Now it’s clear to me, and I managed to build successfully. Thanks again for pointing me in the right direction!

Thanks for your help, I finally managed to build Qt 6.9.2 successfully. 🙏
The problem was that I was trying to build inside the source folder instead of doing an out-of-source build.

Please, next time listen to people answering you. We told you five times to do an out of source build beside the documentation also showing that.

I don’t understand why it is not working anymore.

cmake --install .                                                      
-- Install configuration: "Debug"
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6BuildInternals/Qt6BuildInternalsConfig.cmake
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6BuildInternals/Qt6BuildInternalsConfigVersion.cmake
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6BuildInternals/Qt6BuildInternalsConfigVersionImpl.cmake
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6BuildInternals/QtBuildInternalsExtra.cmake
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6BuildInternals/QtStandaloneTestTemplateProject
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6BuildInternals/QtStandaloneTestTemplateProject/CMakeLists.txt
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6BuildInternals/QtStandaloneTestTemplateProject/Main.cmake
-- Installing: /Users/kevin/Projects/qt6-sdk/lib/cmake/Qt6/qt.toolchain.cmake
-- Installing: /Users/kevin/Projects/qt6-sdk/mkspecs/qconfig.pri
CMake Error at qtbase/cmake_install.cmake:62 (file):
  file INSTALL cannot find
  "/Users/kevin/Projects/qt-everywhere-src-6.9.2/qtbase/mkspecs/qmodule.pri":
  No such file or directory.
Call Stack (most recent call first):
  cmake_install.cmake:42 (include)

From the output of cmake --build . above, you can also see that no files are actually built at all.

On a side note, do you realize that several people wrote that you should do out of source builds but you persist not to follow that suggestion ?

./configure -submodules qtbase,qtimageformats -debug -prefix ../qt6-sdk
cmake build . -DCMAKE_CXX_FLAGS="-Wno-error=unused-but-set-variable"

But the output is only:

./configure -submodules qtbase,qtimageformats -debug -prefix ../qt6-sdk
+ /Users/kevin/Projects/qt-everywhere-src-6.9.2/qtbase/configure -top-level -debug -prefix ../qt6-sdk -submodules qtbase,qtimageformats -- -DBUILD_qtbase=ON -DBUILD_qtimageformats=ON
'/usr/local/bin/cmake' '-DQT_BUILD_SUBMODULES=qtbase;qtimageformats' '-DBUILD_qtbase=ON' '-DBUILD_qtimageformats=ON' '-DQT_INTERNAL_CALLED_FROM_CONFIGURE:BOOL=TRUE' '-DCMAKE_INSTALL_PREFIX=../qt6-sdk' '-DCMAKE_BUILD_TYPE=Debug' '-G' 'Ninja' '/Users/kevin/Projects/qt-everywhere-src-6.9.2'
-- The CXX compiler identification is AppleClang 17.0.0.17000013
-- The C compiler identification is AppleClang 17.0.0.17000013
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- The ASM compiler identification is AppleClang
-- Found assembler: /usr/bin/cc
-- Checking dependencies of submodule 'qtbase'
-- Checking dependencies of submodule 'qtimageformats'
-- Configuring submodule 'qtbase'
-- [QtBase] CMAKE_BUILD_TYPE was already explicitly set to: 'Debug'
-- Configuring submodule 'qtimageformats'

-- Configuration summary has been written to /Users/kevin/Projects/qt-everywhere-src-6.9.2/config.summary

-- 

Qt is now configured for building. Just run 'cmake --build . --parallel'

Once everything is built, you must run 'cmake --install .'
Qt will be installed into '/Users/kevin/Projects/qt6-sdk'

To configure and build other Qt modules, you can use the following convenience script:
        /Users/kevin/Projects/qt6-sdk/bin/qt-configure-module

If reconfiguration fails for some reason, try removing 'CMakeCache.txt' from the build directory
Alternatively, you can add the --fresh flag to your CMake flags.

