[Requirement Machine] Fix the order of terms with pack element symbols to always appear on the left-hand side of the same-element rewrite rule.

Author: simanerush

lib/AST/RequirementMachine/Term.cpp

@@ -136,17 +136,30 @@ static std::optional<int> shortlexCompare(const Symbol *lhsBegin,
                                           RewriteContext &ctx) {
   // First, compare the number of name and pack element symbols.
   unsigned lhsNameCount = 0;
+  unsigned lhsPackElementCount = 0;
   for (auto *iter = lhsBegin; iter != lhsEnd; ++iter) {
     if (iter->getKind() == Symbol::Kind::Name)
       ++lhsNameCount;
+
+    if (iter->getKind() == Symbol::Kind::PackElement)
+      ++lhsPackElementCount;
   }
 
   unsigned rhsNameCount = 0;
+  unsigned rhsPackElementCount = 0;
   for (auto *iter = rhsBegin; iter != rhsEnd; ++iter) {
     if (iter->getKind() == Symbol::Kind::Name)
       ++rhsNameCount;
+
+    if (iter->getKind() == Symbol::Kind::PackElement)
+      ++rhsPackElementCount;
   }
 
+  // A term with more pack element symbols orders after a term with
+  // fewer pack element symbols.
+  if (lhsPackElementCount != rhsPackElementCount)
+    return lhsPackElementCount > rhsPackElementCount ? 1 : -1;
+
   // A term with more name symbols orders after a term with fewer name symbols.
   if (lhsNameCount != rhsNameCount)
     return lhsNameCount > rhsNameCount ? 1 : -1;