Copying 'params' from every target delegate's Invoke onto the lambda broke
the Newtonsoft.Json round-trip: each plain '(object[] args) => ...' bound to
a 'params' delegate came back as '(params object[] args)', and below C# 12
the fallback rendered that as '([ParamArray] object[] args)', which no C#
version compiles (CS0674, plus CS8400 for lambda attributes before C# 10).
The lambda's own metadata is the faithful record for named delegate types:
current Roslyn emits ParamArrayAttribute and the default value on the
closure method exactly when the source spelled them, and where Roslyn 4.14
omits the attribute the plain parameter list is equivalent anyway. Only a
compiler-synthesized delegate type has to be spelled through the lambda's
declaration, so that is the one place Invoke is still consulted. Without a
legal pre-C# 12 spelling, the modifiers are now dropped instead of being
turned into attributes.
Assisted-by: Claude:claude-fable-5:Claude Code
Christoph Wille