Special case NamedTuple.From for arguments derived from Tuple

[aherlihy]

Fixes #22036

Some questions:

• NamedTuple.From(<TupleN>) works as is, so maybe there's a better way to fully normalize/evaluate the arguments to NamedTuple.From at the start of fieldsOf, instead of special casing types derived from Tuple?
• is there a better way to extract the expected keys (ConstantType(Constant(_<n>)) than just hard coding "_"?

[smarter]

NamedTuple.From() works as is, so maybe there's a better way to fully normalize/evaluate the arguments to NamedTuple.From at the start of fieldsOf, instead of special casing types derived from Tuple?

TupleN works because each TupleN class is a case class. In theory I'd expect summon[Mirror.Of[type]] to be more generic, but in practice it currently only works with tuples of arity <= 22 too:

scala3/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

Line 479 in f7e5df5

val reason = s"it reduces to a tuple with arity $arity, expected arity <= $maxArity"

(which surprises me, I thought we got rid of arbitrary limitations like that).
Mirror.Of is also less generic because it doesn't work with case classes with multiple parameter lists, whereas NamedTuple.From works (by only returning the first parameter list).

is there a better way to extract the expected keys (ConstantType(Constant()) than just hard coding ""?

I don't think so since Constant takes a String and not a Name.

[bishabosha]

(which surprises me, I thought we got rid of arbitrary limitations like that).

no one decided how it should work if there isnt an actual accessor to back up the name, for TupleXXL there are no _23 methods