Build fails with LTO due to ODR violations

[eli-schwartz]

I tried to build with the following *FLAGS to optimize the build: -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing

Note the -Werror=* flags are used to help detect cases where the compiler tries to optimize by assuming Undefined Behavior cannot exist in the source code -- if it does exist, ordinarily the code would be miscompiled, and this says to make the miscompilation a fatal error.

I got this error:

libtool: link: x86_64-pc-linux-gnu-g++  -fPIC -DPIC -shared -nostdlib /usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../lib64/crti.o /usr/lib/gcc/x86_64-pc-linux-gnu/13/crtbeginS.o  src/base/.libs/libctemplate_la-arena.o src/.libs/libctemplate_la-per_expand_data.o src/.libs/libctemplate_la-template.o src/.libs/libctemplate_la-template_annotator.o src/.libs/libctemplate_la-template_cache.o src/.libs/libctemplate_la-template_dictionary.o src/.libs/libctemplate_la-template_modifiers.o src/.libs/libctemplate_la-template_namelist.o src/.libs/libctemplate_la-template_pathops.o src/.libs/libctemplate_la-template_string.o src/htmlparser/.libs/libctemplate_la-htmlparser.o src/htmlparser/.libs/libctemplate_la-jsparser.o src/htmlparser/.libs/libctemplate_la-statemachine.o   -Wl,--as-needed -L/usr/lib/gcc/x86_64-pc-linux-gnu/13 -L/usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../x86_64-pc-linux-gnu/lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/13/../../.. -lstdc++ -lm -lc -lgcc_s /usr/lib/gcc/x86_64-pc-linux-gnu/13/crtendS.o /usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../lib64/crtn.o  -march=native -fstack-protector-all -O2 -fdiagnostics-color=always -frecord-gcc-switches -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=format-security -Wl,-O1 -Wl,-z -Wl,pack-relative-relocs -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing -Wl,--defsym=__gentoo_check_ldflags__=0   -pthread -Wl,-soname -Wl,libctemplate.so.3 -Wl,-retain-symbols-file -Wl,.libs/libctemplate.exp -o .libs/libctemplate.so.3.0.0
./src/ctemplate/template_cache.h:61:8: error: type 'struct TemplateCache' violates the C++ One Definition Rule [-Werror=odr]
   61 | class  TemplateCache {
      |        ^
./src/ctemplate/template_cache.h:61:8: note: a different type is defined in another translation unit
   61 | class  TemplateCache {
      |        ^
./src/ctemplate/template_cache.h:357:16: note: the first difference of corresponding definitions is field 'mutex_'
  357 |   Mutex* const mutex_;
      |                ^
./src/ctemplate/template_cache.h:357:16: note: a field of same name but different type is defined in another translation unit
  357 |   Mutex* const mutex_;
      |                ^
./src/base/mutex.h:168:7: note: type name 'ctemplate::Mutex' should match type name 'Mutex'
  168 | class Mutex {
      |       ^
./src/ctemplate/template_cache.h:48:7: note: the incompatible type is defined here
   48 | class Mutex;
      |       ^
lto1: some warnings being treated as errors
lto-wrapper: fatal error: x86_64-pc-linux-gnu-g++ returned 1 exit status
compilation terminated.
/usr/libexec/gcc/x86_64-pc-linux-gnu/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status
make[1]: *** [Makefile:1779: libctemplate.la] Error 1

Originally reported downstream: https://bugs.gentoo.org/924637
Full build log: build.log

[OlafvdSpek]

How is a forward declaration of the type incompatible?

[eli-schwartz]

There is nothing in this error message that is intrinsically about a forward declaration? The compiler is simply asserting the claim that they are different types with different names. One is ctemplate::Mutex and the other is Mutex.

template_cache.h.in declares Mutex one line below a namespace for ctemplate; mutex.h declares Mutex inside of a ctemplate namespace.

[OlafvdSpek]

class Mutex; is the forward declaration. It's not about the name. If it had a different name it'd be a different type.
I'll check what's going on.