RFC: Generic Constraints

docs/generic-constraints.md

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+# Generic Constraints
+## Summary
+Introduce syntax to extend/constrain the type of a generic.
+## Motivation
+Luau currently does not provide a way in order to constrain the type of a generic.
+```luau
+local qux = { foo = 10, bar = "string" }
+
+local function getProperty<T>( object: T, key: keyof<T> )
+  return object[key]
+end
+
+local foo = getProperty( qux, "foo" )
+-- foo: number | string
+local bar = getProperty( qux, "bar" )
+-- bar: number | string
+```
+Type interference believes that either value could be a number or string as keyof<T> is too broad.
+
+```luau
+local function callbackProperty<T>( object: T, key: keyof<T>, callback: (index<T, keyof<T>>) -> () )
+....
+```
+It is impossible to tell whether or not the key variable is the same as the one being used to index T in the callback, and thus Luau infers that it could be a number | string.
+
+## Design
+The design of this would take inspiration from TypeScript's `extends`, where instead of the additional keyword, we would use `&`.
+```luau
+local qux = { foo = 10, bar = "string" }
+
+local function getProperty<T, K & keyof<T>>( object: T, key: K )
+  return object[key]
+end
+
+local foo = getProperty( qux, "foo" )
+-- foo: number
+local bar = getProperty( qux, "bar" )
+-- bar: string
+```
+This would correctly infer the type of the each key's value.
+```luau
+local qux = { foo = 10, bar = "string" }
+
+local function callbackProperty<T, K & keyof<T>>( object: T, key: K, callback: (index<T, K>) -> () )
+  callback( object[key] )
+end
+
+callbackProperty( qux, "foo", function( foo )
+  -- foo: number -- this is expected!
+end)
+```
+This would also be correctly inferred as index<T, K> would correctly result in the type of the field.
+
+An alternative syntax could use `:` instead of `&`. However, this would not match the current semantics of `:` and using `&` implies a union of types.
+```luau
+local function getProperty<T, K: keyof<T>>( object: T, key: K ) .... end
+local function callbackProperty<T, K: keyof<T>>( object: T, key: K, callback: (index<T, K>) -> () ) .... end
+```
+## Drawbacks
+- I am not personally familiar with the internals of the typechecker, but this has a chance to further complicate type inference.
+- Adding an extra use to `&` could make its usage more confusing to novices.
+## Alternatives
+- Don't do this; this would make it impossible for functions like above to be able to be inferred correctly. Just let people explicitly type their variables instead of inferring types. This makes code more verbose and would likely not allow for full optimization.
+- Use overloaded functions as previously mentioned, but this would not allow the usage of a generic, and would thus require users to add a new overload for each key.