[lewis6991]

Assignments, arguments, and returns use different rules to decide whether a value fits an expected type. Local exceptions duplicate logic and can accept invalid values. A union can pass even when some of its possible values do not fit.

For example, these three uses must all fail because value can be an integer:

---@param value string | integer
---@param consume fun(text: string)
---@return string
local function example(value, consume)
    ---@type string
    local text = value -- error: assignment
    consume(value)     -- error: argument
    return value       -- error: return
end

Use one directional compatibility check for all three contexts. Every source union member must fit at least one target alternative. Check callbacks by the arguments they must accept and the results they can produce, including optional and variadic slots. Keep strict subtype checks separate for casts and conditional types. This gives the checkers one place to define and enforce value compatibility.

The shared check needs accurate input types. Preserve integer results and complete return sequences, including an expanding final call. Retain object fields from annotations and resolve inherited index types with their generic arguments and key domains. Widen inferred literals for reassignment, while keeping declared literal types exact. These changes let valid code pass without local checker exceptions.

Member guards also need to describe the correct object. Track object identity through local aliases and discard a guard after a write can change the guarded value. This prevents a nil check on one object from being reused for another object or after an affected field changes.

Reuse resolved syntax nodes and cache cast paths to avoid repeated tree walks during flow analysis of long expressions.