Trying to add plain project and converting it to library is futile.
I now have main SUBDIR project with "added library".
The library is build as SUBDIR internal library and added as such to main SUDBIR project. Works as expected.
I'll we transferring "btscanner" code example piece by piece to my library, hopefully learning more about Bluetooth in the process.

I would like to express my appreciation to all who helped , albeit unsuccessfully , to fix this problem.

CASE closed

No problem using the real library.
Same problem using the converted library.

Now the error is what has been suspect from get go

So a same error / story but now official.
Hope I find the missing link soon...

@AnneRanch said in Need help seeing my include:

It looks as linker is creating .a (shared ?) library.

That's an archive, a.k.a. a static library.

now SUDDIRS adds .pro

SUBDIRS is simply a tree-like build for qmake. It doesn't have to do with code at all. Each project is built separately and all considerations for stand-alone projects apply. The INCLUDEPATH is necessary for the compiler so it has the declarations avaialble, without it names (i.e. identifiers) mean nothing to it. LIBS is necessary for the linker, whenever it starts to resolve the objects it needs to know what symbol is defined in which binary (library, object file, executable).

HOWEVER, I now have another issue
if I add
CONFIG += staticlib - as recommended in other forum

Don't do that. Stick to dynamic libraries, it's simpler. Otherwise you need to satisfy the dependencies on the client side for the linking.
Also on linux it's necessary as far as I recall to add the archive to the PRE_TARGETDEPS.

Thanks for sticking with this mess.

I decided to actually check if the .so library is built and where.
Now I need to verify if that is for real as far as SUDIRS go.

I really believe , I am convinced, I have created this myself by adding
TEMPLATE = lib to the project.

I have no issue using / accessing the real library , created by Qt SUDBIRS .

There are copies of libbtscanner .so files :

qe@qe-desktop:~$ find ./ -name "*btscanner.so"
./Qt_5/Examples/Qt-6.2.0/bluetooth/btscanner/libbtscanner.so
./Qt/Examples/Qt-5.15.2/bluetooth/build-btscanner-Desktop-Debug/libbtscanner.so
qe@qe-desktop:~$

It looks as linker is creating .a (shared ?) library.

That's an archive, a.k.a. a static library.

now SUDDIRS adds .pro

SUBDIRS is simply a tree-like build for qmake. It doesn't have to do with code at all. Each project is built separately and all considerations for stand-alone projects apply. The INCLUDEPATH is necessary for the compiler so it has the declarations avaialble, without it names (i.e. identifiers) mean nothing to it. LIBS is necessary for the linker, whenever it starts to resolve the objects it needs to know what symbol is defined in which binary (library, object file, executable).

HOWEVER, I now have another issue
if I add
CONFIG += staticlib - as recommended in other forum

Don't do that. Stick to dynamic libraries, it's simpler. Otherwise you need to satisfy the dependencies on the client side for the linking.
Also on linux it's necessary as far as I recall to add the archive to the PRE_TARGETDEPS.

"normal " code has
header
source file

relations.

now SUDDIRS adds .pro

and default project mimic the

header
source file

relation in .pro file

without changing anything - not sure about that - done that few versions ago

I can "include"
a library header file in my .cpp file - the client reference point to library ( my inherited definition ) - editor(!) will complain if an attempt to include inaccessible file is made (nice real time feature kidos ) ,
intelisense will display ONLY accessible files .

What is suggested that IN ADDITION to C++ relations between client and library

there need to be manually added "stuff " to .pro file where access to libray is desired.

I would expect this to be done by SUBDIR option "add library" .
Easy to verify.

Sounds reasonable.

HOWEVER, I now have another issue
if I add
CONFIG += staticlib - as recommended in other forum

I get bscanner.a = not btscanner.so

So I got two issues right now.
This will take some time to fix stand by...

Here is part of the compiler / linker .
It looks as linker is creating .a (shared ?) library.

I have added -Wl --verbose , much needed tool to track what is happening ,and do not the see "btscanner.so" library build at all.

I am going to see how adding the HEADER and INCLUDE to .pro file make a difference.

