D Templates can take just about anything in the language as an argument (user defined, builtin, doesn't matter) and I can confirm that being able to do this is

D Templates can take just about anything in the language as an argument (user defined, builtin, doesn't matter) and I can confirm that being able to do this is extremely useful i.e. if you write a hybrid allocator you can specify the metaparameters as a template parameter. e.g. struct SmallString(size_t N) { /* etc. */ }