Incorrect type inference in 'match' expression

[developedby]

Reproducing the behavior

Considering the following program

type Box:
  Box {x: Unit}

type Unit:
  Unit

Erase: Box -> None
(Erase (Box/Box x)) = (Erase x)

main: None
main = (Erase (Box/Box Unit/Unit))

This should generate a type error in the Erase function, since it expects a Box value but we're giving it a Unit.

Based on the match expression, the type of Erase should be Box -> var, while according to the call it should be Unit -> var. When unifying these two we expect a type error, which never happens.

The final infered type is Box -> var, which specializes to Box -> None when checking against the annotation.

System Settings

Bend commit: 4d6f461

Additional context

No response

[developedby]

The issue was that the type constructors inside of constructor fields were not being resolved and were kept as variables. So, even though the type of the Box/Box constructor was being updated to Unit -> Box with Unit as a constructor, the type of x was still Unit as a type variable.

This means that all fields inside of match expressions were being inferred variable types for the nullary constructors in their types.

[developedby]

Fixed by #728