IT SHOULD - I have been missing that from get go !!!

/home/qe/Qt_5/6.2.0/gcc_64/libexec/moc -DBTSCANNER_PROJECT_LIBRARY -DQT_QML_DEBUG -DQT_BLUETOOTH_LIB -DQT_NETWORK_LIB -DQT_DBUS_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB --include /media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source/moc_predefs.h -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -I/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtBluetooth -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtNetwork -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtDBus -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I/usr/include/c++/10 -I/usr/include/x86_64-linux-gnu/c++/10 -I/usr/include/c++/10/backward -I/usr/lib/gcc/x86_64-linux-gnu/10/include -I/usr/local/include -I/usr/include/x86_64-linux-gnu -I/usr/include service.h -o moc_service.cpp
g++ -c -pipe -g -fPIC -std=gnu++1z -Wall -Wextra -D_REENTRANT -DBTSCANNER_PROJECT_LIBRARY -DQT_QML_DEBUG -DQT_BLUETOOTH_LIB -DQT_NETWORK_LIB -DQT_DBUS_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtBluetooth -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtNetwork -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtDBus -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o moc_device.o moc_device.cpp
g++ -c -pipe -g -fPIC -std=gnu++1z -Wall -Wextra -D_REENTRANT -DBTSCANNER_PROJECT_LIBRARY -DQT_QML_DEBUG -DQT_BLUETOOTH_LIB -DQT_NETWORK_LIB -DQT_DBUS_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtBluetooth -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtNetwork -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtDBus -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o moc_service.o moc_service.cpp
rm -f libbtscanner.a
ar cqs libbtscanner.a main.o device.o service.o moc_device.o moc_service.o

but I can see the build in compiler output ...

The compiler complains that it doesn't know what the btscanner identifier means. It is missing a declaration or a definition, which usually means that the includes are wrong (somehow). Is this a function?

g++ -Wl,-rpath,/home/qe/Qt_5/6.2.0/gcc_64/lib -shared -Wl,-soname,libbtscanner.so.1 -o libbtscanner.so.1.0.0 main.o device.o service.o moc_device.o moc_service.o /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Bluetooth.so -pthread /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Network.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6DBus.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Widgets.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Gui.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Core.so -lpthread -lGL

That's the linker call, so the library is in /home/qe/Qt_5/6.2.0/gcc_64/lib, assuming that the rpath's set to the build directory.

I did add DEFINES but not sure what the does.

I don't follow. What defines and why?

PS Looks as "tick tack toe" make a mess when copying . I am sure there is a hack to fix that.
[snip]

If that's the project file that's supposed to use the btscanner library, then it is missing the proper INCLUDEPATH and the LIBS variables being filled in. It doesn't matter if that's an application or library (template) it still needs to link against the dependency to be able to use it.

Should look something like this:

HEADERS += \
    TEST_LIBRARY_global.h \
    test_library.h

INCLUDEPATH += /home/qe/<...>/include<...>                   # Wherever the headers reside
LIBS += -L/home/qe/Qt_5/6.2.0/gcc_64/lib -lbtscanner

I decided to post part of the compiler output.
Could you give me an idea where in the output is the resulting library path?

-DQT_DBUS_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -I/home/qe/Qt_5/6.2.0/gcc_64QtBluetooth -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtBluetooth -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtNetwork -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtDBus -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o moc_device.o moc_device.cpp
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DBTSCANNER_PROJECT_LIBRARY -DQT_QML_DEBUG -DQT_BLUETOOTH_LIB -DQT_NETWORK_LIB -DQT_DBUS_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -I/home/qe/Qt_5/6.2.0/gcc_64QtBluetooth -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtBluetooth -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtNetwork -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtDBus -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o moc_service.o moc_service.cpp
rm -f libbtscanner.so.1.0.0 libbtscanner.so libbtscanner.so.1 libbtscanner.so.1.0
g++ -Wl,-rpath,/home/qe/Qt_5/6.2.0/gcc_64/lib -shared -Wl,-soname,libbtscanner.so.1 -o libbtscanner.so.1.0.0 main.o device.o service.o moc_device.o moc_service.o /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Bluetooth.so -pthread /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Network.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6DBus.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Widgets.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Gui.so /home/qe/Qt_5/6.2.0/gcc_64/lib/libQt6Core.so -lpthread -lGL
ln -s libbtscanner.so.1.0.0 libbtscanner.so
ln -s libbtscanner.so.1.0.0 libbtscanner.so.1
ln -s libbtscanner.so.1.0.0 libbtscanner.so.1.0
09:37:11: The process "/usr/bin/make" exited normally.
09:37:11: Elapsed time: 00:22.

