LibWeb: Do not throw exceptions from Promise-type attributes

If an attribute's getter were to throw an exception, we must instead
return a rejected promise. We already supported this behavior for
functions.

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -3729,8 +3729,19 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.getter_callback@)
 {
     WebIDL::log_trace(vm, "@class_name@::@attribute.getter_callback@");
     [[maybe_unused]] auto& realm = *vm.current_realm();
+)~~~");
+
+        // NOTE: Create a wrapper lambda so that if the function steps return an exception, we can return that in a rejected promise.
+        if (attribute.type->name() == "Promise"sv) {
+            attribute_generator.append(R"~~~(
+    auto steps = [&]() -> JS::ThrowCompletionOr<GC::Ptr<WebIDL::Promise>> {
+)~~~");
+        }
+
+        attribute_generator.append(R"~~~(
     [[maybe_unused]] auto* impl = TRY(impl_from(vm));
 )~~~");
+
         if (attribute.extended_attributes.contains("CEReactions")) {
             // 1. Push a new element queue onto this object's relevant agent's custom element reactions stack.
             attribute_generator.append(R"~~~(
@@ -4069,6 +4080,23 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.getter_callback@)
             }
         }
 
+        if (attribute.type->name() == "Promise"sv) {
+            attribute_generator.append(R"~~~(
+        return retval;
+    };
+
+    auto maybe_retval = steps();
+
+    // And then, if an exception E was thrown:
+    // 1. If attribute’s type is a promise type, then return ! Call(%Promise.reject%, %Promise%, «E»).
+    // 2. Otherwise, end these steps and allow the exception to propagate.
+    if (maybe_retval.is_throw_completion())
+        return WebIDL::create_rejected_promise(realm, maybe_retval.error_value())->promise();
+
+    auto retval = maybe_retval.release_value();
+)~~~");
+        }
+
         generate_return_statement(generator, *attribute.type, interface);
 
         attribute_generator.append(R"~~~(

Tests/LibWeb/Text/expected/wpt-import/streams/idlharness.any.txt

@@ -2,8 +2,8 @@ Harness status: OK
 
 Found 229 tests
 
-224 Pass
+228 Pass
-5 Fail
+1 Fail
 Pass	idl_test setup
 Pass	idl_test validation
 Pass	ReadableStreamDefaultReader includes ReadableStreamGenericReader: member names are unique
@@ -44,7 +44,7 @@ Pass	ReadableStreamDefaultReader interface: existence and properties of interfac
 Pass	ReadableStreamDefaultReader interface: existence and properties of interface prototype object's @@unscopables property
 Pass	ReadableStreamDefaultReader interface: operation read()
 Pass	ReadableStreamDefaultReader interface: operation releaseLock()
-Fail	ReadableStreamDefaultReader interface: attribute closed
+Pass	ReadableStreamDefaultReader interface: attribute closed
 Pass	ReadableStreamDefaultReader interface: operation cancel(optional any)
 Pass	ReadableStreamDefaultReader must be primary interface of (new ReadableStream()).getReader()
 Pass	Stringification of (new ReadableStream()).getReader()
@@ -61,7 +61,7 @@ Pass	ReadableStreamBYOBReader interface: existence and properties of interface p
 Pass	ReadableStreamBYOBReader interface: existence and properties of interface prototype object's @@unscopables property
 Pass	ReadableStreamBYOBReader interface: operation read(ArrayBufferView, optional ReadableStreamBYOBReaderReadOptions)
 Pass	ReadableStreamBYOBReader interface: operation releaseLock()
-Fail	ReadableStreamBYOBReader interface: attribute closed
+Pass	ReadableStreamBYOBReader interface: attribute closed
 Pass	ReadableStreamBYOBReader interface: operation cancel(optional any)
 Pass	ReadableStreamBYOBReader must be primary interface of (new ReadableStream({ type: 'bytes' })).getReader({ mode: 'byob' })
 Pass	Stringification of (new ReadableStream({ type: 'bytes' })).getReader({ mode: 'byob' })
@@ -148,9 +148,9 @@ Pass	WritableStreamDefaultWriter interface object name
 Pass	WritableStreamDefaultWriter interface: existence and properties of interface prototype object
 Pass	WritableStreamDefaultWriter interface: existence and properties of interface prototype object's "constructor" property
 Pass	WritableStreamDefaultWriter interface: existence and properties of interface prototype object's @@unscopables property
-Fail	WritableStreamDefaultWriter interface: attribute closed
+Pass	WritableStreamDefaultWriter interface: attribute closed
 Pass	WritableStreamDefaultWriter interface: attribute desiredSize
-Fail	WritableStreamDefaultWriter interface: attribute ready
+Pass	WritableStreamDefaultWriter interface: attribute ready
 Pass	WritableStreamDefaultWriter interface: operation abort(optional any)
 Pass	WritableStreamDefaultWriter interface: operation close()
 Pass	WritableStreamDefaultWriter interface: operation releaseLock()