-- Configuring done (49.2s)
-- Generating done (1.9s)
-- Build files have been written to: /Users/kevin/Projects/qt-everywhere-src-6.9.2

cmake build . -DCMAKE_CXX_FLAGS="-Wno-error=unused-but-set-variable"   
-- Checking dependencies of submodule 'qtbase'
-- Checking dependencies of submodule 'qtimageformats'
-- Configuring submodule 'qtbase'
-- [QtBase] CMAKE_BUILD_TYPE was already explicitly set to: 'Debug'
-- Configuring submodule 'qtimageformats'

-- Configuration summary has been written to /Users/kevin/Projects/qt-everywhere-src-6.9.2/config.summary

-- Configuring done (19.7s)
-- Generating done (1.8s)
-- Build files have been written to: /Users/kevin/Projects/qt-everywhere-src-6.9.2

My config.summary looks like this:

Building for: macx-clang (x86_64, CPU features: cx16 sse3 ssse3 sse4.1)
Compiler: clang (Apple) 17.0.0.17000013
Build options:
  Mode ................................... debug
  Fully optimize release builds (-O3) .... no
  Building shared libraries (alias of  BUILD_SHARED_LIBS)  yes
  Using ccache ........................... no
  Unity Build ............................ no
  Relocatable ............................ yes
  Using precompiled headers .............. yes
  Using Link Time Optimization (LTCG) .... no
  Using Intel Control-flow Enforcement Technology (CET)  yes
  Using Glibc function fortification ..... no
  Using -ftrivial-auto-var-init=pattern .. yes
  Using -fstack-protector-strong ......... yes
  Using -fstack-clash-protection ......... no
  Using libstdc++ assertions ............. no
  Using libc++ hardening ................. yes
  Using -z relro -z now when linking ..... no
  Target compiler supports:
    x86 Intrinsics ....................... Basic VAES AVX512VBMI2
  Sanitizers:
    Addresses ............................ no
    Threads .............................. no
    Memory ............................... no
    Fuzzer (instrumentation only) ........ no
    Undefined ............................ no
  Build parts ............................ libs tools
  Install examples sources ............... no
  App store compliance ................... no
Qt modules and options:
  Qt Concurrent .......................... yes
  Qt D-Bus ............................... yes
  Qt D-Bus directly linked to libdbus .... no
  Qt Gui ................................. yes
  Qt Network ............................. yes
  Qt PrintSupport ........................ yes
  Qt Sql ................................. yes
  Qt Testlib ............................. yes
  Qt Widgets ............................. yes
  Qt Xml ................................. yes
Support enabled for:
  Using pkg-config ....................... no
  Using vcpkg ............................ no
  udev ................................... no
  OpenSSL ................................ no
    Qt directly linked to OpenSSL ........ no
  OpenSSL 1.1 ............................ no
  OpenSSL 3.0 ............................ no
  Using system zlib ...................... yes
  Zstandard support ...................... no
  Thread support ......................... yes
Common build options:
  Linker can resolve circular dependencies  yes
Qt Core:
  backtrace .............................. yes
  C++23 <stacktrace> ..................... no
  DoubleConversion ....................... yes
    Using system DoubleConversion ........ no
  GLib ................................... no
  ICU .................................... no
  std::chrono::tzdb QTZ backend .......... no
  Using system libb2 ..................... no
  Built-in copy of the MIME database ..... yes
  Application permissions ................ yes
  Defaulting legacy IPC to POSIX ......... no
  Tracing backend ........................ <none>
  OpenSSL based cryptographic hash ....... no
  Logging backends:
    journald ............................. no
    syslog ............................... no
    slog2 ................................ no
  PCRE2 .................................. yes
    Using system PCRE2 ................... no
Qt Sql:
  SQL item models ........................ yes
Qt Network:
  getifaddrs() ........................... yes
  IPv6 ifname ............................ yes
  libproxy ............................... no
  SecureTransport ........................ yes
  DTLS ................................... no
  OCSP-stapling .......................... no
  SCTP ................................... no
  Use system proxies ..................... yes
  GSSAPI ................................. yes
  Brotli Decompression Support ........... no
  qIsEffectiveTLD() ...................... yes
    Built-in publicsuffix database ....... yes
    System publicsuffix database ......... no
Core tools:
  Android deployment tool ................ yes
  macOS deployment tool .................. yes
  Windows deployment tool ................ no
  qmake .................................. yes
Qt Gui:
  Accessibility .......................... yes
  Emoji Segmenter ........................ yes
  FreeType ............................... yes
    Using system FreeType ................ no
  HarfBuzz ............................... yes
    Using system HarfBuzz ................ no
