Increase coverage of String.prototype.replace $xy replacement patterns

[gibson042]

• $xy is a valid capture index
• $xy is not a valid capture index but $x is
• neither $xy nor $x is a valid capture index

Ref tc39/ecma262#1426
Ref tc39/ecma262#3157

[ljharb]

Confirmed these tests pass my ES2024 GetSubstitution implementation in es-abstract, and fail on <= ES2023 (as expected)

[ljharb]

update; tc39/ecma262#3157 (comment) should get some coverage too ($01 - $09)

[gibson042]

@ljharb coverage expanded

[ptomato]

This normative change got consensus conditionally in the 2023-09 TC39 plenary meeting. "If there is a mismatch between engines, the presenter should bring the change back to TC39 to discuss next steps." So I guess before we land this, the proponents of the normative change should perform that investigation.

[gibson042]

Done. The only implementation with failures is engine262, which does not implement the single-digit fallback:

• $10 is not a capture index (but $1 is) in /(x)/
• $10 is not a capture index (but $1 is) in /(x)($^)?/
• $10 before 0 is not a capture index (but $1 is) in /(x)/
• $10 before 0 is not a capture index (but $1 is) in /(x)($^)?/
• $20 is not a capture index (but $2 is a failed capture index) in /(x)($^)?/
• $20 is not a capture index (but $2 is) in /((((((((((x))))))))))/
• $20 before 0 is not a capture index (but $2 is a failed capture index) in /(x)($^)?/
• $20 before 0 is not a capture index (but $2 is) in /((((((((((x))))))))))/

$ eshost -se '"foo-x-bar".replace(/(x)/, "|$10|")'
#### ChakraCore, GraalJS, Hermes, JavaScriptCore, Moddable XS, QuickJS, SpiderMonkey, V8
foo-|x0|-bar

#### engine262
foo-|$10|-bar

[ptomato]

Thanks. So does that mean there are no longer any blockers for this?

[gibson042]

Yes, that is my understanding.

[ptomato]

What's the status of this? It looks like it has a positive review from Jordan, but still needs to be autosquashed.

[ljharb]

It's easy to squash it at landing time, but I wanted more reviews before landing it.

[linusg]

LGTM, also passes with my literal implementation of GetSubstitution.