Here is how library project looks when built from scratch by Qt
Notice removal of gui and the
Default rules for deployment.

I did add DEFINES but not sure what the does.

I do not have either in my project I build from "normal" project by adding
TEMPLATE = lib

PS Looks as "tick tack toe" make a mess when copying . I am sure there is a hack to fix that.

QT -= gui

TEMPLATE = lib
DEFINES += TEST_LIBRARY_LIBRARY

CONFIG += c++11

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    test_library.cpp

HEADERS += \
    TEST_LIBRARY_global.h \
    test_library.h

# Default rules for deployment.
unix {
    target.path = /usr/lib
}
!isEmpty(target.path): INSTALLS += target

Check the names and the paths, there should be libbtscanner.so inside $$OUT_PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source or it won't build (drop the trailing / too).

If this library is a static one, you may want to force it within the PRE_TARGETDEPS[1].

[1] https://doc.qt.io/qt-5/qmake-variable-reference.html#pre-targetdeps

unix:!macx: LIBS += -L$$OUT_PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source/ -lbtscanner

INCLUDEPATH += $$PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source
DEPENDPATH += $$PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source

unix:!macx: LIBS += -L$$OUT_PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source/ -lbtscanner

INCLUDEPATH += $$PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source
DEPENDPATH += $$PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source
.

unix:!macx: LIBS += -L$$OUT_PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source/ -lbtscanner

INCLUDEPATH += $$PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source
DEPENDPATH += $$PWD/../../../../../QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source

I still cannot use the library.

The editor does not recognize
"btscanner"
I am still missing something and running out of ideas.

Looking at the defines, I'm pretty sure you forgot to add the module to list in the .pro file (the QT variable).

You found the problem you are the man...
It complies and tun !

AS a poor excuse - I did not think the client using the library needs the module.
Lesson learn.
THANKS

I am not sure the answer is there , but it is a real place to check stuff.

AND I DO APOLOGIZE FOR TAKING SO MUCH MEMORY

I am posting this in case somebody smarter than me can see the problem 

I am not sure the answer is there , but it is a real place to check stuff.