  Fontconfig ............................. no
  Image formats:
    GIF .................................. yes
    ICO .................................. yes
    JPEG ................................. yes
      Using system libjpeg ............... no
    PNG .................................. yes
      Using system libpng ................ no
  Text formats:
    HtmlParser ........................... yes
    CssParser ............................ yes
    OdfWriter ............................ yes
    MarkdownReader ....................... yes
      Using system libmd4c ............... no
    MarkdownWriter ....................... yes
  EGL .................................... no
  OpenVG ................................. no
  OpenGL:
    Desktop OpenGL ....................... yes
    OpenGL ES 2.0 ........................ no
    OpenGL ES 3.0 ........................ no
    OpenGL ES 3.1 ........................ no
    OpenGL ES 3.2 ........................ no
  Vulkan ................................. no
  Metal .................................. yes
  QGraphicsFrameCapture .................. yes
  Session Management ..................... yes
  Multi-threaded image and painting helpers  yes
Features used by QPA backends:
  evdev .................................. no
  libinput ............................... no
  HiRes wheel support in libinput ........ no
  INTEGRITY HID .......................... no
  mtdev .................................. no
  tslib .................................. no
  xkbcommon .............................. no
  vxworksevdev ........................... no
  X11 specific:
    XLib ................................. no
    XCB Xlib ............................. no
    EGL on X11 ........................... no
    xkbcommon-x11 ........................ no
    xcb-sm ............................... no
QPA backends:
  DirectFB ............................... no
  EGLFS .................................. no
  EGLFS details:
    EGLFS OpenWFD ........................ no
    EGLFS i.Mx6 .......................... no
    EGLFS i.Mx6 Wayland .................. no
    EGLFS RCAR ........................... no
    EGLFS EGLDevice ...................... no
    EGLFS GBM ............................ no
    EGLFS VSP2 ........................... no
    EGLFS Mali ........................... no
    EGLFS Raspberry Pi ................... no
    EGLFS X11 ............................ no
  LinuxFB ................................ no
  VNC .................................... no
  VK_KHR_display ......................... no
  QNX:
    lgmon ................................ no
    IMF .................................. no
  XCB:
    Using system-provided xcb-xinput ..... no
    GL integrations:
      GLX Plugin ......................... no
        XCB GLX .......................... no
      EGL-X11 Plugin ..................... no
  Windows:
    Direct 2D ............................ no
    Direct 2D 1.1 ........................ no
    DirectWrite .......................... no
    DirectWrite 3 ........................ no
    DirectWrite COLRv1 Support ........... no
Qt Widgets:
  GTK+ ................................... no
  Styles ................................. Fusion macOS Windows
Qt Testlib:
  Tester for item models ................. yes
  Batch tests ............................ no
Qt PrintSupport:
  CUPS ................................... yes
Qt Sql Drivers:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. no
  OCI (Oracle) ........................... no
  ODBC ................................... no
  PostgreSQL ............................. no
  SQLite ................................. yes
    Using system provided SQLite ......... no
  Mimer .................................. no
Further Image Formats:
  JasPer ................................. no
  MNG .................................... no
  TIFF ................................... yes
    Using system libtiff ................. no
  WEBP ................................... yes
    Using system libwebp ................. no

However, no files are actually built afterwards.
Do you know what I might be missing here?

Specifically, after configuring look at the file it tells you (config.summary). It should have a section like:

Qt Gui:
  Accessibility .......................... yes
  Emoji Segmenter ........................ yes
  FreeType ............................... yes
    Using system FreeType ................ no
  HarfBuzz ............................... yes
    Using system HarfBuzz ................ no
  Fontconfig ............................. no
  Image formats:
    GIF .................................. yes
    ICO .................................. yes
    JPEG ................................. yes
      Using system libjpeg ............... no
    PNG .................................. yes
      Using system libpng ................ no

If it says "yes" for the image formats you care about, you are good to go - but again, you won't see the plugin files until you install the build into the prefix you set.

mkdir -p ~/dev/Qt-build
cd ~/dev/Qt-build
/path/to/QtSources/configure -submodules qtbase qtimageformats -debug
cmake --build . --parallel
cmake --install .

