Update embind nortti test and documentation. NFC

Automatic downcasting of polymorphic types in embind has been broken
without rtti since emscripten-core#10914 landed.

This test update confirms this brokenness rather than fixing it since
this has been the behavior since 2020.

Fixes: emscripten-core#21619

site/source/docs/porting/connecting_cpp_and_javascript/embind.rst

@@ -672,6 +672,8 @@ are available.
 .. note:: *Embind* must understand the fully-derived type for automatic
    downcasting to work.
 
+.. note:: *Embind* does not support this unless RTTI is enabled.
+
 
 Overloaded functions
 ====================

test/core/test_embind_polymorphic_class_no_rtti.cpp

@@ -9,35 +9,64 @@
 #include <stdio.h>
 
 EM_JS(void, calltest, (), {
-  var foo = new Module["Foo"]();
+  var foo = new Module.Foo();
+  var fooRaw = Module.FooRaw();
   console.log("foo.test() returned: " + foo.test());
+  console.log("foo.getClassName() -> " + foo.getClassName());
+  console.log("fooRaw.getClassName() -> " + fooRaw.getClassName());
+#if __has_feature(cxx_rtti)
+  // When RTTI is available we do automatic downcasting
+  assert(foo.getClassName() == "Bar");
+  assert(fooRaw.getClassName() == "Bar");
+#else
+  // When RTTI is not available we cannot do automatic downcasting
+  assert(foo.getClassName() == "Foo");
+  assert(fooRaw.getClassName() == "Foo");
+#endif
   foo.delete();
 });
 
 class Foo {
   public:
   virtual ~Foo() = default;
   virtual int test() = 0;
+  std::string getClassName() {
+    return "Foo";
+  }
 };
 
 class Bar : public Foo {
   public:
   int test() override { return 42; }
+  std::string getClassName() {
+    return "Bar";
+  }
 };
 
 int main(int argc, char** argv){
-  printf("Hello, world.\n");
+  printf("in main\n");
   calltest();
+  printf("calltest done\n");
   return 0;
 }
 
-std::shared_ptr<Foo> MakeFoo() {
+Foo* makeFooRaw() {
+  return new Bar();
+}
+
+std::shared_ptr<Foo> makeFoo() {
   return std::make_shared<Bar>();
 }
 
+using namespace emscripten;
+
 EMSCRIPTEN_BINDINGS(my_module) {
+  function("FooRaw", &makeFooRaw, allow_raw_pointers());
   emscripten::class_<Foo>("Foo")
-    .smart_ptr_constructor("Foo", &MakeFoo)
+    .smart_ptr_constructor("Foo", &makeFoo)
     .function("test", &Foo::test)
+    .function("getClassName", &Foo::getClassName)
     ;
+  class_<Bar, base<Foo>>("Bar")
+    .function("getClassName", &Bar::getClassName);
 };

test/core/test_embind_polymorphic_class_no_rtti.out

@@ -1,2 +1,2 @@
-Hello, world.
+in main
 foo.test() returned: 42

test/test_core.py

@@ -7580,9 +7580,13 @@ def test_embind_no_rtti(self):
     self.emcc_args += ['-lembind', '-fno-rtti', '-DEMSCRIPTEN_HAS_UNBOUND_TYPE_NAMES=0']
     self.do_runf('main.cpp', '418\ndotest returned: 42\n')
 
-  def test_embind_polymorphic_class_no_rtti(self):
-    self.emcc_args += ['-lembind', '-fno-rtti', '-DEMSCRIPTEN_HAS_UNBOUND_TYPE_NAMES=0']
-    self.do_core_test('test_embind_polymorphic_class_no_rtti.cpp')
+  @no_2gb('https://github.com/emscripten-core/emscripten/issues/21633')
+  @parameterized({
+    '': ([],),
+    'no_rtti': (['-fno-rtti', '-DEMSCRIPTEN_HAS_UNBOUND_TYPE_NAMES=0'],),
+  })
+  def test_embind_polymorphic_class(self, args):
+    self.do_core_test('test_embind_polymorphic_class_no_rtti.cpp', emcc_args=args + ['-lembind'])
 
   def test_embind_no_rtti_followed_by_rtti(self):
     src = r'''