[ConstraintSystem] Implement basic type checking for pack expressions.

During prechecking, postfix '...' expressions are rewritten to pack
expansion expressions if the operand contains pack references. References
to packs are replaced with opaque values, and the pack expansion stores
the opaque value bindings to decl refs.

lib/Sema/CSGen.cpp

@@ -1770,6 +1770,13 @@ namespace {
     }
 
     virtual Type visitParenExpr(ParenExpr *expr) {
+      // If the ParenExpr contains a pack expansion, generate a tuple
+      // type containing the pack expansion type.
+      if (CS.getType(expr->getSubExpr())->getAs<PackExpansionType>()) {
+        return TupleType::get({CS.getType(expr->getSubExpr())},
+                              CS.getASTContext());
+      }
+
       if (auto favoredTy = CS.getFavoredType(expr->getSubExpr())) {
         CS.setFavoredType(expr, favoredTy);
       }
@@ -2874,7 +2881,19 @@ namespace {
     }
 
     Type visitPackExpansionExpr(PackExpansionExpr *expr) {
-      llvm_unreachable("not implemented for PackExpansionExpr");
+      for (auto *binding : expr->getBindings()) {
+        auto type = visit(binding);
+        CS.setType(binding, type);
+      }
+
+      auto patternTy = CS.getType(expr->getPatternExpr());
+
+      // FIXME: Use a ShapeOf constraint here.
+      auto *declRef = dyn_cast<DeclRefExpr>(expr->getBindings().front());
+      auto *decl = dyn_cast<VarDecl>(declRef->getDecl());
+      auto shapeType = decl->getType()->getAs<PackExpansionType>()->getCountType();
+
+      return PackExpansionType::get(patternTy, shapeType);
     }
 
     Type visitDynamicTypeExpr(DynamicTypeExpr *expr) {
@@ -2887,7 +2906,6 @@ namespace {
     }
 
     Type visitOpaqueValueExpr(OpaqueValueExpr *expr) {
-      assert(expr->isPlaceholder() && "Already type checked");
       return expr->getType();
     }