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constexpr means running code at compile time. template means duplicating lots of code lots of times. these are not cheap.

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Yeah this is the very first time I am hearing that templates are "extremely cheap". Template instantiation is pretty much where my project spends all of its compilation time.

It depends on what you are instantiating and how often you're doing so. Most people write templates in header files and instantiate them repeatedly in many many TUs.

In many cases it's possible to only declare the template in the header, explicitly instantiate it with a bunch of types in a single TU, and just find those definitions via linker.

On the few times that I have looked at clang traces to try to speed up the build (which has never succeeded) the template instantiation mess largely arose from Abseil or libc++, which I can't do much about.

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