If you want to debug or try some things, you can do a developer build. That way you don't need to re-install Qt every time you try something. Don't use a developer build for building a production version of your application.

I also would not recommend altering the PATH environment variable. Just use the full path to the tools you want to use so you ensure you are using the exact version of Qt you actually want to use.

I followed the instructions from the README that comes with the source package, but after building, I still cannot get any style or image plugins loaded (also, I specifically want a debug build).

The official Qt documentation also does not provide a clear description of how to specify configuration options in detail. I did try to consult an AI assistant, but as you guessed, it did not really understand Qt’s build system.

The options I used: -feature-imageformat_png, -feature-imageformatplugin, -submodules qtimageformats were based on the output of ./configure -list-features and ./configure --help, not AI. However, these flags also don’t seem to work as expected.

So my question is:
👉 Could you please provide me with a typical example configure (or cmake) command for building Qt Core + Gui + Widgets (with debug mode) on macOS?

That would help me a lot to get started in the right direction.

Carefully read the instructions for building on macOS: https://doc.qt.io/qt-6/macos-building.html

Also note that unless you are doing a developer's build from Git, you need to install your build results to the configured prefix to be able to use the Qt you built in an application.

I have downloaded a fresh copy of the Qt 6.9.1 sources and tried to run ./configure again, but the output is only the following:

./configure -feature-imageformat_png -feature-imageformat_jpeg -feature-imageformatplugin -submodules qtimageformats -opensource -confirm-license -debug
+ /qt-everywhere-src-6.9.2/qtbase/configure -top-level -feature-imageformat_png -feature-imageformat_jpeg -feature-imageformatplugin -opensource -confirm-license -debug -submodules qtimageformats -- -DBUILD_qtimageformats=ON
'/usr/local/bin/cmake' '-DQT_BUILD_SUBMODULES=qtimageformats' '-DBUILD_qtimageformats=ON' '-DQT_INTERNAL_CALLED_FROM_CONFIGURE:BOOL=TRUE' '-DFEATURE_imageformat_png=ON' '-DFEATURE_imageformat_jpeg=ON' '-DFEATURE_imageformatplugin=ON' '-DCMAKE_BUILD_TYPE=Debug' '-G' 'Ninja' '/qt-everywhere-src-6.9.2'
-- The CXX compiler identification is AppleClang 17.0.0.17000013
-- The C compiler identification is AppleClang 17.0.0.17000013
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- The ASM compiler identification is AppleClang
-- Found assembler: /usr/bin/cc
-- Checking dependencies of submodule 'qtbase'
-- Checking dependencies of submodule 'qtimageformats'
-- Configuring submodule 'qtbase'
-- [QtBase] CMAKE_BUILD_TYPE was already explicitly set to: 'Debug'
-- Configuring submodule 'qtimageformats'

-- Configuration summary has been written to /qt-everywhere-src-6.9.2/config.summary

-- 

Qt is now configured for building. Just run 'cmake --build . --parallel'

Once everything is built, you must run 'cmake --install .'
Qt will be installed into '/usr/local/Qt-6.9.2'

To configure and build other Qt modules, you can use the following convenience script:
        /usr/local/Qt-6.9.2/bin/qt-configure-module

If reconfiguration fails for some reason, try removing 'CMakeCache.txt' from the build directory
Alternatively, you can add the --fresh flag to your CMake flags.

-- Configuring done (50.7s)
-- Generating done (1.8s)
-- Build files have been written to: /qt-everywhere-src-6.9.2

It seems my ./configure command is not correct.
Could you please provide me with an example of the correct configure command line for building Qt 6.9.1 on macOS (with imageformat and style plugins enabled)?

Thanks in advance!