LibJS: Ensure GetDateTimeFormat returns nonnull for the best format

This is an editorial change in the Temporal proposal. See:
https://github.com/tc39/proposal-temporal/commit/8a52801

Libraries/LibJS/Runtime/Intl/DateTimeFormat.cpp

@@ -373,11 +373,9 @@ Optional<Unicode::CalendarPattern> get_date_time_format(Unicode::CalendarPattern
 
     // 17. If needDefaults is true, then
     if (need_defaults) {
-        // a. If anyPresent is true, return null.
-        if (any_present) {
-            // FIXME: Spec issue: We can hit this when setting `bestFormat`, which should never be null. Don't return for now.
-            //        https://github.com/tc39/proposal-temporal/issues/3049
-        }
+        // a. If anyPresent is true and inherit is RELEVANT, return null.
+        if (any_present && inherit == OptionInherit::Relevant)
+            return {};
 
         // b. For each property name prop of defaultOptions, do
         options.for_each_calendar_field_zipped_with(format_options, default_options, [&](auto const&, auto& format_option) {

Libraries/LibJS/Tests/builtins/Intl/DateTimeFormat/DateTimeFormat.prototype.format.js

@@ -75,6 +75,22 @@ describe("errors", () => {
             "Cannot format Temporal.ZonedDateTime, use Temporal.ZonedDateTime.prototype.toLocaleString"
         );
     });
+
+    test("Temporal fields must overlap formatter", () => {
+        const yearFormatter = new Intl.DateTimeFormat([], { year: "numeric", calendar: "iso8601" });
+        const plainMonthDay = new Temporal.PlainMonthDay(1, 1);
+
+        const dayFormatter = new Intl.DateTimeFormat([], { day: "numeric", calendar: "iso8601" });
+        const plainYearMonth = new Temporal.PlainYearMonth(1972, 1);
+
+        expect(() => {
+            yearFormatter.format(plainMonthDay);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainMonthDay");
+
+        expect(() => {
+            dayFormatter.format(plainYearMonth);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainYearMonth");
+    });
 });
 
 const d0 = Date.UTC(2021, 11, 7, 17, 40, 50, 456);

Libraries/LibJS/Tests/builtins/Intl/DateTimeFormat/DateTimeFormat.prototype.formatRange.js

@@ -112,6 +112,22 @@ describe("errors", () => {
             "Cannot format a date-time range with different date-time types"
         );
     });
+
+    test("Temporal fields must overlap formatter", () => {
+        const yearFormatter = new Intl.DateTimeFormat([], { year: "numeric", calendar: "iso8601" });
+        const plainMonthDay = new Temporal.PlainMonthDay(1, 1);
+
+        const dayFormatter = new Intl.DateTimeFormat([], { day: "numeric", calendar: "iso8601" });
+        const plainYearMonth = new Temporal.PlainYearMonth(1972, 1);
+
+        expect(() => {
+            yearFormatter.formatRange(plainMonthDay, plainMonthDay);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainMonthDay");
+
+        expect(() => {
+            dayFormatter.formatRange(plainYearMonth, plainYearMonth);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainYearMonth");
+    });
 });
 
 const d0 = Date.UTC(1989, 0, 23, 7, 8, 9, 45);

Libraries/LibJS/Tests/builtins/Intl/DateTimeFormat/DateTimeFormat.prototype.formatRangeToParts.js

@@ -112,6 +112,22 @@ describe("errors", () => {
             "Cannot format a date-time range with different date-time types"
         );
     });
+
+    test("Temporal fields must overlap formatter", () => {
+        const yearFormatter = new Intl.DateTimeFormat([], { year: "numeric", calendar: "iso8601" });
+        const plainMonthDay = new Temporal.PlainMonthDay(1, 1);
+
+        const dayFormatter = new Intl.DateTimeFormat([], { day: "numeric", calendar: "iso8601" });
+        const plainYearMonth = new Temporal.PlainYearMonth(1972, 1);
+
+        expect(() => {
+            yearFormatter.formatRangeToParts(plainMonthDay, plainMonthDay);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainMonthDay");
+
+        expect(() => {
+            dayFormatter.formatRangeToParts(plainYearMonth, plainYearMonth);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainYearMonth");
+    });
 });
 
 const d0 = Date.UTC(1989, 0, 23, 7, 8, 9, 45);

Libraries/LibJS/Tests/builtins/Intl/DateTimeFormat/DateTimeFormat.prototype.formatToParts.js

@@ -71,6 +71,22 @@ describe("errors", () => {
             "Cannot format Temporal.ZonedDateTime, use Temporal.ZonedDateTime.prototype.toLocaleString"
         );
     });
+
+    test("Temporal fields must overlap formatter", () => {
+        const yearFormatter = new Intl.DateTimeFormat([], { year: "numeric", calendar: "iso8601" });
+        const plainMonthDay = new Temporal.PlainMonthDay(1, 1);
+
+        const dayFormatter = new Intl.DateTimeFormat([], { day: "numeric", calendar: "iso8601" });
+        const plainYearMonth = new Temporal.PlainYearMonth(1972, 1);
+
+        expect(() => {
+            yearFormatter.formatToParts(plainMonthDay);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainMonthDay");
+
+        expect(() => {
+            dayFormatter.formatToParts(plainYearMonth);
+        }).toThrowWithMessage(TypeError, "Unable to determine format for Temporal.PlainYearMonth");
+    });
 });
 
 const d = Date.UTC(1989, 0, 23, 7, 8, 9, 45);