fix(mf2): Use last rather than first variable name for fallback (unicode-org/message-format-wg#903)

Author: XM5jDcsHTyGJtQqlCi

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

@@ -15,17 +15,9 @@ export function resolveFunctionRef(
   operand: Literal | VariableRef | undefined,
   { name, options }: FunctionRef
 ) {
-  let source: string | undefined;
+  const source = getValueSource(operand) ?? `:${name}`;
   try {
-    let fnInput: [unknown] | [];
-    if (operand) {
-      fnInput = [resolveValue(ctx, operand)];
-      source = getValueSource(operand);
-    } else {
-      fnInput = [];
-    }
-    source ??= `:${name}`;
-
+    const fnInput = operand ? [resolveValue(ctx, operand)] : [];
     const rf = ctx.functions[name];
     if (!rf) {
       throw new MessageError('unknown-function', `Unknown function :${name}`);
@@ -57,7 +49,6 @@ export function resolveFunctionRef(
     return res;
   } catch (error) {
     ctx.onError(error);
-    source ??= getValueSource(operand) ?? `:${name}`;
     return fallback(source);
   }
 }

mf2/messageformat/src/resolve/resolve-value.ts

@@ -26,7 +26,9 @@ export function getValueSource(
 export function getValueSource(value: Literal | VariableRef | undefined) {
   switch (value?.type) {
     case 'literal':
-      return '|' + value.value + '|';
+      return (
+        '|' + value.value.replaceAll('\\', '\\\\').replaceAll('|', '\\|') + '|'
+      );
     case 'variable':
       return '$' + value.name;
     default:

mf2/messageformat/src/resolve/resolve-variable.ts

@@ -84,6 +84,7 @@ export function resolveVariableRef(ctx: Context, ref: VariableRef) {
   let type = typeof value;
   if (type === 'object') {
     const mv = value as MessageValue;
+    if (mv.type === 'fallback') return fallback(source);
     if (ctx.localVars.has(mv)) return mv;
     if (value instanceof Number) type = 'number';
     else if (value instanceof String) type = 'string';