[ConstraintSystem] TypeSimplifier: If pattern and shape are packs - produce pattern type

The `transformWithPosition` would handle its flattening.

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -3793,13 +3793,14 @@ struct TypeSimplifier {
     }
 
     if (auto expansion = dyn_cast<PackExpansionType>(type.getPointer())) {
+      auto patternType = expansion->getPatternType();
       // First, let's check whether pattern type has all of the type variables
       // that represent packs resolved, otherwise we don't have enough information
       // to flatten this pack expansion type.
       //
       // Note that we don't actually need to do deep transformation here
       // because pack variables can only appear in structural positions.
-      if (expansion->getPatternType().findIf([&](Type type) {
+      if (patternType.findIf([&](Type type) {
             if (auto *typeVar = type->getAs<TypeVariableType>()) {
               if (typeVar->getImpl().canBindToPack())
                 return GetFixedTypeFn(typeVar)->is<TypeVariableType>();
@@ -3813,6 +3814,12 @@ struct TypeSimplifier {
       auto countType = expansion->getCountType().transform(
           TypeSimplifier(CS, GetFixedTypeFn));
 
+      // If both pattern and count are resolves, let's just return
+      // the pattern type for `transformWithPosition` to take care
+      // of the rest.
+      if (patternType->is<PackType>() && countType->is<PackType>())
+        return patternType;
+
       if (auto countPack = countType->getAs<PackType>()) {
         SmallVector<Type, 4> elts;
         ActivePackExpansions.push_back({false, 0});