RFC: Reserve name ( syntax.

rfcs/TEMPLATE.md

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-# Feature name
-
-## Summary
-
-One paragraph explanation of the feature.
-
-## Motivation
-
-Why are we doing this? What use cases does it support? What is the expected outcome?
-
-## Design
-
-This is the bulk of the proposal. Explain the design in enough detail for somebody familiar with the language to understand, and include examples of how the feature is used.
-
-## Drawbacks
-
-Why should we *not* do this?
-
-## Alternatives
-
-What other designs have been considered? What is the impact of not doing this?

rfcs/reserve-name-followed-by-parens.md

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+# Reserve ``<name> `('`` syntax in type annotations
+
+## Summary
+
+We propose to reserve the syntax ``<name> `('`` in type annotations, regardless of where they show up, e.g. a type or a type pack.
+
+## Motivation
+
+Lua and by extension Luau's syntax is very free form, which means that when the parser finishes parsing a node, it doesn't try to look for a semi-colon or any termination token e.g. a `{` to start a block, or `;` to end a statement, or a newline, etc. It just immediately invokes the next parser to figure out how to parse the next node based on the remainder's starting token.
+
+That feature is sometimes quite troublesome when we want to add new syntax.
+
+We have cases where we want to add a new contextual keyword before type annotations, e.g. `setmetatable(...` and `keyof T`. By itself, some of these looks innocent, but type annotations support parentheses, so `keyof (({ x: number } & { x: string | number }) | { y: string })` may be parsed incorrectly.
+
+Note: this RFC does not propose any new contextual keywords.
+
+## Drawbacks
+
+This is a breaking change syntactically, e.g. there may already be code that looks like
+
+```
+local function f(t, u): MyType
+  (t or u):m()
+end
+```
+
+If `MyType` ever becomes part of the grammar, oops, the return type of `f` is now parsed like `MyType(t` and becomes invalid syntax when it encounters `or`.
+
+To workaround this, the correct syntax is
+
+```
+local function f(t, u): (MyType)
+  (t or u):m()
+end
+```
+
+## Alternatives
+
+Only allow these syntax when used inside parentheses e.g. `(name(...))`, but this makes it inconsistent with the existing `typeof(...)` type annotation, and changing that over is also breaking change. It's pretty clear that anything inbetween not doing this RFC or doing some backward-compatible option adds warts to the grammar.
+
+Support backtracking in the parser, so if `: MyType(t or u):m()` is invalid syntax, revert and parse `MyType` as a type, and `(t or u):m()` as an expression statement. Even so, this option is terrible for:
+  1. parsing performance (backtracking means losing progress on invalid input),
+  2. user experience (why was this annotation parsed as `X(...)` instead of `X` followed by a statement `(...)`),
+  3. has false positives (`foo(bar)(baz)` may be parsed as `foo(bar)` as the type annotation and `(baz)` is the remainder to parse)