Error out on rval `emscripten::val` assignment Trying to assign properties via naturally-looking syntax `obj["foo"] = ...` bites users because, rather than assigning a JS property as they might expect, it actually changes only a `val` instance that temporarily resides on the stack and isn't accessible afterwards. This change makes such assignment a compilation error, making it easier to catch the misuse.

diff --git a/system/include/emscripten/val.h b/system/include/emscripten/val.h
index 413bee7..bb0ed2b 100644
--- a/system/include/emscripten/val.h
+++ b/system/include/emscripten/val.h

@@ -410,14 +410,14 @@
             return handle;
         }
 
-        val& operator=(val&& v) {
+        val& operator=(val&& v) & {
             internal::_emval_decref(handle);
             handle = v.handle;
             v.handle = 0;
             return *this;
         }
 
-        val& operator=(const val& v) {
+        val& operator=(const val& v) & {
             internal::_emval_incref(v.handle);
             internal::_emval_decref(handle);
             handle = v.handle;

diff --git a/tests/embind/test_val_assignment.cpp b/tests/embind/test_val_assignment.cpp
new file mode 100644
index 0000000..51fdda7
--- /dev/null
+++ b/tests/embind/test_val_assignment.cpp

@@ -0,0 +1,8 @@
+#include <emscripten/val.h>
+
+using emscripten::val;
+
+int main() {
+  val obj = val::object();
+  obj["foo"] = val(42);
+}

diff --git a/tests/test_core.py b/tests/test_core.py
index 9329433..e2427f3 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py

@@ -6973,6 +6973,10 @@
     self.emcc_args += ['--bind']
     self.do_run_in_out_file_test('embind/test_val.cpp')
 
+  def test_embind_val_assignment(self):
+    err = self.expect_fail([EMCC, test_file('embind/test_val_assignment.cpp'), '--bind', '-c'])
+    self.assertContained('candidate function not viable: expects an lvalue for object argument', err)
+
   @no_wasm2js('wasm_bigint')
   def test_embind_i64_val(self):
     self.set_setting('WASM_BIGINT')