ban type parameters for macros (#2976)

Author: jakemac53

working/macros/feature-specification.md

@@ -4,11 +4,6 @@ Authors: Jacob MacDonald, Bob Nystrom
 
 Status: **Work In Progress**
 
-### Changelog
-
-- *2022/01/25:* Specify that identifiers in strings can only refer to local
-  declarations.
-
 ## Introduction
 
 The [motivation][] document explains why we are working on static
@@ -416,6 +411,11 @@ constructors are invoked, and their limitations.
 -  All macros must implement at least one of the `Macro` interfaces.
 -  Macros cannot be abstract.
 -  Macro classes cannot be generated by other macros.
+-  Macro classes cannot contain generic type parameters.
+    - It is possible that in the future we could allow some restricted form of
+      generic type parameters for macro classes, but it gets tricky because the
+      types in the user code instantiating the macro are not necessarily present
+      in the macros own transitive imports.
 
 *Note: The Macro API is still being designed, and lives [here][api].*