Fixing generated code under Windows.

By the standard, std::function can only capture class functions, and not
class methods. For some reasons, this works under gcc however. Visual
Studio rejects it. std::mem_fn is proper template to use.

src/compiler/cpp_generator.cc

@@ -828,9 +828,7 @@ void PrintSourceService(grpc::protobuf::io::Printer *printer,
           "    new ::grpc::RpcMethodHandler< $ns$$Service$::Service, "
           "$Request$, "
           "$Response$>(\n"
-          "        std::function< ::grpc::Status($ns$$Service$::Service*, "
-          "::grpc::ServerContext*, const $Request$*, $Response$*)>("
-          "&$ns$$Service$::Service::$Method$), this),\n"
+          "        std::mem_fn(&$ns$$Service$::Service::$Method$), this),\n"
           "    new $Request$, new $Response$));\n");
     } else if (ClientOnlyStreaming(method)) {
       printer->Print(
@@ -840,10 +838,7 @@ void PrintSourceService(grpc::protobuf::io::Printer *printer,
           "    ::grpc::RpcMethod::CLIENT_STREAMING,\n"
           "    new ::grpc::ClientStreamingHandler< "
           "$ns$$Service$::Service, $Request$, $Response$>(\n"
-          "        std::function< ::grpc::Status($ns$$Service$::Service*, "
-          "::grpc::ServerContext*, "
-          "::grpc::ServerReader< $Request$>*, $Response$*)>("
-          "&$ns$$Service$::Service::$Method$), this),\n"
+          "        std::mem_fn(&$ns$$Service$::Service::$Method$), this),\n"
           "    new $Request$, new $Response$));\n");
     } else if (ServerOnlyStreaming(method)) {
       printer->Print(
@@ -853,10 +848,7 @@ void PrintSourceService(grpc::protobuf::io::Printer *printer,
           "    ::grpc::RpcMethod::SERVER_STREAMING,\n"
           "    new ::grpc::ServerStreamingHandler< "
           "$ns$$Service$::Service, $Request$, $Response$>(\n"
-          "        std::function< ::grpc::Status($ns$$Service$::Service*, "
-          "::grpc::ServerContext*, "
-          "const $Request$*, ::grpc::ServerWriter< $Response$>*)>("
-          "&$ns$$Service$::Service::$Method$), this),\n"
+          "        std::mem_fn(&$ns$$Service$::Service::$Method$), this),\n"
           "    new $Request$, new $Response$));\n");
     } else if (BidiStreaming(method)) {
       printer->Print(
@@ -866,10 +858,7 @@ void PrintSourceService(grpc::protobuf::io::Printer *printer,
           "    ::grpc::RpcMethod::BIDI_STREAMING,\n"
           "    new ::grpc::BidiStreamingHandler< "
           "$ns$$Service$::Service, $Request$, $Response$>(\n"
-          "        std::function< ::grpc::Status($ns$$Service$::Service*, "
-          "::grpc::ServerContext*, "
-          "::grpc::ServerReaderWriter< $Response$, $Request$>*)>("
-          "&$ns$$Service$::Service::$Method$), this),\n"
+          "        std::mem_fn(&$ns$$Service$::Service::$Method$), this),\n"
           "    new $Request$, new $Response$));\n");
     }
   }