how can this be the ~5th iteration of a very wide-spread use-case and still contain a footgun?

The API looks like it's following best-practice, with a specific result type that also contains specific error information and yet, that's not enough and you still end up with edge-cases where things look like they're fine when they aren't.

I suppose the reason is that sometimes you want to parse more than just the number. For example, numbers separated by commas. In that case you will have to call the function repeatedly from your own parsing routine and advance the pointer.

Yes, if you are parsing in-place numbers in an existing string you do not necessarily know a-priori where the number end. You could have extra logic to look ahead to the end of the number before passing it from_chars, but a) it would require an additional pass and b) you could end up replicating part of the implementation of from chars.

from_chars is supposed to be a low lever routine, it must be possible to use it correctly, but it is not necessarily the most ergonomic.

I don't see a foot gun, you just need to check if you have consumed the whole input. Which is the norm in nearly any streaming api

I didn't see a foot gun either, but somehow my foot went missing.

I get that this is a super low level API, but yet, my expectation about an API that parses a buffer with length to a number and which has a specific enum for error cases as its return type would be that when asked to parse "9not a number" that I would get an error response and not a "the number you asked me to parse is 9 and everything went well" as the result.

The whole reason for returning a result enum would be so I don't forget to also check somewhere else whether something could possibly have gone wrong.

Especially when there is a specific result type.

But now you need a different API to compute where a number starts and end and that API must use exactly the same definition of number.

There could be an enum value for "read a partial number"

In what way is it streaming? It takes at least the entire numeric string as a string in memory, you can't stream in more data as needed

That is a good question. The C++ stdlib has some truly bizarre APIs. I wonder if they should freeze std and work on std2.

from_chars is the correct API here. When you're parsing decimal numbers you want to do it with streaming semantics.

Hm but there's nothing streaming about it? You need the entire numeric string in memory

I think they meant the other way around: you can have a single string in memory containing multiple numbers, and process that string as a stream.