Specify that non-plain primary parameters are immutable

accepted/future-releases/primary-constructors/feature-specification.md

@@ -4,7 +4,7 @@ Author: Erik Ernst
 
 Status: Accepted
 
-Version: 1.12
+Version: 1.13
 
 Experiment flag: declaring-constructors
 
@@ -191,10 +191,12 @@ that the instance variable declaration which is induced by this declaring
 constructor parameter is `final`.
 
 In the case where the declaration is an `extension type`, the modifier
-`final` on the representation variable can be specified or omitted. Note
-that an extension type declaration is specified to use a primary
-constructor (it is not supported to declare the representation variable
-using a normal instance variable declaration):
+`final` on the representation variable can be specified or omitted. It is
+an error to specify the modifier `var` on the representation variable.
+
+An extension type declaration is specified to use a primary constructor (it
+is not supported to declare the representation variable using a normal
+instance variable declaration):
 
 ```dart
 // Using a primary constructor.
@@ -745,6 +747,12 @@ main() {
 }
 ```
 
+It is a compile-time error if an assignment to a primary parameter occurs
+in the initializing expression of a non-late instance variable, or in the
+initializer list of the body part of a primary constructor.
+
+*This includes expressions like `p++` where the assignment is implicit.*
+
 The following errors apply to formal parameters of a primary constructor.
 Let _p_ be a formal parameter of a primary constructor in a class, mixin
 class, enum, or extension type declaration _D_ named `C`:
@@ -840,15 +848,16 @@ positional or named parameter remains optional; if it has a default value
   unchanged from _L_ to _L2_ *(this is a plain, non-declaring parameter)*.
 - Otherwise, a formal parameter (named or positional) of the form `var T p`
   or `final T p` where `T` is a type and `p` is an identifier is replaced
-  in _L2_ by `this.p`, along with its default value, if any. Next, a
-  semantic instance variable declaration corresponding to the syntax `T p;`
-  or `final T p;` is added to _D2_. It includes the modifier `final` if the
-  parameter in _L_ has the modifier `final` and _D_ is not an `extension
-  type` decaration; if _D_ is an `extension type` declaration then the name
-  of `p` specifies the name of the representation variable. In all cases, if
-  `p` has the modifier `covariant` then this modifier is removed from the
-  parameter in _L2_, and it is added to the instance variable declaration
-  named `p`.
+  in _L2_ by `this.p`, along with its default value, if any. If the
+  parameter has the modifier `var` and _D_ is an extension type declaration
+  then a compile-time error occurs. Otherwise, a semantic instance variable
+  declaration corresponding to the syntax `T p;` or `final T p;` is added
+  to _D2_. It includes the modifier `final` if the parameter in _L_ has the
+  modifier `final` and _D_ is not an `extension type` decaration; if _D_ is
+  an `extension type` declaration then the name of `p` specifies the name
+  of the representation variable. In all cases, if `p` has the modifier
+  `covariant` then this modifier is removed from the parameter in _L2_, and
+  it is added to the instance variable declaration named `p`.
 
 If there is an initializer list following the formal parameter list _L_
 then _k2_ has an initializer list with the same elements in the same order.
@@ -899,6 +908,11 @@ of declaration, and the constructor might be non-const).
 
 ### Changelog
 
+1.13 - November 25, 2025
+
+* Specify that assignments to primary parameters in initialization code is
+  an error.
+
 1.12 - November 6, 2025
 
 * Eliminate in-body declaring constructors. Revert to the terminology where