CMake 3.17 deletes Qt translations when updating the TS files - qt

Question

Update: Simplified problem
With CMake 3.16.8 or less, the following workflow was working, but 3.17+ deletes the Qt's TS files.
The idea is taken from Professional CMake, 7th Edition by Craig Scott
Requirements
cmake 3.17+, Qt 5.9+, ninja
Concept
To update the translation files for Qt, UPDATE_TRANSLATIONS can be enabled. When finished, disable it.
CMake 3.17+ deletes the updated TS files.
Adjust pathes in the script update_translation.sh to automate the workflow.
Steps to reproduce
git clone https://github.com/Macintron/QtTranslationDemo.git # or create files from below in QtTranslationDemo.
mkdir build # NOT in QtTranslationDemo
cd build
cmake -G Ninja -DCMAKE_PREFIX_PATH=[pathToQtDir/5.15.1/clang_64] ../QtTranslationDemo
ninja
./demo
Output should be:
Demo
de.qm: 85 bytes
Language: 'Deutsch'
greeting: ''
en.qm: 85 bytes
Language: 'English'
greeting: ''
now the problem starts:
cmake -DUPDATE_TRANSLATIONS=ON ../QtTranslationDemo
ninja demoTranslations
cmake -DUPDATE_TRANSLATIONS=OFF ../QtTranslationDemo
# new strings ("greeting") should haven been added to the ts files, but TS files are deleted instead!
Why are the TS files get deleted? Is this a bug in CMake 3.17+ or a bug in my workflow?
build info
The build directory must NOT be inside the sources.
Source files
CMakeLists.txt
cmake_minimum_required(VERSION 3.17)
project(QtTranslationDemo LANGUAGES CXX)
find_package(Qt5
COMPONENTS
LinguistTools Core
REQUIRED
)
option(UPDATE_TRANSLATIONS "Enable rescanning sources to update .ts files" OFF)
set(_tsFiles en.ts de.ts)
if(UPDATE_TRANSLATIONS)
message("** parse sources for new translations")
qt5_create_translation(_qmFiles
main.cpp
${_tsFiles}
)
else()
message("** update qm files")
qt5_add_translation(_qmFiles
${_tsFiles}
)
endif()
add_custom_target(demoTranslations DEPENDS ${_qmFiles})
add_executable(demo
main.cpp
${_qmFiles}
)
target_link_libraries(demo
PUBLIC
Qt5::Core
)
main.cpp
#include <QTranslator>
#include <QDir>
#include <QStringList>
static const char* language = QT_TRANSLATE_NOOP("#default", "Language");
static const char* greeting = QT_TRANSLATE_NOOP("#default", "greeting"); // not in ts files now!
int main(int argc, char *argv[])
{
qDebug("Demo");
QStringList qms = QDir("./").entryList({"*.qm"});
for (const auto& qm: qms) {
qDebug("%s: %lld bytes", qPrintable(qm), QFileInfo("./" + qm).size());
QTranslator appTranslator;
if (appTranslator.load(qm, "./")) {
qDebug(" Language: '%s'", qPrintable(appTranslator.translate("#default", language)));
qDebug(" greeting: '%s'", qPrintable(appTranslator.translate("#default", greeting)));
} else {
qWarning("Failed to load %s", qPrintable(qm));
}
}
return 0;
}
en.ts
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1" language="en" sourcelanguage="en">
<context>
<name>#default</name>
<message>
<source>Language</source>
<translation>English</translation>
</message>
</context>
</TS>
de.ts
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1" language="de" sourcelanguage="en">
<context>
<name>#default</name>
<message>
<source>Language</source>
<translation>Deutsch</translation>
</message>
</context>
</TS>
update_translation.sh
#!/bin/sh
set -e
thisdir=$(dirname $0)
qt_root=/usr/local/Trolltech/Qt5.15/5.15.1/clang_64
#cmaketool=/usr/local/Cellar/cmake/3.16.8/bin/cmake
cmaketool=cmake
# easier setup
rm -Rf en.qm de.qm CMakeFiles CMakeCache.txt cmake_install.cmake build.ninja .ninja_* > /dev/null 2>&1
git checkout $thisdir/de.ts $thisdir/en.ts # restore files
$cmaketool --version
$cmaketool -G Ninja -DCMAKE_PREFIX_PATH=$qt_root ../QtTranslationDemo
ninja
./demo
rm -f demo
echo "Starting CMake with DUPDATE_TRANSLATIONS..."
$cmaketool -DUPDATE_TRANSLATIONS=ON $thisdir
echo
echo "Updating translation files..."
ninja demoTranslations
echo
echo "Restore CMake configuration..."
$cmaketool -DUPDATE_TRANSLATIONS=OFF $thisdir
ninja
./demo
exit 0

Answer 1

This was a generic CMake problem, caused by the cleandead feature. It should be fixed in CMake 3.19.2.
Thank you all for your contributions to solve the problem.