Issue #21293 has been updated by shyouhei (Shyouhei Urabe). Yes... This is a macro expansion glitch. Basically if `zero` was an integer constant expression (which it isn't, by the way, in your diff), `__buiitin_constant_p` in [this] line would be true, and `rb_define_method` macro would be expanded as expected. This normally works because a C function tends to have compile-time fixed arity. People do not need to pass dynamic value here. But if they do... It breaks, like you reported. `rb_define_method` is a rusty API. Its design dates back to when matz still used K&R C. C evolved since then. We have coped with the language updates but are having hard times recently. Maybe either we want a "modern" C API, or we want to stop following the C standard (which many projects do anyway). [this]: https://github.com/ruby/ruby/blob/994dadfbf4d090e73f996bafcc9d3d64892a57c6/i... ---------------------------------------- Bug #21293: C23/GCC 15 build breakage with rb_define_method() and friends https://bugs.ruby-lang.org/issues/21293#change-112845 * Author: alanwu (Alan Wu) * Status: Open * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- GCC 15 defaults to `-std=gnu23`, which breaks rb_define_method() in some corner cases: ```diff diff --git a/object.c b/object.c index 5a379e9..958f421 100644 --- a/object.c +++ b/object.c @@ -4613,7 +4613,8 @@ InitVM_Object(void) rb_cFalseClass = rb_define_class("FalseClass", rb_cObject); rb_cFalseClass_to_s = rb_fstring_enc_lit("false", rb_usascii_encoding()); rb_vm_register_global_object(rb_cFalseClass_to_s); - rb_define_method(rb_cFalseClass, "to_s", rb_false_to_s, 0); + int zero = 0; + rb_define_method(rb_cFalseClass, "to_s", rb_false_to_s, zero); rb_define_alias(rb_cFalseClass, "inspect", "to_s"); rb_define_method(rb_cFalseClass, "&", false_and, 1); rb_define_method(rb_cFalseClass, "|", false_or, 1); ``` Applying the above is fine prior to C23, but on GCC 15 it triggers a build error: ```text compiling object.c In file included from ./include/ruby/ruby.h:27, from constant.h:13, from object.c:22: object.c: In function 'InitVM_Object': ./include/ruby/internal/anyargs.h:288:135: error: passing argument 3 of 'rb_define_method_m3' from incompatible pointer type [-Wincompatible-pointer-types] 288 | #define rb_define_method(klass, mid, func, arity) RBIMPL_ANYARGS_DISPATCH_rb_define_method((arity), (func))((klass), (mid), (func), (arity)) | ^~~~~~ | | | VALUE (*)(VALUE) {aka long unsigned int (*)(long unsigned int)} object.c:4617:5: note: in expansion of macro 'rb_define_method' 4617 | rb_define_method(rb_cFalseClass, "to_s", rb_false_to_s, kek); | ^~~~~~~~~~~~~~~~ ./include/ruby/internal/anyargs.h:277:21: note: expected 'VALUE (*)(void)' {aka 'long unsigned int (*)(void)'} but argument is of type 'VALUE (*)(VALUE)' {aka 'long unsigned int (*)(long unsigned int)'} 277 | RBIMPL_ANYARGS_DECL(rb_define_method, VALUE, const char *) | ^~~~~~~~~~~~~~~~ ./include/ruby/internal/anyargs.h:252:41: note: in definition of macro 'RBIMPL_ANYARGS_DECL' 252 | RBIMPL_ANYARGS_ATTRSET(sym) static void sym ## _m3(__VA_ARGS__, VALUE(*)(ANYARGS), int); \ | ^~~ object.c:1605:1: note: 'rb_false_to_s' declared here 1605 | rb_false_to_s(VALUE obj) | ^~~~~~~~~~~~~ At top level: cc1: note: unrecognized command-line option '-Wno-self-assign' may have been intended to silence earlier diagnostics cc1: note: unrecognized command-line option '-Wno-parentheses-equality' may have been intended to silence earlier diagnostics cc1: note: unrecognized command-line option '-Wno-constant-logical-operand' may have been intended to silence earlier diagnostics make: *** [Makefile:464: object.o] Error 1 ``` This also happens for C method that takes a large number of arguments. This because in C23 `void foo();` no longers means "foo takes an unspecified number of arguments", and there is no way to implement a working `ANYARGS` AFAIK. Not something people would run into in practice, I hope. (related: #21286 but this one is not Windows specific) -- https://bugs.ruby-lang.org/