fix template default errors and other warnings

[nathanhhughes]

@Schmluk switched to use clang for some testing I'm doing downstream and found out that redeclaring template defaults is a compiler error (and found some other warnings). Might be worth considering a clang-based github action workflow at some point

[Schmluk]

LGTM!

Interesting that clang throws more warnings than GCC. For adding clang CI would you make the build fail on warnings? Or just to make sure it also compiles with clang? Feel free to add an additional check for clang compilation if you think that helps!

[nathanhhughes]

Just to make sure it compiles (though the extra warnings clang offers are nice / kinda useful). I'll make a note to add the clang workflow, but it'll likely be a little while before I actually get around to it