feat(mf2): Add duplicate-variant error (unicode-org/message-format-wg#853)

Author: XM5jDcsHTyGJtQqlCi

packages/mf2-messageformat/src/data-model/validate.ts

@@ -28,6 +28,9 @@ import { visit } from './visit.js';
  * - **Duplicate Option Name**:
  *   The same _identifier_ appears as the name of more than one _option_ in the same _expression_.
  *
+ * - **Duplicate Variant**:
+ *   The same list of _keys_ is used for more than one _variant_.
+ *
  * @returns The sets of runtime `functions` and `variables` used by the message.
  * @beta
  */
@@ -52,6 +55,7 @@ export function validate(
   const functions = new Set<string>();
   const localVars = new Set<string>();
   const variables = new Set<string>();
+  const variants = new Set<string>();
 
   let setArgAsDeclared = true;
   visit(msg, {
@@ -116,6 +120,11 @@ export function validate(
     variant(variant) {
       const { keys } = variant;
       if (keys.length !== selectorCount) onError('key-mismatch', variant);
+      const strKeys = JSON.stringify(
+        keys.map(key => (key.type === 'literal' ? key.value : 0))
+      );
+      if (variants.has(strKeys)) onError('duplicate-variant', variant);
+      else variants.add(strKeys);
       missingFallback &&= keys.every(key => key.type === '*') ? null : variant;
     }
   });

packages/mf2-messageformat/src/errors.ts

@@ -32,6 +32,7 @@ export class MessageSyntaxError extends MessageError {
     | 'bad-input-expression'
     | 'duplicate-declaration'
     | 'duplicate-option'
+    | 'duplicate-variant'
     | 'extra-content'
     | 'key-mismatch'
     | 'parse-error'
@@ -64,6 +65,7 @@ export class MessageDataModelError extends MessageSyntaxError {
   declare type:
     | 'duplicate-declaration'
     | 'duplicate-option'
+    | 'duplicate-variant'
     | 'key-mismatch'
     | 'missing-fallback'
     | 'missing-selector-annotation';

test/mfwg-test-utils.ts

@@ -47,6 +47,7 @@ export function testName({ src, locale, params }: Test) {
 const dataModelErrors = [
   'duplicate-declaration',
   'duplicate-option-name',
+  'duplicate-variant',
   'missing-fallback-variant',
   'missing-selector-annotation',
   'variant-key-mismatch'