use C++98-compatible template brackets

Status Job ID Name Coverage
  Test
passed #306
Linux
clang-c++11

00:12:21

passed #308
Linux
clang-c++98

00:10:07

passed #305
Linux
gcc-c++11

00:19:11

passed #307
Linux
gcc-c++98

00:13:43

passed #309
Apple
macos-c++11

00:05:49

passed #310
Apple
macos-c++98

00:05:32