After looking into the reported examples a bit, I believe one possible root cause of these issues is that (?!\G) is not working in end patterns.

[WILY12424]

          After looking into the reported examples a bit, I believe one possible root cause of these issues is that `(?!\G)` is not working in `end` patterns.

This would explain the following issues:

Swift

class A: B {
  let x: Int
}

protocol Foo {
  associatedtype T: Equatable
  var x: String { get }
}

protocol Foo {
  associatedtype T = Int
  var x: String { get }
}

protocol Foo {
  associatedtype T: Equatable = Int
  var x: String { get }
}

• Reported at: Bug with protocols and associatedtype in GitHub jtbandes/swift-tmlanguage#8, Enums and other decls members have incorrect highlighting jtbandes/swift-tmlanguage#10, Highlighting breaks after multiple protocol inheritance and with @main jtbandes/swift-tmlanguage#12, Broken highlighting at the start of declarations jtbandes/swift-tmlanguage#13
• (?!\G) used here: https://github.com/jtbandes/swift-tmlanguage/blob/f839cb6fde7c5afa1e08c38625c95b80bb05c626/Swift.tmLanguage.yaml#L880

gitconfig

[good]
b = c
[bad]

• Reported at: Replace Swift grammar github-linguist/linguist#6603 (comment)
• (?!\G) used here: https://github.com/Alhadis/language-etc/blob/f3e265b2c155eded4a1808d4b75136ffbd6406b5/grammars/gitconfig.cson#L317

TOML

[good]
b = c  # comment
[bad]

• Reported at: Replace Swift grammar github-linguist/linguist#6603 (comment)
• (?!\G) used here: https://github.com/textmate/toml.tmbundle/blob/e82b64c1e86396220786846201e9aa3f0a2d9ca2/Syntaxes/TOML.tmLanguage#L46-L47

Originally posted by @jtbandes in github-linguist/linguist#6668 (comment)