Only allow super parameters in class declarations. (#2148)

* Only allow super parameters in `class` declarations.

Makes it explicit that you cannot use super-parameters in an `enum` declaration constructor..

Author: lrhn

working/1855 - super parameters/proposal.md

@@ -1,6 +1,6 @@
 # Dart Super-Initializer Parameters
 
-Author: [email protected]<br>Version: 1.2
+Author: [email protected]<br>Version: 1.3
 
 ## Background and Motivation
 
@@ -46,9 +46,9 @@ _That is, exactly the same grammar as initializing formals, but with `super` ins
 
 ## Semantics
 
-It’s a **compile-time error** if a super-parameter in any declaration other than a non-redirecting generative constructor.
+It’s a **compile-time error** if a super-parameter declaration occurs in any declaration other than a non-redirecting generative constructor of a class declaration.
 
-_All non-redirecting generative constructors have a super-constructor invocation at the end of their initializer list. If none is written (or there is even no initializer list), the default is an invocation of `super()`, targeting the unnamed superclass constructor. It’s a compiler-time error if the superclass does not have the specified constructor, or it’s not a generative constructor._
+_All non-redirecting generative constructors of a class declaration have a super-constructor invocation at the end of their initializer list. If none is written (or there is even no initializer list), the default is an invocation of `super()`, targeting the unnamed superclass constructor. It’s a compiler-time error if the superclass does not have the specified constructor, or it’s not a generative constructor. The new enhanced `enum` declarations, on the other hand, do have non-redirecting generative constructors, but those cannot not have any super-invocations, and they also have no known superclass constructor to forward parameters to._
 
 It’s a **compile-time error** if `var` occurs as the first token of a `<superFormalParameter>` production. (It’s generally a compile-time error if `const` or `late` occurs in a parameter declaration, this also applies to super-parameters).
 
@@ -176,3 +176,5 @@ class C extends B {
 1.1: Don’t allow both positional super parameters and explicit positional arguments. Inherit default value.
 
 1.2: Don’t do inference (and implicit coercion) on the implicit arguments.
+
+1.3: Make it explicit that you can't use super parameters in the new enum declarations.