parser fix

Author: catamorphism

icu4c/source/i18n/messageformat2_parser.cpp

@@ -1973,7 +1973,8 @@ void Parser::parseSelectors(UErrorCode& status) {
     // "Backtracking" is required here. It's not clear if whitespace is
     // (`[s]` selector) or (`[s]` variant)
     while (isWhitespace(peek()) || peek() == DOLLAR) {
-        parseOptionalWhitespace();
+        int32_t whitespaceStart = index;
+        parseRequiredWhitespace(status);
         // Restore precondition
         CHECK_BOUNDS(status);
         if (peek() != DOLLAR) {
@@ -2004,6 +2005,9 @@ void Parser::parseSelectors(UErrorCode& status) {
             break;                                 \
         }                                          \
 
+    // Parse required whitespace before first variant
+    parseRequiredWhitespace(status);
+
     // Parse variants
     while (isWhitespace(peek()) || isBidiControl(peek()) || isKeyStart(peek())) {
         // Trailing whitespace is allowed