patch 9.0.2170: Vim9: no support for const/final class/objects vars

Problem:  Vim9: no support for const/final class/objects vars
Solution: Support final and const object and class variables

closes: #13655

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/tags

@@ -4505,7 +4505,10 @@ E1401	vim9class.txt	/*E1401*
 E1402	vim9class.txt	/*E1402*
 E1403	vim9class.txt	/*E1403*
 E1407	vim9class.txt	/*E1407*
+E1408	vim9class.txt	/*E1408*
+E1409	vim9class.txt	/*E1409*
 E141	message.txt	/*E141*
+E1410	vim9class.txt	/*E1410*
 E142	message.txt	/*E142*
 E143	autocmd.txt	/*E143*
 E144	various.txt	/*E144*
@@ -9096,6 +9099,8 @@ o_V	motion.txt	/*o_V*
 o_object-select	motion.txt	/*o_object-select*
 o_v	motion.txt	/*o_v*
 object	vim9class.txt	/*object*
+object-const-variable	vim9class.txt	/*object-const-variable*
+object-final-variable	vim9class.txt	/*object-final-variable*
 object-motions	motion.txt	/*object-motions*
 object-select	motion.txt	/*object-select*
 objects	index.txt	/*objects*

runtime/doc/todo.txt

@@ -1,4 +1,4 @@
-*todo.txt*      For Vim version 9.0.  Last change: 2023 Jun 08
+*todo.txt*      For Vim version 9.0.  Last change: 2023 Dec 14
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -122,7 +122,6 @@ Upcoming larger works:
 
 Further Vim9 improvements:
 - Classes and Interfaces. See |vim9-classes|
-  - "final" object members - can only be set in the constructor.
   - Cannot use class type of itself in the method (Issue #12369)
   - Getting member of variable with "any" type should be handled at runtime.
     Remove temporary solution from #12096 / patch 9.0.1375.

runtime/doc/vim9class.txt

@@ -364,6 +364,78 @@ super class.  Depending on the class where the member is used the
 corresponding class member will be used.  The type of the class member in a
 child class can be different from that in the super class.
 
+					*object-final-variable* *E1409*
+The |:final| keyword can be used to make a class or object variable a
+constant.  Examples: >
+
+    class A
+	final v1 = [1, 2]		# final object variable
+	public final v2 = {x: 1}	# final object variable
+	static final v3 = 'abc'		# final class variable
+	public static final v4 = 0z10	# final class variable
+    endclass
+<
+A final variable can be changed only from a constructor function.  Example: >
+
+    class A
+	final v1: list<number>
+	def new()
+	    this.v1 = [1, 2]
+	enddef
+    endclass
+    var a = A.new()
+    echo a.v1
+<
+Note that the value of a final variable can be changed.  Example: >
+
+    class A
+	public final v1 = [1, 2]
+    endclass
+    var a = A.new()
+    a.v1[0] = 6			# OK
+    a.v1->add(3)		# OK
+    a.v1 = [3, 4]		# Error
+<
+							*E1408*
+Final variables are not supported in an interface.  A class or object method
+cannot be final.
+
+					*object-const-variable*
+The |:const| keyword can be used to make a class or object variable and the
+value a constant.  Examples: >
+
+    class A
+	const v1 = [1, 2]		# const object variable
+	public const v2 = {x: 1}	# const object variable
+	static const v3 = 'abc'		# const class variable
+	public static const v4 = 0z10	# const class variable
+    endclass
+<
+A const variable can be changed only from a constructor function. Example: >
+
+    class A
+	const v1: list<number>
+	def new()
+	    this.v1 = [1, 2]
+	enddef
+    endclass
+    var a = A.new()
+    echo a.v1
+<
+A const variable and its value cannot be changed.  Example: >
+
+    class A
+	public const v1 = [1, 2]
+    endclass
+    var a = A.new()
+    a.v1[0] = 6			# Error
+    a.v1->add(3)		# Error
+    a.v1 = [3, 4]		# Error
+<
+							 *E1410*
+Const variables are not supported in an interface.  A class or object method
+cannot be a const.
+
 ==============================================================================
 
 4.  Using an abstract class			*Vim9-abstract-class*
@@ -982,8 +1054,6 @@ function declaration syntax for class/object variables and methods.  Vim9 also
 reuses the general function declaration syntax for methods.  So, for the sake
 of consistency, we require "var" in these declarations.
 
-This also allows for a natural use of "final" and "const" in the future.
-
 
 Using "ClassName.new()" to construct an object ~