Consider adding a note to MergeAdjacent to mention this method should be updated when used.

It looks LanguageParser.ParseLambdaExpression() also uses MergeAdjacent() for SyntaxKind.EqualsGreaterThanToken. If there's a potential incremental parsing bug there, we should open an issue.

Originally posted by @cston in #59977 (comment)

Consider adding an assert to MergeAdjacent(): Debug.Assert(IsFabricatedToken(kind));

Originally posted by @cston in #59977 (comment)