gcc fails to obey #pragma once with precompiled headers (via cmake)

I have a weird problem with gcc trying to include a file twice in one compilation unit, even though there's #pragma once in that file.

The files needed to reproduce the issue are:

CMakeLists.txt

cmake_minimum_required( VERSION 3.16 )project( example )add_library( example SHARED src/example/class_a.cpp )target_include_directories( example PRIVATE src )target_precompile_headers( example PRIVATE src/example/pch.h )

src/example/class_a.cpp

#include "class_a.h"

src/example/class_a.h

#pragma onceclass class_a { };

src/example/pch.h

#pragma once#include <example/class_a.h>

Trying to compile the project leads to:

[ 33%] Building CXX object CMakeFiles/example.dir/cmake_pch.hxx.gch[ 66%] Building CXX object CMakeFiles/example.dir/src/example/class_a.cpp.oIn file included from /src/example/class_a.cpp:1:/src/example/class_a.h:6:7: error: redefinition of ‘class class_a’    6 | class class_a      |       ^~~~~~~In file included from /src/example/pch.h:2,                 from CMakeFiles/example.dir/cmake_pch.hxx:5,                 from <command-line>:/src/example/class_a.h:6:7: note: previous definition of ‘class class_a’    6 | class class_a      |       ^~~~~~~

If I switch the #pragma once guards to old-fashioned #ifndef XX, it compiles fine.

Also it compiles without errors if I change to "class_a.h" in pch.h file.

Am I doing something really wrong here? It really feels like a gcc bug not being able to figure out that #pragma once means "only once".