SUMMARY
tag rv20.08.1+, git clone 2020-09-28
generators/poppler/generator_pdf.h:116:53:
error: 'OutlineItem' is not a member of 'Poppler'"
In poppler-qt5.h
  QVector< OutlineItem > children () const

I don't understand why QVector<Poppler::OutlineItem> produces a compile error.
I last successfully compiled tag v20.4.3, which did not include OutlineItem.

Using kdesrc-build. 
Cmake error log also shows ‘class Poppler::FormFieldChoice’ has no member named ‘choicesWithExportValues’
    6 |     ffc->choicesWithExportValues();

STEPS TO REPRODUCE
Cmake error log:
/usr/bin/c++ -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NO_DEBUG -D_GNU_SOURCE -D_LARGEFILE64_SOURCE -isystem /usr/include/poppler/qt5 -isystem /usr/include/poppler -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtCore -isystem /usr/lib64/qt5/mkspecs/linux-g++ -isystem /usr/include/qt5/QtGui -pipe -fno-operator-names -fno-exceptions -Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type -Wvla -Wdate-time -Wsuggest-override -Wlogical-op -DHAVE_POPPLER_0_87 -fPIC -std=c++14 

C++ v10.2.0
The system is: Linux - 5.4.68 - x86_64

Configured with: ../configure --prefix=/usr --libdir=/usr/lib64 --mandir=/usr/man --infodir=/usr/info --enable-shared --enable-bootstrap --enable-languages=ada,brig,c,c++,d,fortran,go,lto,objc,obj-c++ --enable-threads=posix --enable-checking=release --enable-objc-gc --with-system-zlib --enable-libstdcxx-dual-abi --with-default-libstdcxx-abi=new --disable-libstdcxx-pch --disable-libunwind-exceptions --enable-__cxa_atexit --disable-libssp --enable-gnu-unique-object --enable-plugin --enable-lto --disable-install-libiberty --disable-werror --with-gnu-ld --with-isl --verbose --with-arch-directory=amd64 --disable-gtktest --enable-clocale=gnu --disable-multilib --target=x86_64-slackware-linux --build=x86_64-slackware-linux --host=x86_64-slackware-linux

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.19.4
KDE Frameworks Version: 5.72.0
Qt Version: 5.15.1