CMake: Fix target-less call of qt_add_lupdate

A call to qt_add_lupdate like the following would lead to an error when
updating translations, because we generated an invalid JSON file and fed
it to lupdate.

    qt_add_lupdate(
        TS_FILES du_de.ts
        SOURCES main.cpp one.cpp two.cpp
    )

Fix this by ensuring that we don't generate subprojects entries in the
JSON file if there aren't any subprojects (== targets).

Fixes: QTBUG-123011
Change-Id: Ic6465654375a7bfb7d387a6951f8f30408cae452
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit c12750987f2ca474f0016d553e508164957ba659)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 66bed97bfd33875e8412cba5ea6a41b69a83365a)

2 files changed, 27 insertions, 17 deletions

diff --git a/src/linguist/GenerateLUpdateProject.cmake b/src/linguist/GenerateLUpdateProject.cmake
index 6d8277bed..64c383867 100644
--- a/src/linguist/GenerateLUpdateProject.cmake
+++ b/src/linguist/GenerateLUpdateProject.cmake
@@ -80,13 +80,15 @@ get_filename_component(project_root "${lupdate_project_file}" DIRECTORY)
 
 # Make relative paths absolute to the project root
 set(path_variables sources include_paths translations)
-foreach(i RANGE 1 ${lupdate_subproject_count})
-    list(APPEND path_variables
-        subproject${i}_include_paths
-        subproject${i}_sources
-        subproject${i}_excluded
-    )
-endforeach()
+if(lupdate_subproject_count GREATER 0)
+    foreach(i RANGE 1 ${lupdate_subproject_count})
+        list(APPEND path_variables
+            subproject${i}_include_paths
+            subproject${i}_sources
+            subproject${i}_excluded
+        )
+    endforeach()
+endif()
 foreach(path_var IN LISTS path_variables)
     set(absolute_${path_var} "")
     foreach(path IN LISTS lupdate_${path_var})
@@ -113,21 +115,23 @@ set(content "{
   \"translations\": ${json_translations},
   \"subProjects\": [
 ")
-foreach(i RANGE 1 ${lupdate_subproject_count})
-    prepare_json_sources(json_sources ${absolute_subproject${i}_sources})
-    list_to_json_array("${absolute_subproject${i}_include_paths}" json_include_paths)
-    list_to_json_array("${absolute_subproject${i}_excluded}" json_sources_exclusions)
-    string(APPEND content "    {
+if(lupdate_subproject_count GREATER 0)
+    foreach(i RANGE 1 ${lupdate_subproject_count})
+        prepare_json_sources(json_sources ${absolute_subproject${i}_sources})
+        list_to_json_array("${absolute_subproject${i}_include_paths}" json_include_paths)
+        list_to_json_array("${absolute_subproject${i}_excluded}" json_sources_exclusions)
+        string(APPEND content "    {
       \"projectFile\": \"${lupdate_subproject${i}_source_dir}/CMakeLists.txt\",
       \"includePaths\": ${json_include_paths},
       \"sources\": ${json_sources},
       \"excluded\": ${json_sources_exclusions}
     }")
-    if(i LESS lupdate_subproject_count)
-        string(APPEND content ",")
-    endif()
-    string(APPEND content "\n")
-endforeach()
+        if(i LESS lupdate_subproject_count)
+            string(APPEND content ",")
+        endif()
+        string(APPEND content "\n")
+    endforeach()
+endif()
 string(APPEND content "  ]
 }
 ")
diff --git a/tests/auto/cmake/linguist/test_translation_api/CMakeLists.txt b/tests/auto/cmake/linguist/test_translation_api/CMakeLists.txt
index 68835b2ef..781927f37 100644
--- a/tests/auto/cmake/linguist/test_translation_api/CMakeLists.txt
+++ b/tests/auto/cmake/linguist/test_translation_api/CMakeLists.txt
@@ -140,6 +140,12 @@ qt6_add_translations(app6
     TS_FILES myobject_cs.ts
     SOURCES myi18nobject.cpp)
 
+# Explicitly specify SOURCES with target-less qt_add_lupdate.
+qt6_add_lupdate(
+    TS_FILES myobject_sk.ts
+    SOURCES myi18nobject.cpp
+)
+
 # Explicitly specify INCLUDE_DIRECTORIES.
 create_app(app7)
 qt6_add_translations(app7