How to export a C API from my C++ shared lib that my C-only executable can link without linker error "undefined reference"?

[metaleap]

Trying to link a C++ shared lib with a minimal (1 function) extern "C" API into a C executable — my meson.build begins like so:

project('test',['cpp', 'c'],default_options: ['cpp_std=c++20', 'c_std=c23', 'cpp_rtti=false'])

The C++ shared lib:

// mylib.h
#pragma once
#ifdef __cplusplus
    extern "C"
#endif /* isocpp.org/wiki/faq/mixing-c-and-cpp#call-cpp */
    void myTest();

// mylib.cpp
#include <cstdio>
void myTest() {
  printf("Hello!\n");
}

# meson.build
mylib = shared_library(
    'mylib',
    sources: ['mylib.cpp'],
)

The C executable:

# meson.build
executable(
    'test.exec',
    link_with: [mylib],
    implicit_include_directories: true,
    # link_language: 'c', # problem persists with this both on or off
    sources: ['main.c'],
)

// main.c
#include "mylib.h"
int main() {
  myTest();
}

The problem:

The meson compile step ends with this linker error:

[1/2] Linking target test.exec
FAILED: test.exec 
c++  -o test.exec test.exec.p/main.c.o -Wl,--as-needed -Wl,--no-undefined '-Wl,-rpath,$ORIGIN/' -Wl,-rpath-link,/home/_/c/c/cWicked/.build/debug/ -Wl,--start-group mylib.so /usr/lib/libSDL2.so -Wl,--end-group -pthread
/usr/bin/ld: test.exec.p/main.c.o: in function `main':
/home/_/c/c/cWicked/.build/debug/../../main.c:4:(.text+0x5): undefined reference to `myTest'

Well, the executable has a link_with and the main.c has an #include, what could still be missing here then?

[metaleap]

#include "mylib.h" was missing in mylib.cpp...