RFC: Constrained Generics

rfcs/constrained-generics.md

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+# Constrained Generics
+
+## Summary
+
+Allows you to give a function generic a constrained type.
+
+## Motivation
+
+Right now, when you create a generic function, the generic paramter is assumed to be everything. This creates problems, especially with tables
+```lua
+local function SomethingWithAGenericTable<T>(tab: T)
+  for _, v in t do --Cannot call non-function T (T is not a table)
+
+  end
+end
+```
+
+This constraining is mainly for concrete table types and functions, since types like `number`, `string`, and `boolean` are literal types.
+
+## Design
+
+The way to implement this is to have new syntax under the generic's brackets: `<T: typedef>`. This would constrain the type from the generic def and give the typechecker better hints.
+```lua
+local function SomethingWithAGenericTable<T: {[any]: any}>(tab: T)
+  for _, v in t do
+    print(v)
+  end
+end
+
+SomethingWithAGenericTable({meow = 5}) --OK
+SomethingWithAGenericTable(5) --not OK
+```
+
+## Drawbacks
+
+This adds unusual syntax, as well as adding to the complexity of the language.
+
+## Alternatives
+
+Do nothing, and use the `assert(type(T) == T)` syntax, though this creates extra bytecode.