LibJS: Start implementing Temporal.PlainYearMonth

This commit adds the PlainYearMonth object itself, its constructor and
prototype (currently empty), and the CreateTemporalYearMonth and
ISOYearMonthWithinLimits abstract operations.

Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthConstructor.cpp

@@ -0,0 +1,109 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Temporal/Calendar.h>
+#include <LibJS/Runtime/Temporal/PlainYearMonth.h>
+#include <LibJS/Runtime/Temporal/PlainYearMonthConstructor.h>
+
+namespace JS::Temporal {
+
+// 9.1 The Temporal.PlainYearMonth Constructor, https://tc39.es/proposal-temporal/#sec-temporal-plainyearmonth-constructor
+PlainYearMonthConstructor::PlainYearMonthConstructor(GlobalObject& global_object)
+    : NativeFunction(vm().names.PlainYearMonth.as_string(), *global_object.function_prototype())
+{
+}
+
+void PlainYearMonthConstructor::initialize(GlobalObject& global_object)
+{
+    NativeFunction::initialize(global_object);
+
+    auto& vm = this->vm();
+
+    // 9.2.1 Temporal.PlainYearMonth.prototype, https://tc39.es/proposal-temporal/#sec-temporal-plainyearmonth-prototype
+    define_direct_property(vm.names.prototype, global_object.temporal_plain_year_month_prototype(), 0);
+
+    define_direct_property(vm.names.length, Value(2), Attribute::Configurable);
+}
+
+// 9.1.1 Temporal.PlainYearMonth ( isoYear, isoMonth [ , calendarLike [ , referenceISODay ] ] ), https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth
+Value PlainYearMonthConstructor::call()
+{
+    auto& vm = this->vm();
+
+    // 1. If NewTarget is undefined, throw a TypeError exception.
+    vm.throw_exception<TypeError>(global_object(), ErrorType::ConstructorWithoutNew, "Temporal.PlainYearMonth");
+    return {};
+}
+
+// 9.1.1 Temporal.PlainYearMonth ( isoYear, isoMonth [ , calendarLike [ , referenceISODay ] ] ), https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth
+Value PlainYearMonthConstructor::construct(FunctionObject& new_target)
+{
+    auto& vm = this->vm();
+    auto& global_object = this->global_object();
+
+    auto iso_year = vm.argument(0);
+    auto iso_month = vm.argument(1);
+    auto calendar_like = vm.argument(2);
+    auto reference_iso_day = vm.argument(3);
+
+    // 2. If referenceISODay is undefined, then
+    if (reference_iso_day.is_undefined()) {
+        // a. Set referenceISODay to 1𝔽.
+        reference_iso_day = Value(1);
+    }
+
+    // 3. Let y be ? ToIntegerOrInfinity(isoYear).
+    auto y = iso_year.to_integer_or_infinity(global_object);
+    if (vm.exception())
+        return {};
+
+    // 4. If y is +∞ or -∞, throw a RangeError exception.
+    if (Value(y).is_infinity()) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::TemporalInvalidPlainYearMonth);
+        return {};
+    }
+
+    // 5. Let m be ? ToIntegerOrInfinity(isoMonth).
+    auto m = iso_month.to_integer_or_infinity(global_object);
+    if (vm.exception())
+        return {};
+
+    // 6. If m is +∞ or -∞, throw a RangeError exception.
+    if (Value(m).is_infinity()) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::TemporalInvalidPlainYearMonth);
+        return {};
+    }
+
+    // 7. Let calendar be ? ToTemporalCalendarWithISODefault(calendarLike).
+    auto* calendar = to_temporal_calendar_with_iso_default(global_object, calendar_like);
+    if (vm.exception())
+        return {};
+
+    // 8. Let ref be ? ToIntegerOrInfinity(referenceISODay).
+    auto ref = reference_iso_day.to_integer_or_infinity(global_object);
+    if (vm.exception())
+        return {};
+
+    // 9. If ref is +∞ or -∞, throw a RangeError exception.
+    if (Value(ref).is_infinity()) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::TemporalInvalidPlainYearMonth);
+        return {};
+    }
+
+    // IMPLEMENTATION DEFINED: This is an optimization that allows us to treat these doubles as normal integers from this point onwards.
+    // This does not change the exposed behaviour as the call to CreateTemporalYearMonth will immediately check that these values are valid
+    // ISO values (for years: -273975 - 273975, for months: 1 - 12, for days: 1 - 31) all of which are subsets of this check.
+    if (!AK::is_within_range<i32>(y) || !AK::is_within_range<u8>(m) || !AK::is_within_range<u8>(ref)) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::TemporalInvalidPlainYearMonth);
+        return {};
+    }
+
+    // 10. Return ? CreateTemporalYearMonth(y, m, calendar, ref, NewTarget).
+    return create_temporal_year_month(global_object, y, m, *calendar, ref, &new_target);
+}
+
+}