Walk into pack expansions in this verifier.

Fixes rdar://107664237

Author: rjmccall

lib/SIL/IR/TypeLowering.cpp

@@ -2750,6 +2750,7 @@ bool TypeConverter::visitAggregateLeaves(
   auto isAggregate = [](CanType ty) {
     return isa<SILPackType>(ty) ||
            isa<TupleType>(ty) ||
+           isa<PackExpansionType>(ty) ||
            ty.getEnumOrBoundGenericEnum() ||
            ty.getStructOrBoundGenericStruct();
   };
@@ -2781,6 +2782,10 @@ bool TypeConverter::visitAggregateLeaves(
                                  tupleIndex);
               ++tupleIndex;
             });
+      } else if (auto expansion = dyn_cast<PackExpansionType>(ty)) {
+        insertIntoWorklist(expansion.getPatternType(),
+                           origTy.getPackExpansionPatternType(),
+                           field, index);
       } else if (auto *decl = ty.getStructOrBoundGenericStruct()) {
         for (auto *structField : decl->getStoredProperties()) {
           auto subMap = ty->getContextSubstitutionMap(&M, decl);

test/SILGen/variadic-generic-tuples.swift

@@ -331,3 +331,6 @@ func testFancyTuple_concrete() {
 func testFancyTuple_pack<each T>(values: repeat each T) {
   FancyTuple<Int, String, repeat each T, Bool>(x: (1, "hi", repeat each values, false)).makeTuple()
 }
+
+// rdar://107664237
+func f<each T>() -> (repeat Array<each T>) {}