Spec bug: What happens with a `g,C` mapping?

[nicolo-ribaudo]

Even including #179, I think our spec is just broken in this case. In https://tc39.es/ecma426/#sec-DecodeBase64VLQ we consume g, then the loop continues and it calls ConsumeBase64ValueAt, which tries to decode , as a base64 value.

#180 fixes it by making it an error explicitly, but I'm opening this issue to make sure it's tested.

What do realm implementations do? Do they error, or do they recover by just assuming that the user meant A instead of g (which is the same bit pattern but without the continuation bit)?

[szuend]

Chrome DevTools will throw an error nowadays: https://source.chromium.org/chromium/chromium/src/+/main:third_party/devtools-frontend/src/front_end/core/sdk/SourceMap.ts;l=816;drc=6ca4ad9f4aac1de91c8b5095b8fdb558ad9e7c5e

Before 2024 it would silently ignore it and treat , as 0. Because the charCode for , is 44, which was mapped to 0:
https://source.chromium.org/chromium/_/chromium/devtools/devtools-frontend/+/1e817df1218f0a513dbf08488928b33e5add0dea:front_end/core/sdk/SourceMap.ts;l=820;drc=7758591a0d658b29b92945801fed871a65bf4bf3

This would effectively treat g as A.

[jridgewell]

Nitpicking, but it's impossible to hit , in this case because it's already been split out to create segments.

We can encounter an incomplete segment = "g" (eg, in mappings: "g", mappings: "g;" or mappings: "g,A" after splitting lines and segments). I raised this during the initial review, and it's why we added EOL checking.

[jridgewell]

What do realm implementations do? Do they error, or do they recover by just assuming that the user meant A instead of g (which is the same bit pattern but without the continuation bit)?

If it's at the end of the mappings, it'll turn into 0 (and further reads will get 0s until we hit 4/5 tuple length). If it's in the middle of a the mappings, it'll turn , or ; into a 0 but then read into the next segment to fill the 4/5 tuple length.

I don't specifically handle it at all 😬. Sourcemaps are machine generated, and I don't wanna slow down everyone for handwritten validation.