Gem won't compile in ruby 4.x

[mvastola]

So I'm getting this when I try to install the gem in ruby 4:

mvastola@vastdesk:~/tmp/test-repo$ gem install bitset
Building native extensions. This could take a while...
ERROR:  Error installing bitset:
ERROR: Failed to build gem native extension.

current directory: /usr/local/rvm/gems/ruby-4.0.3/gems/bitset-1.2.0/ext/bitset
/usr/local/rvm/rubies/ruby-4.0.3/bin/ruby extconf.rb
creating Makefile

current directory: /usr/local/rvm/gems/ruby-4.0.3/gems/bitset-1.2.0/ext/bitset
make -j33 DESTDIR\= sitearchdir\=./.gem.20260506-2887983-k7rqf0 sitelibdir\=./.gem.20260506-2887983-k7rqf0 clean

current directory: /usr/local/rvm/gems/ruby-4.0.3/gems/bitset-1.2.0/ext/bitset
make -j33 DESTDIR\= sitearchdir\=./.gem.20260506-2887983-k7rqf0 sitelibdir\=./.gem.20260506-2887983-k7rqf0
compiling bitset.c
bitset.c: In function ‘get_bitset’:
bitset.c:48:5: warning: ‘rb_data_object_get_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
48 |     Data_Get_Struct(obj, Bitset, bs);
|     ^~~~~~~~~~~~~~~
In file included from /usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core.h:27,
from /usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/ruby.h:29,
from /usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby.h:38,
from bitset.c:1:
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:328:1: note: declared here
328 | rb_data_object_get_warning(VALUE obj)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_alloc’:
bitset.c:54:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
54 |     obj = Data_Wrap_Struct(klass, 0, bitset_free, bitset_new());
|     ^~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_intersect’:
bitset.c:224:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
224 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_union’:
bitset.c:244:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
244 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_difference’:
bitset.c:264:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
264 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_xor’:
bitset.c:284:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
284 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_not’:
bitset.c:301:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
301 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_from_s’:
bitset.c:331:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
331 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_dup’:
bitset.c:406:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
406 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_values_at’:
bitset.c:520:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
520 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
bitset.c: In function ‘rb_bitset_reverse’:
bitset.c:536:5: warning: ‘rb_data_object_wrap_warning’ is deprecated: by TypedData [-Wdeprecated-declarations]
536 |     return Data_Wrap_Struct(cBitset, 0, bitset_free, new_bs);
|     ^~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/core/rdata.h:296:1: note: declared here
296 | rb_data_object_wrap_warning(VALUE klass, void *ptr, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/ruby.h:27:
bitset.c: In function ‘Init_bitset’:
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/anyargs.h:291:135: error: passing argument 3 of ‘rb_define_method_00’ from incompatible pointer type [-Wincompatible-pointer-types]
291 | #define rb_define_method(klass, mid, func, arity)           RBIMPL_ANYARGS_DISPATCH_rb_define_method((arity), (func))((klass), (mid), (func),(arity))
|                                                                                                                                       ^~~~~~
|                                                                                                                                       |
|                                                                                                                                       VALUE (*)(VALUE,  VALUE) {aka long unsigned int (*)(long unsigned int,  long unsigned int)}
bitset.c:605:5: note: in expansion of macro ‘rb_define_method’
605 |     rb_define_method(cBitset, "size", rb_bitset_size, 0);
|     ^~~~~~~~~~~~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/anyargs.h:280:21: note: expected ‘VALUE (*)(VALUE)’ {aka ‘long unsigned int (*)(long unsigned int)’} but argument is of type ‘VALUE (*)(VALUE,  VALUE)’ {aka ‘long unsigned int (*)(long unsigned int,  long unsigned int)’}
280 | RBIMPL_ANYARGS_DECL(rb_define_method, VALUE, const char *)
|                     ^~~~~~~~~~~~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/anyargs.h:258:41: note: in definition of macro ‘RBIMPL_ANYARGS_DECL’
258 | RBIMPL_ANYARGS_ATTRSET(sym) static void sym ## _00(__VA_ARGS__, VALUE(*)(VALUE), int); \
|                                         ^~~
bitset.c:78:14: note: ‘rb_bitset_size’ declared here
78 | static VALUE rb_bitset_size(VALUE self, VALUE len) {
|              ^~~~~~~~~~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/anyargs.h:291:135: error: passing argument 3 of ‘rb_define_method_00’ from incompatible pointer type [-Wincompatible-pointer-types]
291 | #define rb_define_method(klass, mid, func, arity)           RBIMPL_ANYARGS_DISPATCH_rb_define_method((arity), (func))((klass), (mid), (func),(arity))
|                                                                                                                                       ^~~~~~
|                                                                                                                                       |
|                                                                                                                                       VALUE (*)(VALUE,  VALUE) {aka long unsigned int (*)(long unsigned int,  long unsigned int)}
bitset.c:649:5: note: in expansion of macro ‘rb_define_method’
649 |     rb_define_method(cBitset, "reverse", rb_bitset_reverse, 0);
|     ^~~~~~~~~~~~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/anyargs.h:280:21: note: expected ‘VALUE (*)(VALUE)’ {aka ‘long unsigned int (*)(long unsigned int)’} but argument is of type ‘VALUE (*)(VALUE,  VALUE)’ {aka ‘long unsigned int (*)(long unsigned int,  long unsigned int)’}
280 | RBIMPL_ANYARGS_DECL(rb_define_method, VALUE, const char *)
|                     ^~~~~~~~~~~~~~~~
/usr/local/rvm/rubies/ruby-4.0.3/include/ruby-4.0.0/ruby/internal/anyargs.h:258:41: note: in definition of macro ‘RBIMPL_ANYARGS_DECL’
258 | RBIMPL_ANYARGS_ATTRSET(sym) static void sym ## _00(__VA_ARGS__, VALUE(*)(VALUE), int); \
|                                         ^~~
bitset.c:524:14: note: ‘rb_bitset_reverse’ declared here
524 | static VALUE rb_bitset_reverse(VALUE self, VALUE index_array) {
|              ^~~~~~~~~~~~~~~~~
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:251: bitset.o] Error 1

make failed, exit code 2

Gem files will remain installed in /usr/local/rvm/gems/ruby-4.0.3/gems/bitset-1.2.0 for inspection.
Results logged to /usr/local/rvm/gems/ruby-4.0.3/extensions/x86_64-linux/4.0.0/bitset-1.2.0/gem_make.out

This is on Ubuntu 25.10 with ruby 4.0.3. Happy to provide any more info that might help.

Here is the gem_make.out for fake of completeness, but it appears to be identical to the console output I pasted above.

[mvastola]

UPDATE: following some research, it seems that this error is a duplicate of #7 (and is therefore fixed by #8), with the exception that in ruby 4 this is the case regardless of compiler. It appears this error CAN NOT be bypassed on gcc, however, by using the equivalent of the clang workaround (gem install bitset -- --with-cflags=-Wno-error=incompatible-pointer-types) because this leads to undefined symbols for some reason. (It using the --with-cflags argument somehow causes some other flags to not be added.)

Could you please merge #8 and cut a new release?