使用泛型从打字稿中的对象属性推断枚举

Question

我有一个场景，我想根据模式 ID 验证命令并返回类型化的验证结果。为此，我需要能够推断架构 id 的枚举值。

我知道我可以为此使用泛型，例如

TSchemaId extends SchemaId

，并且它有效。然而，当我尝试从对象内部推断它时，它的行为并不像我想象的那样。

这就是我所拥有的

enum SchemaId {
  SCHEMA_1 = 'SCHEMA_1',
  SCHEMA_2 = 'SCHEMA_2',
}

function inferSchemaId<TSchemaId extends SchemaId> (schemaId: TSchemaId): TSchemaId {
  return schemaId
}

function inferObjectSchemaId<
    TSchemaId extends SchemaId,
    TObjectWithSchemaId extends { schemaId: TSchemaId },
> (objectWithSchemaId: TObjectWithSchemaId): TSchemaId {
  return objectWithSchemaId.schemaId
}

const si1 = inferSchemaId(SchemaId.SCHEMA_1)                     // SchemaId.SCHEMA_1
const si2 = inferObjectSchemaId({ schemaId: SchemaId.SCHEMA_1 }) // SchemaId

如上所示，

si1

正确缩小为

SchemaId.SCHEMA_1

，但

si2

无法缩小。

为什么会这样？我错过了什么？

Answer 1

奇怪的是不仅返回类型（SchemaId）而且还推断准确的值。我已经做了一个可行的案例：

enum SchemaId {
  SCHEMA_1 = 'SCHEMA_1',
  SCHEMA_2 = 'SCHEMA_2',
}

function inferSchemaId<TSchemaId extends SchemaId> (schemaId: TSchemaId): TSchemaId {
  return schemaId
}

type ObjectWithSchemaId<T extends SchemaId> = { schemaId: T }

type InferSchemaIdFromObj<S> = S extends ObjectWithSchemaId<infer T>? T : never;

function inferObjectSchemaId<
    TObjectWithSchemaId extends ObjectWithSchemaId<SchemaId>,
> (objectWithSchemaId: TObjectWithSchemaId): InferSchemaIdFromObj<TObjectWithSchemaId> {
  return objectWithSchemaId.schemaId as InferSchemaIdFromObj<TObjectWithSchemaId>
}

const si1 = inferSchemaId(SchemaId.SCHEMA_1)                     // SchemaId.SCHEMA_1
const si2 = inferObjectSchemaId({ schemaId: SchemaId.SCHEMA_1 }) // SchemaId.SCHEMA_1

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