CMakeLists.txt

cmake_minimum_required(VERSION 3.22)
get_filename_component(ProjectId ${CMAKE_CURRENT_SOURCE_DIR} NAME)
string(REPLACE " " "_" ProjectId ${ProjectId})
string(REGEX REPLACE "[^a-zA-Z0-9_]" "" ProjectId ${ProjectId})
project(${ProjectId})

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

find_package(Qt6 REQUIRED COMPONENTS Core Quick QmlWorkerScript)

qt_standard_project_setup()

set(CMAKE_CXX_CLANG_TIDY "clang-tidy-18")

set(SRC_DIR ${CMAKE_SOURCE_DIR}/src)
set(INCLUDE_DIR ${CMAKE_SOURCE_DIR}/include)

qt_add_executable(${ProjectId} ${SRC_DIR}/main.cpp)

qt_add_qml_module(${ProjectId}
    URI project
    VERSION 1.0
    QML_FILES
        qml/Main.qml
        SOURCES ${INCLUDE_DIR}/header.h

)


target_include_directories(${ProjectId} PUBLIC ${INCLUDE_DIR})
target_link_libraries(${ProjectId} PRIVATE Qt6::Core Qt6::Quick Qt6::QmlWorkerScript)

include(GNUInstallDirs)
install(TARGETS ${ProjectId}
    BUNDLE DESTINATION .
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
)

main.cpp (where errors occur)

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "header.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:project/qml/Main.qml"));
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreationFailed,
        &app,
        []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    engine.load(url);
    return app.exec();
}

Errors example: identifier "QGuiApplication" is undefined;
incomplete type "QObject" is not allowed;
incomplete type "const QUrl" is not allowed;
no instance of function template "QtPrivate::qMakeStringPrivate" matches the argument list;
name followed by '::' must be a class or namespace name.

BUT everything compiles and builds fine.

Project is built in the Docker container. Qt packages I get:

qt6-base-dev 
qt6-base-dev-tools 
qt6-declarative-dev 
qt6-declarative-dev-tools 
qt6-httpserver-dev 
qt6-qmllint-plugins 
libqt6quickcontrols2-6

I tried using target_include_directories explicitly, but problem remained the same. I’m not sure but it looks like error for QGuiApplication disappears when I visit source files of types that cause errors, but everything comes back as it was when I close those files.