Legacy parser accepts invalid key path syntax

[AnthonyLatsis]

Description

An expression starting with \[ is apparently necessarily invalid and correctly rejected by the new Swift parser.

Reproduction

let _ = \[0]

Expected behavior

Parser error.

Environment

Swift 6.2-dev (d620886)

Additional information

No response

[ahoppen]

\[0] is diagnosed during type checking when using the C++ parser. While I agree that the additional new Swift parser generated errors for code that C++ parser accepted error message is not ideal, this only affects assert toolchains, doesn’t make invalid code valid or the other way round and the error message is transitional, so we we’re not planning to fix this.