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Record coercion can construct a private record from outside its module

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  • #9125 来自 @cknitt —— 已合并

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4/5
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新手友好度
52/100
Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
活跃
技术栈
ocaml
领域
compilers

调研方向

Start by tracing the record coercion type-checking path in the ReScript compiler and compare it with the handling of mutable from #9063, fixed by #9086. Check both examples in this issue: coercion from a plain record and from another module’s private record should be rejected, while coercion out to a plain record should remain allowed. Done means those cases behave as expected and the compiler’s relevant regression tests pass.

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描述

A record coerces structurally into a private record with the same fields, so code outside a module can construct values of that module's private record type, although private means they can only be built inside it.

From a plain record:

module Blob: {
  type t = private {size: float}
} = {
  type t = {size: float}
}

type plain = {size: float}
let fake = (p: plain): Blob.t => (p :> Blob.t)   // compiles: `return p`

From another module's private record with the same fields, which that module constructs freely:

module Fake: {
  type t = private {size: float}
  let make: float => t
} = {
  type t = {size: float}
  let make = size => {size: size}
}

let forged = (Fake.make(1.) :> Blob.t)   // compiles: `{size: 1}`

Expected: a coercion into a private record type is refused, unless the source is that same type. Coercing out of a private record, into a plain record with the same fields, stays allowed: reading a private record is fine.

This is a prerequisite for @instanceOf on private records (#9058): there, a forged value would be dispatched as an instance of a class it isn't. It is the same family as #9063 (fixed by #9086), where record coercion ignored mutable.

Found on master 3562ce6b80.

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