Record coercion can turn an immutable field into a mutable one #9063

What happens

:> between record types accepts a target whose field is mutable when the source's field is not:

type ro = {x: float}
type rw = {mutable x: float}

let toRw = (r: ro) => (r :> rw)

compiles on master (1a49d91) to

function toRw(r) {
  return r;
}

So a value typed ro, whose x the type system treats as immutable, can be written through the coerced view:

let r: ro = {x: 0.}
let w = (r :> rw)
w.x = 1.   // compiles; r.x is now 1.

Where

Record_coercion.check_record_fields (compiler/ml/record_coercion.ml) compares field names, optionality and runtime names, and passes the field types on for subtyping; it does not compare mutability.

Expected

A coercion should not grant write access the source does not have: a mutable target field requires a mutable source field. Where both are mutable, the field type is read and written, so it has to be equal (invariant) rather than a subtype.