,7 +26,9 @@ set(CMAKE_FIND_LIBRARY_SUFFIXES ".so" ".a")
 
 set(CMAKE_AUTOGEN_ORIGIN_DEPENDS ON)
 set(CMAKE_AUTOMOC_COMPILER_PREDEFINES ON)
-set(CMAKE_AUTOMOC_PATH_PREFIX ON)
+if(NOT DEFINED CMAKE_AUTOMOC_PATH_PREFIX)
+  set(CMAKE_AUTOMOC_PATH_PREFIX OFF)
+endif()
 set(CMAKE_AUTOMOC_MACRO_NAMES "Q_OBJECT" "Q_GADGET" "Q_NAMESPACE")
 
 # basically all general purpose OSs support shared libs
diff --git a/Tests/QtAutogen/MocIncludeSymlink/CMakeLists.txt b/Tests/QtAutogen/MocIncludeSymlink/CMakeLists.txt
index 1627b39..8b11b46 100644
--- a/Tests/QtAutogen/MocIncludeSymlink/CMakeLists.txt
+++ b/Tests/QtAutogen/MocIncludeSymlink/CMakeLists.txt
@@ -61,6 +61,7 @@ macro(buildMocInclude sourceDir binaryDir)
     "${sourceDir}"
     MocInclude
     CMAKE_FLAGS "-DQT_TEST_VERSION=${QT_TEST_VERSION}"
+                "-DCMAKE_AUTOMOC_PATH_PREFIX=ON"
                 "-DCMAKE_AUTOGEN_VERBOSE=${CMAKE_AUTOGEN_VERBOSE}"
                 "-DQT_QMAKE_EXECUTABLE:FILEPATH=${QT_QMAKE_EXECUTABLE}"
     OUTPUT_VARIABLE output