I've only cursory of Zig, so bear with me. Does this mean that Zig templates are like C++, that is, duck typing? Or is there a notion of constraining the type arguments?

It's duck typed but it's not based on a funky and limited syntax, but rather types at comptime become normal arguments to functions that you can then inspect using normal Zig code.

I wrote a blog post about comptime if you want to learn more: https://kristoff.it/blog/what-is-zig-comptime/

It's duck typed but let's say your "template" takes an integer and divides by two depending on some boolean (perhaps you need to return a lookup table of sometimes half the bitwidth of an integer type). That function called by your template at compile time to maybe-divide-by-two is in the same language as runtime zig, and you could even conceivably call that very same function as a compiled entity in the runtime context.

You can constrain type arguments with a compile-time `if` check, and generate a compile error on violation.