Compiler warns about missing `val::promise_type::unhandled_exception` with `-fno-exceptions`

[stevenwdv]

Given:

coro.cpp:

#include <emscripten/bind.h>
#include <emscripten/val.h>

static emscripten::val myCoro() { co_return 42; }

EMSCRIPTEN_BINDINGS(myBindings) {
  emscripten::function("myCoro", myCoro);
}

Compiling with -fno-exceptions, meaning __cpp_exceptions is not defined:

em++ -v -std=c++20 -lembind -fno-exceptions ./coro.cpp -o./coro.js

 "/home/swdv/emsdk/upstream/bin/clang++" -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/swdv/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -v -std=c++20 -fno-exceptions -c ./coro.cpp -o /tmp/emscripten_temp_sg7wryg7/coro.o
clang version 23.0.0git (https:/github.com/llvm/llvm-project 15cf81b13fd50e17092220f4730cb256a9a0da27)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/swdv/emsdk/upstream/bin
Build config: +assertions
 (in-process)
 "/home/swdv/emsdk/upstream/bin/clang-23" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -main-file-name coro.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/home/swdv/webtest -v -fcoverage-compilation-dir=/home/swdv/webtest -resource-dir /home/swdv/emsdk/upstream/lib/clang/23 -D EMSCRIPTEN -isysroot /home/swdv/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/lib/clang/23/include -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include -std=c++20 -fdeprecated-macro -ferror-limit 19 -fmessage-length=336 -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fignore-exceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_sg7wryg7/coro.o -x c++ ./coro.cpp
clang -cc1 version 23.0.0git based upon LLVM 23.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
 /home/swdv/emsdk/upstream/lib/clang/23/include
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
./coro.cpp:4:24: warning: 'promise_type' is required to declare the member 'unhandled_exception()' when exceptions are enabled [-Wcoroutine-missing-unhandled-exception]
    4 | static emscripten::val myCoro() { co_return 42; }
      |                        ^
/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/emscripten/val.h:725:12: note: 'promise_type' defined here                                                                                                                                                                                                                           
  725 | class val::promise_type {
      |            ^
1 warning generated.                                                                                                                                                                                                                                                                                                                            
 /home/swdv/emsdk/upstream/bin/clang --version
 /home/swdv/emsdk/node/22.16.0_64bit/bin/node /home/swdv/emsdk/upstream/emscripten/tools/compiler.mjs - --symbols-only
 /home/swdv/emsdk/upstream/bin/wasm-ld -o ./coro.wasm /tmp/tmp3c8hmsowlibemscripten_js_symbols.so --strip-debug --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=__getTypeName --export=__wasm_call_ctors --export=_emscripten_stack_restore --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --export-table -z stack-size=65536 --no-growable-memory --initial-heap=16777216 --no-entry --stack-first --table-base=1 -lembind-rtti /tmp/emscripten_temp_sg7wryg7/coro.o -L/home/swdv/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -L/home/swdv/emsdk/upstream/emscripten/src/lib -lGL-getprocaddr -lal -lhtml5 -lstubs-debug -lnoexit -lc-debug -ldlmalloc-debug -lcompiler_rt -lc++-debug-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr
 /home/swdv/emsdk/upstream/bin/llvm-objcopy ./coro.wasm ./coro.wasm '--remove-section=llvm.func_attr.annotate.*' --remove-section=producers --remove-section=name '--remove-section=.debug*'
 /home/swdv/emsdk/node/22.16.0_64bit/bin/node /home/swdv/emsdk/upstream/emscripten/tools/compiler.mjs -

Note the -Wcoroutine-missing-unhandled-exception warning, which warns that if exceptions were enabled (which they are not), then you'd need to implement unhandled_exception (which is conditionally undefined here).

Consider silencing this or implementing unhandled_exception to just call std::terminate. Also feel free to close this issue if you think users should just add -Wno-coroutine-missing-unhandled-exception.

Version

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 5.0.2-git (8551146)
clang version 23.0.0git (llvm/llvm-project@15cf81b)