AND I DO APOLOGIZE FOR TAKING SO MUCH MEMORY 






<pre>11:05:54: Running steps for project TEST_SUB...
11:05:54: Starting: "/usr/bin/make" clean -j4
rm -f moc_predefs.h
rm -f moc_mainwindow.cpp
rm -f ui_mainwindow.h
rm -f main.o mainwindow.o moc_mainwindow.o
rm -f *~ core *.core
11:05:54: The process "/usr/bin/make" exited normally.
11:05:55: Starting: "/home/qe/Qt_5/6.2.0/gcc_64/bin/qmake" /media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/FIRST_SUB/FIRST_SUB.pro -spec linux-g++ CONFIG+=debug CONFIG+=qml_debug
11:05:55: The process "/home/qe/Qt_5/6.2.0/gcc_64/bin/qmake" exited normally.
11:05:55: Starting: "/usr/bin/make" -f /media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/build-TEST_SUB-Desktop_Qt_6_2_0_GCC_64bit-Debug/FIRST_SUB/Makefile qmake_all
make: Nothing to be done for 'qmake_all'.
11:05:55: The process "/usr/bin/make" exited normally.
11:05:55: Starting: "/usr/bin/make" -j4
/home/qe/Qt_5/6.2.0/gcc_64/libexec/uic ../../TEST_SUB/FIRST_SUB/mainwindow.ui -o ui_mainwindow.h
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../TEST_SUB/FIRST_SUB -I. -I../../TEST_SUB/TEST_PROJECT -I../../TEST_SUB/TEST_LIBRARY -I/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source -IQTDIR -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o main.o ../../TEST_SUB/FIRST_SUB/main.cpp
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../TEST_SUB/FIRST_SUB -I. -I../../TEST_SUB/TEST_PROJECT -I../../TEST_SUB/TEST_LIBRARY -I/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source -IQTDIR -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o mainwindow.o ../../TEST_SUB/FIRST_SUB/mainwindow.cpp
g++ -pipe -g -std=gnu++1z -Wall -Wextra -dM -E -o moc_predefs.h /home/qe/Qt_5/6.2.0/gcc_64/mkspecs/features/data/dummy.cpp
/home/qe/Qt_5/6.2.0/gcc_64/libexec/moc -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB --include /media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/build-TEST_SUB-Desktop_Qt_6_2_0_GCC_64bit-Debug/FIRST_SUB/moc_predefs.h -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/FIRST_SUB -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/TEST_PROJECT -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/TEST_LIBRARY -I/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/FIRST_SUB/QTDIR -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I/usr/include/c++/10 -I/usr/include/x86_64-linux-gnu/c++/10 -I/usr/include/c++/10/backward -I/usr/lib/gcc/x86_64-linux-gnu/10/include -I/usr/local/include -I/usr/include/x86_64-linux-gnu -I/usr/include ../../TEST_SUB/FIRST_SUB/mainwindow.h -o moc_mainwindow.cpp
In file included from ../../TEST_SUB/FIRST_SUB/main.cpp:14:
/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source/device.h:56:10: fatal error: qbluetoothlocaldevice.h: No such file or directory
   56 | #include <qbluetoothlocaldevice.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [Makefile:1084: main.o] Error 1
make: *** Waiting for unfinished jobs....
11:06:03: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project TEST_SUB (kit: Desktop Qt 6.2.0 GCC 64bit)
When executing step "Make"
11:06:03: Elapsed time: 00:09.
</pre>

<pre>11:05:54: Running steps for project TEST_SUB...
11:05:54: Starting: "/usr/bin/make" clean -j4
rm -f moc_predefs.h
rm -f moc_mainwindow.cpp
rm -f ui_mainwindow.h
rm -f main.o mainwindow.o moc_mainwindow.o
rm -f *~ core *.core
11:05:54: The process "/usr/bin/make" exited normally.
11:05:55: Starting: "/home/qe/Qt_5/6.2.0/gcc_64/bin/qmake" /media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/FIRST_SUB/FIRST_SUB.pro -spec linux-g++ CONFIG+=debug CONFIG+=qml_debug
11:05:55: The process "/home/qe/Qt_5/6.2.0/gcc_64/bin/qmake" exited normally.
11:05:55: Starting: "/usr/bin/make" -f /media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/build-TEST_SUB-Desktop_Qt_6_2_0_GCC_64bit-Debug/FIRST_SUB/Makefile qmake_all
make: Nothing to be done for 'qmake_all'.
11:05:55: The process "/usr/bin/make" exited normally.
11:05:55: Starting: "/usr/bin/make" -j4
/home/qe/Qt_5/6.2.0/gcc_64/libexec/uic ../../TEST_SUB/FIRST_SUB/mainwindow.ui -o ui_mainwindow.h
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../TEST_SUB/FIRST_SUB -I. -I../../TEST_SUB/TEST_PROJECT -I../../TEST_SUB/TEST_LIBRARY -I/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source -IQTDIR -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o main.o ../../TEST_SUB/FIRST_SUB/main.cpp
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../TEST_SUB/FIRST_SUB -I. -I../../TEST_SUB/TEST_PROJECT -I../../TEST_SUB/TEST_LIBRARY -I/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source -IQTDIR -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I. -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -o mainwindow.o ../../TEST_SUB/FIRST_SUB/mainwindow.cpp
g++ -pipe -g -std=gnu++1z -Wall -Wextra -dM -E -o moc_predefs.h /home/qe/Qt_5/6.2.0/gcc_64/mkspecs/features/data/dummy.cpp
/home/qe/Qt_5/6.2.0/gcc_64/libexec/moc -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB --include /media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/build-TEST_SUB-Desktop_Qt_6_2_0_GCC_64bit-Debug/FIRST_SUB/moc_predefs.h -I/home/qe/Qt_5/6.2.0/gcc_64/mkspecs/linux-g++ -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/FIRST_SUB -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/TEST_PROJECT -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/TEST_LIBRARY -I/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source -I/media/qe/TSET_QT_LABEL/TEST_FOLDER/TEST_SUB_MAIN/TEST_SUB_PROJECT/TEST_SUB/FIRST_SUB/QTDIR -I/home/qe/Qt_5/6.2.0/gcc_64/include -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtWidgets -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtGui -I/home/qe/Qt_5/6.2.0/gcc_64/include/QtCore -I. -I/usr/include/c++/10 -I/usr/include/x86_64-linux-gnu/c++/10 -I/usr/include/c++/10/backward -I/usr/lib/gcc/x86_64-linux-gnu/10/include -I/usr/local/include -I/usr/include/x86_64-linux-gnu -I/usr/include ../../TEST_SUB/FIRST_SUB/mainwindow.h -o moc_mainwindow.cpp
In file included from ../../TEST_SUB/FIRST_SUB/main.cpp:14:
/media/qe/TSET_QT_LABEL/QT_6/QT6_PRO/CAT/SUB_PRO/EXAMPLES_COPY/QT_5/Examples/Qt-6.2.0/bluetooth/btscanner_source/device.h:56:10: fatal error: qbluetoothlocaldevice.h: No such file or directory
56 | #include <qbluetoothlocaldevice.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [Makefile:1084: main.o] Error 1
make: *** Waiting for unfinished jobs....
11:06:03: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project TEST_SUB (kit: Desktop Qt 6.2.0 GCC 64bit)
When executing step "Make"
11:06:03: Elapsed time: 00:09.
</pre>

