Feature Request: Typestate / Self-Type Refinement for Method Calls

Problem

Lua is a highly dynamic language, and many real-world Lua frameworks mutate object capabilities at runtime.

This pattern is especially common in:

• game engines
• UI frameworks
• ECS architectures
• builder APIs
• runtime mixin systems
• userdata bindings from native engines

Currently, LuaLS/LuaCats cannot properly express:

"after calling this method, self now has additional capabilities/methods"

This becomes a major DX limitation for frameworks that dynamically extend objects.

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Real-world Example

Consider a UI framework where all elements start as a generic UIElement.

---@class UIElement
local UIElement = {}

Calling:

element:setupUIImage()

injects image-related functionality into the element at runtime:

element:setImage(...)

However, LuaLS still sees element as only UIElement.

The only current workaround is manual casting:

element:setupUIImage()

---@cast element UIImage
element:setImage(material)

This works technically, but becomes repetitive and hurts DX significantly in large codebases.

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Comparison to TypeScript

TypeScript supports type refinement after method calls through assertion signatures such as:

asserts this is SomeType

This allows APIs to safely refine object types after capability-changing methods.

A good real-world example is discord.js, where Interaction objects can be refined into subtypes through methods like:

interaction.isButton()
interaction.isChatInputCommand()

After refinement, TypeScript understands the new subtype automatically.

Lua frameworks often use similar runtime patterns, but LuaLS currently cannot model them.

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Proposed Solution

Introduce a LuaCats annotation for self-type refinement / capability injection.

Possible syntax examples:

---@injects UIImage
function UIElement:setupUIImage() end

or:

---@mutates self UIImage
function UIElement:setupUIImage() end

or:

---@becomes UIImage
function UIElement:setupUIImage() end

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Expected Behavior

After:

element:setupUIImage()

LuaLS would understand:

element :: UIElement & UIImage

allowing:

element:setImage(...)

without requiring manual casts.

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Why This Matters

This would greatly improve support for:

• dynamic Lua architectures
• runtime capability injection
• engine bindings
• UI frameworks
• builder APIs
• fluent APIs
• mixin systems

while keeping Lua's dynamic nature intact.

This feature would also reduce:

• repetitive ---@cast
• noisy annotations
• inaccurate supersets in base classes
• degraded autocomplete quality

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Additional Notes

This feature does not need to become full typestate analysis or advanced dependent typing.

Even a pragmatic flow-sensitive refinement system limited to:

• direct method calls
• current scope
• explicit annotations only

would already provide massive DX improvements.

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Example Use Case

---@class UIImage
---@field setImage fun(self: UIImage, material: string)

---@class UIElement

---@injects UIImage
function UIElement:setupUIImage() end

local element = UIElement.new()

element:setupUIImage()

element:setImage("icon")

Expected:

• autocomplete works
• diagnostics recognize setImage
• no manual cast required

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Thanks for all the amazing work on LuaLS and LuaCats.
This would significantly improve tooling support for dynamic Lua patterns commonly used in real-world frameworks and game engines.

A issue não nomeia arquivos de implementação nem testes. Comece revisando como o LuaLS atualmente lida com ---@cast explícitos e chamadas de métodos; em seguida, avalie o escopo de refinamento proposto: chamadas diretas, o escopo atual e anotações explícitas. O trabalho estará concluído quando houver um design de anotações definido, com refinamento de capacidades sensível ao fluxo, autocompletar e diagnósticos funcionando sem um cast manual.