set(CMAKE_CXX_COMPILER g++) cause error when sub directory added

Question

I have an empty project with some dummy CMakeLists.txts only, below is the structure of my directory and CMake files.

dummy
 |
 +- CMakeLists.txt
 |
 +- sub
     |
     +- CMakeListx.txt

# dummy/CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(dummy)

set(CMAKE_CXX_COMPILER g++)       # #1
add_subdirectory(sub)

# dummy/sub/CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(sub)                      # #2

I do this because I want project sub be ready to be built standalone, while building it as a part of project dummy is also desirable.

The fact is, it doesn't work.

Trying to generate build files of project dummy gives me an error:

#####
CMake Error at sub/CMakeLists.txt:2 (project):
  The CMAKE_CXX_COMPILER:

    g++

  is not a full path and was not found in the PATH.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.


-- Configuring incomplete, errors occurred!
#####

g++ is not the case, it's the my default compiler and placed in PATH appropriately.

But if I change line #1 to set($ENV{CXX} g++) (set env CXX), or remove line #2, build files can be generated successfully.

But what's the matter? Am I misunderstanding some basic concepts of CMake?

Setting a compiler **after** the `project()` call is wrong in most cases. See [that my answer](https://stackoverflow.com/a/63944545/3440745) why it is wrong. See other answers to the question for proper way of setting a compiler in CMake. — Tsyvarev, Jun 24 '21 at 14:26

set(CMAKE_CXX_COMPILER g++) cause error when sub directory added

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