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> 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.

I like const because I can look at a function signature and know that nothing downstream is mutating a parameter. I can also write a function that returns a const reference to a member and know that nobody in the future will ever break my invariants.

This isn't about "oops, I didn't mean to mutate that." This is about rapidly being able to reason about the correctness of some code that is leveraging const-qualified code.

> "preventing themselves from accidentally mutating a variable" (when has that ever happened?)

I can't count the number of times I've seen someone new to the language use map::operator[] without realizing that it's a mutating operation.

It’s extra delightful that vector::operator[] isn’t mutating, so you can change a vector to a map and get rather different semantics.

map::operator[] should just be [[deprecated]] - even if you want to mutate the map it's probably not the best way to do what you want.

I kinda like it on occasion. Works like pythons defaultdict. Like, if you wanna count something:

    for (const auto &thing: collection) {
        counts[thing]++;
    }

Works nicely, you don't have to check if it's already there before ++ing it. As long as you know that's what operator[] does, it comes in handy more than I would've expected.

Yeah. It has its uses. You could accomplish the same with the rather verbose `counts.try_emplace(thing).first->second++` but nobody wants to type that (even if it is more explicit about what it's doing).

Another popular use case is something along the lines of:

    std::unique_ptr<T>& ptr = ptrs[key];
    if (ptr != nullptr) {
      ptr = std::make_unique<T>(...);
    }

That said, I don't know what behavior I'd want if maps didn't automatically insert an element when the key was absent. UB (as with vector)? Throw an exception? Report the incident to Stroustrop? All these options feel differently bad.

> when has that ever happened?

Maybe it's not a concern in C-family languages, but rust's culture of defaulting to let and only using mut when it's specifically required does feel very pleasant and ergonomic when I'm in that headspace.

> (when has that ever happened?)

to me, pretty much a few times per week at least