-isystem does not suppress warnings from macros

[elbeno]

Given a header provoke_warning.hpp:

enum struct E { A, B, C };

namespace N {
  using enum E;
}

#define PROVOKE_WARNING using enum E;

And a source file main.cpp:

#include <provoke_warning.hpp>

struct S {
  PROVOKE_WARNING;
};

When compile with:
clang -std=c++17 -isystem . -c main.cpp

We get:

main.cpp:4:3: warning: using enum declaration is a C++20 extension [-Wc++20-extensions]
    4 |   PROVOKE_WARNING;
      |   ^
./provoke_warning.hpp:7:31: note: expanded from macro 'PROVOKE_WARNING'
    7 | #define PROVOKE_WARNING using enum E;
      |                               ^                                

Notice that the warning we would get from the using enum inside the namespace is not emitted (because of -isystem). However the use of the macro still gives a warning despite the fact that the macro came from a header included under -isystem.

--

I see why this could be difficult to remedy, and I already see the counterarguments: "it's a macro! It's in your code, not the library code!" So it's understandable why this happens, but that doesn't make it correct or expected.

This comes up in real code: catchorg/Catch2#2910