Invalid application of incomplete type 'Test::Impl'

Please have a look at directory structure first [ attached at the bottom of the question end].

Following are my Cmake files :

1. Main cmake

    cmake_minimum_required(VERSION 3.9)  
    set (CMAKE_CXX_STANDARD 14)
   
    add_executable (test main.cc)  
   
    target_include_directories(test PUBLIC test_include_interface)
   
    target_link_libraries(test PUBLIC test_test)
   

2. Test cmake :

    add_library(test_include_interface INTERFACE)
   
    target_include_directories(test_include_interface INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
   
    add_library(test_test STATIC test_interface.h
                                        test.h
                                        test.cc)
   
    target_include_directories(test_test INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
   

test.h

#pragma once

#include "test_interface.h"

#include <memory>

class Test : public ITest {
public:
    Test();

private:
    class Impl;
    std::unique_ptr<Impl> impl_;

};

test.cc

#include "test.h"


class Test::Impl {
public:
    Impl() {}
    ~Impl() {}
};

Test::Test() : impl_{std::make_unique<Impl>()} {}

Error :

In file included from /usr/include/c++/9/memory:80,
                 from /home/vkd0726/test/test/test.h:9,
                 from /home/vkd0726/test/main.cc:2:
/usr/include/c++/9/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = Test::Impl]’:
/usr/include/c++/9/bits/unique_ptr.h:292:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = Test::Impl; _Dp = std::default_delete<Test::Impl>]’
test/test/test.h:11:7:   required from here
/usr/include/c++/9/bits/unique_ptr.h:79:16: error: invalid application of ‘sizeof’ to incomplete type ‘Test::Impl’
   79 |  static_assert(sizeof(_Tp)>0,
      |                ^~~~~~~~~~~
make[2]: *** [CMakeFiles/test.dir/build.make:63: CMakeFiles/test.dir/main.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:76: CMakeFiles/test.dir/all] Error 2
make: *** [Makefile:84: all] Error 2

Please help me understand this error and how to resolve it ? Is it a problem with the Cmake or code itself. I have seen such "Impl" design pattern use at many places but unable to write a test snippet. What is wrong here ?

Directory structure