cmake_minimum_required(VERSION 3.3.0)
project(myproject C CXX)

# The following works: 
# set(CMAKE_AUTOMOC ON)

find_package(Qt5Widgets REQUIRED)

set(SOURCES MainWindow.cpp main.cpp)

# This line is broken with Qt 5.12.7: 
qt5_wrap_cpp(SOURCES MainWindow.h)

add_executable(myapp ${SOURCES} MainWindow.h)
target_link_libraries(myapp Qt5::Widgets)

CMake Error at C:/Qt/5.12.7/msvc2017_64/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:131 (file):
  file failed to open for writing (Invalid argument):

    Z:/qt5_wrap_test/build/moc_Z:/qt5_wrap_test/build/MainWindow.cpp_parameters
Call Stack (most recent call first):
  C:/Qt/5.12.7/msvc2017_64/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:181 (qt5_create_moc_command)
  CMakeLists.txt:7 (qt5_wrap_cpp)