implicit summoning of records.Keys for singleton subtype of HList fail sporadically about shapeless HOT 3 CLOSED

The last two lines are not syntactically equivalent. The inferred type is

inferKeys[FieldType["a", Int] :: FieldType["b", Int] :: HNil](record2)

It doesn't work with the singleton type (which is a subtype) because Keys is invariant.
You could use Keys[_ >: T] instead.

from shapeless.

@joroKr21 so type parameter cannot be inferred as singleton type even if the variable is declared so? Wow

The irony is that this correctly demonstrates the invariance you described:

type RR = record.type

val record2: RR =  ...

Really? The compiler is really mangled.

Regardless, my impression so far is that: it should be a valid improvement to make Keys contravariant to the input type, what do you think?

from shapeless.

If we apply variance it should be consistently to all type classes. But Scala 2 implicit resolution has quirks when it comes to variance, esp. contravariance. That would also be too big of a can of worms to open in Shapeless 2 I'm afraid. And since it has an easy workaround (use Keys[_ >: T] if you need contravariance) I think it's not worth the effort.

from shapeless.