> I can directly inspect the result of the preprocessor makes macros easier to debug

Tangential note but: why does this require a separate re-invocation of the compiler with the '-E' flag? Say I have this silly example with a comma instead of the semicolon inside W() macro:

    #define W(a, b)  b, int a
    #define X(a, b)  W(b, a)

    int kekw(int x) {
        int z = X(23, k) = x;
        return z + k;
    }

When I shove it into GCC without -E, it says

    <source>: In function 'int kekw(int)':
    <source>:1:21: error: expected unqualified-id before 'int'
        1 | #define W(a, b)  b, int a
          |                     ^~~
    <source>:2:18: note: in expansion of macro 'W'
        2 | #define X(a, b)  W(b, a)
          |                  ^
    <source>:5:13: note: in expansion of macro 'X'
        5 |     int z = X(23, k) = x;
          |             ^
    <source>:6:16: error: 'k' was not declared in this scope
        6 |     return z + k;
          |       

instead of, you know, showing me the expanded text so that I could see what's actually wrong with the generated variable declarations? Something like

    <source>: In function 'int kekw(int)':
    <source>:1:21: error: expected unqualified-id before 'int'
        1 | #define W(a, b)  b, int a
          |                     ^~~
    <source>:2:18: note: in expansion of macro 'W'
        2 | #define X(a, b)  W(b, a)
          |                  ^
    <source>:5:13: note: in expansion of macro 'X'
        5 |     int z = X(23, k) = x;
          |             ^
    <source>:5:-: note: the expanded source text is
          |     int z = 23, int k = x;
          |                 ^~~
    <source>:6:16: error: 'k' was not declared in this scope
        6 |     return z + k;
          |       

perhaps? I understand that tracking the lines/columns inside the expanded text is highly non-trivial (although collapsing them all onto the position of the top-level macro is already just as non-trivial), but still. Show me what the source that you actually can't compile, not the three-steps removed precursor to it — I sure as hell can't expand macros in my head.

Also, what is this first error message about in the first place? Why the hell does it expect unqualified-id before "int"? If you put "int" after the unqualified-id, it still would be illegal; and an unqualified-id is not the only thing that can follow the comma in this context anyway.