[类型 / Types] 评估用户泛型与互递归 ADT / evaluate user generics and mutually recursive ADTs

中文说明

版本、顺序与当前进度

版本待定 / P2 / deferred。有限直接自递归和 #262 依赖排序不等于用户泛型/互递归完成。只有真实复用模型与预算明确后才安排具体版本；#264 查询绑定不依赖本 issue。

目标

独立评估用户定义泛型和互递归 named ADT，不把内建 option/list 或已支持的直接自递归当作完成。

范围与验收

• Result a b、Tree a、互递归类型组的身份、有限可构造性与错误定位。
• 定义 monomorphization/运行时表示、codec、schema hash、migration、索引和 Rust/wire 映射。
• 有真实复用场景和容量预算后才进入实现。
• RFC 明确选择、拒绝项和兼容策略；不得引入无界值或隐式结构类型等价。

English Description

Version, sequence, and current progress

Version TBD / P2 / deferred. Direct self-recursion and #262 dependency ordering do not complete user generics/mutual recursion. Assign a concrete release only after real reuse and budgets are established; #264 does not depend on this issue.

Goal

Evaluate user-defined generics and mutually recursive named ADTs independently, without treating built-in option/list or direct self-recursion as completion.

Scope and Acceptance

• Cover identity, finite constructibility, and diagnostics for Result a b, Tree a, and mutually recursive groups.
• Define monomorphization/runtime representation, codecs, schema hashes, migrations, indexes, and Rust/wire mapping.
• Implementation starts only after real reuse scenarios and capacity budgets exist.
• The RFC records selections, rejections, and compatibility without unbounded values or implicit structural equivalence.

Start by reviewing the existing handling of built-in option/list types and direct self-recursion, then compare the requirements for Result a b, Tree a, and mutually recursive groups. The RFC is done when it records representation, codecs, schema hashes, migrations, indexes, Rust/wire mapping, selected and rejected approaches, compatibility rules, reuse scenarios, and capacity budgets.