Missing signature argument to addFunction when using EM_JS (+ MAIN_MODULE & -fexceptions)

[ptc-lfruehstueck]

When built with the right combination of flags, the following code produces output that fails to load in the browser with this error:

test.js:648 Aborted(Assertion failed: Missing signature argument to addFunction: function HelloJS(message) { console.log(UTF8ToString(message)); })
abort	@	test.js:648
assert	@	test.js:239
addFunction	@	test.js:1342
reportUndefinedSymbols	@	test.js:4529
loadDylibs	@	test.js:4540
receiveInstance	@	test.js:788
receiveInstantiationResult	@	test.js:811
createWasm	@	test.js:837
await in createWasm		
(anonymous)	@	test.js:5714

Example code:

#include <emscripten.h>
#include <string>

class MyClass
{
public:
    MyClass();
    std::string something;
};

EM_JS(void, HelloJS, (const char* message),
    {
        console.log(UTF8ToString(message));
    }
);

MyClass::MyClass()
{
    HelloJS("test");
}

int main() 
{
    MyClass instance;
    return 0;
}

Build command:

em++ -s MAIN_MODULE=2 -fexceptions -c -o test.o test.cpp
em++ -s MAIN_MODULE=2 -fexceptions -o test.html test.o

Any one of the following changes "fixes" the problem:

• Removing -s MAIN_MODULE=1/2
• Removing -fexceptions (or using -fwasm-exceptions instead)
• Adding -flto
• And the weirdest part: Removing the std::string something class member or changing it to a simple type like int.
  • Other stdlib types (e.g. std::vector<int>) also trigger the issue.
  • Having a std::string instance in e.g. the main function and not as a class member does not trigger the issue.

I have seen other issues/discussions (#16997, #14959) about related problems, but those seem to deal with existing/library JS code, not JS code coming from EM_JS().

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.15 (09f52557f0d48b65b8c724853ed8f4e8bf80e669)
clang version 22.0.0git (https:/github.com/llvm/llvm-project 3388d40684742e950b3c5d1d2dafe5a40695cfc1)
Target: wasm32-unknown-emscripten
Thread model: posix

(the same behavior exists on older versions like 4.0.10)