LibWeb: Always return a rejected Promise for functions which throw

We were previously throwing an exception if the generated code was
throwing an exception before it hit the implementation of the interface.
Instead, we are meant to catch any exception, and wrap that in a
rejected promise.

For example, this was impacting the fixed test in this commit as an
exception was being thrown when invoking WebIDL::convert_to_int<T>
as the given number was out of range, and the [EnforceRange]
extended attribute decorates that attribute.

This same type of case is seen for a few tests in WPT.

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

@@ -1992,6 +1992,14 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@function.name:snakecase@@overload_suffi
     [[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 (function.return_type->name() == "Promise"sv) {
+        function_generator.append(R"~~~(
+    auto steps = [&realm, &vm]() -> JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> {
+        (void)realm;
+)~~~");
+    }
+
     if (is_static_function == StaticFunction::No) {
         function_generator.append(R"~~~(
     auto* impl = TRY(impl_from(vm));
@@ -2053,6 +2061,26 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@function.name:snakecase@@overload_suffi
 )~~~");
     }
 
+    if (function.return_type->name() == "Promise"sv) {
+        // https://webidl.spec.whatwg.org/#dfn-create-operation-function
+        // If we had an exception running the steps and are meant to return a Promise, wrap that exception in a rejected promise.
+        function_generator.append(R"~~~(
+        return retval;
+    };
+
+    auto maybe_retval = steps();
+
+    // And then, if an exception E was thrown:
+    // 1. If op has a return type that is a promise type, then return ! Call(%Promise.reject%, %Promise%, «E»).
+    // 2. Otherwise, end these steps and allow the exception to propagate.
+    // NOTE: We know that this is a Promise return type statically by the IDL.
+    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, *function.return_type, interface);
 
     function_generator.append(R"~~~(

Tests/LibWeb/Text/expected/wpt-import/streams/readable-byte-streams/read-min.any.txt

@@ -6,11 +6,10 @@ Rerun
 
 Found 24 tests
 
-23 Pass
+24 Pass
-1 Fail
 Details
 Result	Test Name	MessagePass	ReadableStream with byte source: read({ min }) rejects if min is 0	
-Fail	ReadableStream with byte source: read({ min }) rejects if min is negative	
+Pass	ReadableStream with byte source: read({ min }) rejects if min is negative	
 Pass	ReadableStream with byte source: read({ min }) rejects if min is larger than view's length (Uint8Array)	
 Pass	ReadableStream with byte source: read({ min }) rejects if min is larger than view's length (Uint16Array)	
 Pass	ReadableStream with byte source: read({ min }) rejects if min is larger than view's length (DataView)