feat(mf2): Always isolate when u:dir is set (unicode-org/message-format-wg#942)

Author: XM5jDcsHTyGJtQqlCi

mf2/messageformat/src/message-value.ts

@@ -0,0 +1,16 @@
+import type { MessageExpressionPart } from './formatted-parts.js';
+
+export const BIDI_ISOLATE = Symbol('bidi-isolate');
+
+export interface MessageValue {
+  readonly type: string;
+  readonly source: string;
+  readonly locale: string;
+  readonly dir?: 'ltr' | 'rtl' | 'auto';
+  readonly options?: Readonly<object>;
+  selectKey?: (keys: Set<string>) => string | null;
+  toParts?: () => MessageExpressionPart[];
+  toString?: () => string;
+  valueOf?: () => unknown;
+  [BIDI_ISOLATE]?: boolean;
+}

mf2/messageformat/src/messageformat.ts

@@ -15,7 +15,7 @@ import {
   string,
   time
 } from './functions/index.js';
-import type { MessageValue } from './message-value.js';
+import { BIDI_ISOLATE, type MessageValue } from './message-value.js';
 import { formatMarkup } from './resolve/format-markup.js';
 import type { MessageFunctionContext } from './resolve/function-context.js';
 import { resolveExpression } from './resolve/resolve-expression.js';
@@ -136,7 +136,7 @@ export class MessageFormat {
           if (typeof mv.toString === 'function') {
             if (
               this.#bidiIsolation &&
-              (this.#dir !== 'ltr' || mv.dir !== 'ltr')
+              (this.#dir !== 'ltr' || mv.dir !== 'ltr' || mv[BIDI_ISOLATE])
             ) {
               const pre = mv.dir === 'ltr' ? LRI : mv.dir === 'rtl' ? RLI : FSI;
               res += pre + mv.toString() + PDI;
@@ -176,7 +176,7 @@ export class MessageFormat {
             const mp = mv.toParts();
             if (
               this.#bidiIsolation &&
-              (this.#dir !== 'ltr' || mv.dir !== 'ltr')
+              (this.#dir !== 'ltr' || mv.dir !== 'ltr' || mv[BIDI_ISOLATE])
             ) {
               const pre = mv.dir === 'ltr' ? LRI : mv.dir === 'rtl' ? RLI : FSI;
               parts.push({ type: 'bidiIsolation', value: pre }, ...mp, {

mf2/messageformat/src/resolve/function-context.ts

@@ -36,7 +36,7 @@ export class MessageFunctionContext {
       const dir = String(resolveValue(ctx, dirOpt));
       if (dir === 'ltr' || dir === 'rtl' || dir === 'auto') {
         this.dir = dir;
-      } else {
+      } else if (dir !== 'inherit') {
         const msg = 'Unsupported value for u:dir option';
         const optSource = getValueSource(dirOpt);
         ctx.onError(new MessageResolutionError('bad-option', msg, optSource));

mf2/messageformat/src/resolve/resolve-function-ref.ts

@@ -7,6 +7,7 @@ import type {
 import { MessageError } from '../errors.js';
 import type { Context } from '../format-context.js';
 import { fallback } from '../functions/fallback.js';
+import { BIDI_ISOLATE } from '../message-value.js';
 import { MessageFunctionContext } from './function-context.js';
 import { getValueSource, resolveValue } from './resolve-value.js';
 
@@ -24,7 +25,7 @@ export function resolveFunctionRef(
     }
     const msgCtx = new MessageFunctionContext(ctx, source, options);
     const opt = resolveOptions(ctx, options);
-    const res = rf(msgCtx, opt, ...fnInput);
+    let res = rf(msgCtx, opt, ...fnInput);
     if (
       res === null ||
       (typeof res !== 'object' && typeof res !== 'function') ||
@@ -36,6 +37,7 @@ export function resolveFunctionRef(
         `Function :${name} did not return a MessageValue`
       );
     }
+    if (msgCtx.dir) res = { ...res, dir: msgCtx.dir, [BIDI_ISOLATE]: true };
     if (msgCtx.id && typeof res.toParts === 'function') {
       return {
         ...res,