[PreCheckExpr] Gate PackExpansionExpr creation behind the variadic generics flag.

Author: hborla

lib/Sema/CSDiagnostics.cpp

@@ -7059,6 +7059,7 @@ bool ExtraneousCallFailure::diagnoseAsError() {
     }
   }
 
+  anchor.dump();
   auto diagnostic =
       emitDiagnostic(diag::cannot_call_non_function_value, getType(anchor));
   removeParensFixIt(diagnostic);

lib/Sema/PreCheckExpr.cpp

@@ -1378,7 +1378,8 @@ namespace {
       // references to PackExpansionExpr.
       if (auto *postfixExpr = dyn_cast<PostfixUnaryExpr>(expr)) {
         auto *op = dyn_cast<OverloadedDeclRefExpr>(postfixExpr->getFn());
-        if (op && op->getDecls()[0]->getBaseName().getIdentifier().isExpansionOperator()) {
+        if (op && Ctx.LangOpts.hasFeature(Feature::VariadicGenerics) &&
+            op->getDecls()[0]->getBaseName().getIdentifier().isExpansionOperator()) {
           auto *operand = postfixExpr->getOperand();
           if (auto *expansion = getPackExpansion(DC, operand, op->getLoc())) {
             return Action::Continue(expansion);