Named Tuples don't propagate singleton types as widely as tuples

[odersky]

Compiler version

3.5-RC1, with #19174

Minimized example

import language.experimental.namedTuples

type TripleSingle = ("Lausanne", 1000, 140000)
type CitySingle = (name: "Lausanne", zip: 1000, pop: 140000)

@main def Test =
  val tripleSingle: TripleSingle = ("Lausanne", 1000, 140000) // OK
  val citySingle: CitySingle = (name = "Lausanne", zip = 1000, pop = 140000) // error

Output

-- [E007] Type Mismatch Error: named-tuple-single-ops.scala:8:31 ---------------
8 |  val citySingle: CitySingle = (name = "Lausanne", zip = 1000, pop = 140000)
  |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                           Found:    (name : String, zip : Int, pop : Int)
  |                           Required: CitySingle
  |
  | longer explanation available when compiling with `-explain`
1 error found

Expectation

Singleton types in an expected named tuple type should be propagated to the right-hand side just as in the case for normal tuples.

This currently fails because named tuples expand to typelevel operations that perform too much widening. It's not yet clear whether we want to pursue a solution specific to named tuples or whether we want to improve the type-level possibilities for precise types first and then build on that.

[soronpo]

This could be a deal breaker for Named Tuples to make it to 3.5 as non-experimental. I'm not sure the compatibility story allows to just support it later.

[odersky]

Named tuples will be only experimental in 3.5, anyway.

[bishabosha]

it looks as is this was fixed by #20497 - latest nightly compiles this