Please pass the string directly to log::error, without std::ostringstream{} <<. (ostringstream is only necessary when a string needs to be built by << insertions). So please just:

log::error("File IO not supported in WebAssembly builds.");

BTW, maybe an itkExceptionMacro("File IO not supported in WebAssembly builds."); would be more appropriate, right?

Yes, we could do both -- WebAssembly has limited C++ exception support at the moment, so it will help to also log an error message.

Thanks, can you please explain what you mean by limited C++ exception support?

The current Emscripten toolchain adds a JavaScript shim for C++ exceptions, and I added a shim for C++ exceptions in WASI that calls abort();. There is newer, experimental WASM-exceptions in some newer toolchains, but I have not enabled them by default.

Does WASI support multi-threading? Specifically, will this ComputeImageExtremaFilter run using multiple threads in parallel?

Currently, the default WASI toolchain does not support multi-threading. This is being improved, but we need these for now.

Sorry, still not very happy about having #ifndef __wasi__ all over the place. Would it be an idea to have a "dummy" mutex class (that does not do anything) for single-threaded builds.

#ifdef ELX_SINGLE_THREADED_BUILD
  using ElastixMutexType = DummyMutex;  // Mutex-like dummy type that does nothing
#else
  using ElastixMutexType = std::mutex;
#endif 

Sorry, I do not see how a dummy mutex type addresses the std::lock_guard and the mutex class members.

std::lock_guard should be able to support any "mutex-like" type, that has a lock() and an unlock() member function. So for example, would the following code compile on WASI? https://godbolt.org/z/oP5T6Pn35 Can you please try that out?

#define ELX_SINGLE_THREADED // For WebAssembly Support

#include <mutex>

// Mutex for single-threading, does nothing.
class SingleThreadedMutex
{
  public:
    void lock() {}
    void unlock() {}
};

// The MutexType type alias:
#ifdef ELX_SINGLE_THREADED
using MutexType = SingleThreadedMutex;
#else
using MutexType = std::mutex;
#endif

// Use case:
MutexType mutexVariable;
std::lock_guard<MutexType> lockGuard(mutexVariable);

@thewtex Would it be helpful to you if I would implement such a SingleThreadedMutex for elastix, that would replace std::mutex when a CMake option like ELX_SINGLE_THREADED would be ON?

Sorry to disagree about the subject line, "BUG: Remove unused itkImageFileCastWriter includes". Each of the three hxx files modified by this commit does use "itkImageFileCastWriter.h". Specifically, each of them has a call to itk::WriteCastedImage. It's certainly not a bug.

The way the include was added causes build errors when the include needs to avoided and causes build errors. This is a bug. BUG here does not refer to the use case referenced.