Bug #21629: Ruby-3.4.7 prints -Wdefault-const-init-field-unsafe warnings on clang / llvm 21

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x86_64-darwin16]
• Backport: 3.2: REQUIRED, 3.3: REQUIRED, 3.4: REQUIRED

[debo (David Bohman)]

Here is the error I encountered. This is using clang version 21.1.3 as the compiler toolchain on Darwin.

DYLD_LIBRARY_PATH=.:../../.. ASAN_OPTIONS=detect_leaks=0 "clang -I../../../.ext/include/x86_64-darwin16 -I../../../../ruby-3.4.7/include -I../../../../ruby-3.4.7/ext/-test-/public_header_warnings -I/usr/local/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -fstack-protector-strong -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fdeclspec  -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wextra-tokens -Wdeprecated-declarations -Wdivision-by-zero -Wdiv-by-zero -Wimplicit-function-declaration -Wimplicit-int -Wpointer-arith -Wshorten-64-to-32 -Wwrite-strings -Wold-style-definition -Wmissing-noreturn -Wno-cast-function-type -Wno-constant-logical-operand -Wno-long-long -Wno-missing-field-initializers -Wno-overlength-strings -Wno-parentheses-equality -Wno-self-assign -Wno-tautological-compare -Wno-unused-parameter -Wno-unused-value -Wunused-variable -Wmisleading-indentation -Wundef  -fno-common -pipe  -D_TEST_OK -Werror -c conftest.c"
In file included from conftest.c:1:
In file included from ../../../../ruby-3.4.7/include/ruby.h:38:
In file included from ../../../../ruby-3.4.7/include/ruby/ruby.h:28:
In file included from ../../../../ruby-3.4.7/include/ruby/internal/arithmetic.h:24:
In file included from ../../../../ruby-3.4.7/include/ruby/internal/arithmetic/char.h:29:
../../../../ruby-3.4.7/include/ruby/internal/core/rstring.h:398:24: error: default initialization of an object of type 'struct RString' with const member leaves the object uninitialized [-Werror,-Wdefault-const-init-field-unsafe]
  398 |         struct RString retval;
      |                        ^
../../../../ruby-3.4.7/include/ruby/internal/core/rbasic.h:86:17: note: member 'klass' declared 'const' here
   86 |     const VALUE klass;
      |                 ^
1 error generated.
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: int main(int argc, char **argv)
4: {
5:   return !!argv[argc];
6: }
/* end */

The configuration is the following:

CC=clang CXX=clang++ CPPFLAGS=-I/usr/local/include LDFLAGS=-L/usr/local/lib ../ruby-3.4.7/configure --enable-shared --with-rdoc=ri,html

[debo (David Bohman)]

Let me know if you would like me to attach the entire build log.

[debo (David Bohman)]

Note that this is a tarball build.

[k0kubun (Takashi Kokubun)]

• Status changed from Open to Feedback
• ruby -v changed from ruby 3.4.6 (2025-09-16 revision dbd83256b1) +PRISM [x86_64-darwin16] to ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [arm64-darwin16]

Let me know if you would like me to attach the entire build log.

Please do. I'm not sure how an error in one of the things checked by configure (conftest.c) leads to a build failure from this log.

I confirmed that I can build the same tarball of Ruby 3.4.7 with Apple clang 17.0.0 on darwin25. You might want to consider testing it with Apple clang installed by XCode command-line tools and/or upgrading your macOS to a non-EOL version (Apple's support of darwin16/macOS 10.12 has ended in 2019).

[k0kubun (Takashi Kokubun)]

• ruby -v changed from ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [arm64-darwin16] to ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x86_64-darwin16]

[k0kubun (Takashi Kokubun)]

• Subject changed from Ruby-3.4.7 fails to build using clang / llvm to Ruby-3.4.7 fails to build using clang / llvm 21
• Status changed from Feedback to Open

I'll see if I can reproduce it with clang 21 on my end too

[debo (David Bohman)]

• Subject changed from Ruby-3.4.7 fails to build using clang / llvm 21 to Ruby-3.4.7 fails to build using clang / llvm

Note that I was using llvm-21.1.1 as the compiler toolchain when I built ruby-3.4.6. I'll try building ruby-3.4.6 again.

