> the main point was that "give me a constant value" should be at the usage site, not at the function definition.
The issue is, not everything can be done at compile time, and so “I can use this at compile time” becomes part of the signature because you want to ensure that it will continue to be able to be used that way. Without it, changes in your function could easily break your callers.
Exactly right. There's a huge benefit to encode the ability for compile-time evaluation in the signature of the function itself. Much better than doing it "ad-hoc", like how template instantiation does it. Sometimes it will work, sometimes it doesn't. constexpr functions always work.
The issue is, not everything can be done at compile time, and so “I can use this at compile time” becomes part of the signature because you want to ensure that it will continue to be able to be used that way. Without it, changes in your function could easily break your callers.