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Fleshed out example a bit more

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ajeffrey@roblox.com 2021-09-20 15:26:07 -05:00
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11
rfcs/property-getter-setter.md
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@ -141,8 +141,17 @@ function h(): Animal? return x.p end
```
it is not obvious what type to infer for `x`, should it be `{ p: Dog? }` or `{ p: Animal? }`?
With either of those types, the following code does not typecheck:
```lua
local t1: { p: Animal? } = { p: nil }
local t2: { p: Dog? } = { p: nil }
x = t1 -- Does not typecheck if x: { p: Dog? }
x = t2 -- Does not typecheck if x: { p: Amimal? }
```
If there are separate getter and setter types, there is a most general type, which is
`{ get p: Animal?, set p: Dog? }`.
`{ get p: Animal?, set p: Dog? }`, which allows this program to typecheck.
With separate getter and setter types, every position in a type is either
covariant or contravariant, and *there are no more uses of invariance*.