boot2

Playing with the boostrap
git clone https://git.ryansepassi.com/git/boot2.git
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210-pp-arg-prescan.c (624B)

      1 /* Per C11 ยง6.10.3.1: function-like macro arguments must be
      2  * macro-expanded BEFORE substitution into the body, except when
      3  * the parameter is the operand of `#` or `##`.
      4  *
      5  * `M(M(1))` for `#define M(x) ((x) + (x))`:
      6  *   With prescan: inner M(1) -> ((1) + (1)); outer expands to
      7  *     ((((1) + (1))) + (((1) + (1)))) = 4.
      8  *   Without prescan: outer body becomes ((M(1)) + (M(1))) where M
      9  *     is in every substituted token's hide-set, so the inner Ms
     10  *     never expand and the parser sees calls to undeclared `M`.
     11  */
     12 
     13 #define M(x) ((x) + (x))
     14 
     15 int main(void) {
     16     int v = M(M(1));
     17     return v == 4 ? 0 : 1;
     18 }