And you post is pointing that way.
I have a copy of compiler output and I may be able to prove this theory .

Many thanks for your post, appreciate it .

Now I have to make sure 5.15.2-2 IS USED by BOTH library and the client.

qe@qe-desktop:~$ dpkg --list | grep libqt5bluetooth5
ii libqt5bluetooth5:amd64 5.15.2-2 amd64 Qt Connectivity Bluetooth module
ii libqt5bluetooth5-bin 5.15.2-2 amd64 Qt Connectivity Bluetooth module helper binaries
qe@qe-desktop:~$

/usr/include/x86_64-linux-gnu/qt5

This is the system include directory. Meaning that you're building with a Qt provided by your package manager.

A recommended fix is to add

The recommendation's wrong. The system include dir is always searched as part of the build process. The headers are installed by your system package manager when you install the proper packages.
If dpkg --list | grep libqt5bluetooth5 returns empty then you don't have the bluetooth module installed. Use apt-get, synaptic or whatever tool you use to manage your packages to install it.

How do I find base path for bluetooth ?

There's no such base path for the system provided module. It's under the Qt includes (as provided by qmake like above) under a directory named after the module. A piece of advice:

#include <qbluetoothaddress.h>

should be:

#include <QBluetoothAddress>

and also for the rest of the includes.

There's no QTDIR environment variable, it used be set in Qt4 times but it since had been dropped. The Qt includes directory can be obtained by the qmake used to build the project:

qmake -query QT_INSTALL_HEADERS

OK, sounds too simple .
I get this
What is missing in command?

qe@qe-desktop:~$ qmake -query QT_INSTALL_HEADERS
/usr/include/x86_64-linux-gnu/qt5
qe@qe-desktop:~$

A recommended fix is to add
INCLUDEPATH += (Base Path to openssl includes)
to project file .

How do I find base path for bluetooth ?

qmake -query QT_INSTALL_HEADERS

I have added

INCLUDEPATH = "$(QTDIR)QtBluetooth"

to pro file with no change .
I like to identify the include tree/ chain for the bluetooth includes.
I am working on that.

include
you should add the AdditionalIncludeDirectories like "$(QTDIR)QtBluetooth"