[debo (David Bohman)]

• File ruby-log.txt ruby-log.txt added

Attaching the build log for ruby-3.4.7.

[k0kubun (Takashi Kokubun)]

• File ruby-log-2.txt ruby-log-2.txt added
• Status changed from Open to Feedback

Attaching the build log for ruby-3.4.7.

Could you also include the logs of configure and the commands you ran (both configure and make)? As an example, I've attached mine.

As shown in the attached log, it succeeded with llvm-21. So this is not an issue of Ruby 3.4.7 with LLVM 21 on a still-supported macOS version. As such, I strongly encourage you to test it after upgrading your macOS to a non-EOL version.

[k0kubun (Takashi Kokubun)]

• Subject changed from Ruby-3.4.7 fails to build using clang / llvm to Ruby-3.4.7 prints -Wdefault-const-init-field-unsafe warnings on clang / llvm21
• Status changed from Feedback to Open
• Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: REQUIRED

To me, the direct cause of the build failure seems to be -Werror added by your environment. I don't know what part of llvm-21 or darwin-16 on Intel Mac leads to adding -Werror, but it's unfortunately normal that new compiler versions show warnings on Ruby builds, so you shouldn't let warnings fail your Ruby build.

I'll repurpose this ticket for fixing the -Wdefault-const-init-field-unsafe warning. But the fact that you have -Werror in your build environment seems like a separate problem.

[k0kubun (Takashi Kokubun)]

• Subject changed from Ruby-3.4.7 prints -Wdefault-const-init-field-unsafe warnings on clang / llvm21 to Ruby-3.4.7 prints -Wdefault-const-init-field-unsafe warnings on clang / llvm 21

[debo (David Bohman)]

• Subject changed from Ruby-3.4.7 prints -Wdefault-const-init-field-unsafe warnings on clang / llvm 21 to Ruby-3.4.7 fails to build using clang / llvm

I don't understand. The ruby build is what added the -Werror to the compile that failed.

[nobu (Nobuyoshi Nakada)]

• Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: REQUIRED to 3.2: REQUIRED, 3.3: REQUIRED, 3.4: REQUIRED

[k0kubun (Takashi Kokubun)]

• Subject changed from Ruby-3.4.7 fails to build using clang / llvm to Ruby-3.4.7 prints -Wdefault-const-init-field-unsafe warnings on clang / llvm 21

The ruby build is what added the -Werror to the compile that failed.

Could you point out the code that does it? As I said before, it didn't in my environment (Ruby 3.4.7, macOS 26, llvm-21), so I can't find the thing that I can't reproduce. You still haven't provided the logs of configure that I asked for, so it's also making it harder to figure out.

[rhenium (Kazuki Yamaguchi)]

The attached ruby-log.txt seems to show the build was successful, but the test case meant to catch this kind of problem failed, as intended:

*** Following extensions are not compiled:
-test-/public_header_warnings:
	Could not be configured. It will not be installed.
	/tera/tera/debo/Projects/ruby/ruby-3.4.7/ext/-test-/public_header_warnings/extconf.rb:14: baseline compiler warning test failed
	Check ext/-test-/public_header_warnings/mkmf.log for more details.
*** Fix the problems, then remove these directories and try again if you want.

[debo (David Bohman)]

• File ruby-config-log.txt ruby-config-log.txt added

Here is the configure log.

[debo (David Bohman)]

I just built ruby with make -j <n> where <n> is the number of cpu threads available on this machine.

[debo (David Bohman)]

The fact that the ruby build gets these warnings seems to be new in clang 21.

[debo (David Bohman)]

You are correct, it does build and install:

ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x86_64-darwin16]