patch 8.2.2066: Vim9: assignment with += doesn't work

Problem:    Vim9: assignment with += doesn't work.
Solution:   Do not see the "+" as an addition operator.

src/eval.c

@@ -2769,10 +2769,11 @@ eval5(char_u **arg, typval_T *rettv, evalarg_T *evalarg)
 	int	    vim9script = in_vim9script();
 
 	// "." is only string concatenation when scriptversion is 1
+	// "+=" and "-=" are assignment
 	p = eval_next_non_blank(*arg, evalarg, &getnext);
 	op = *p;
 	concat = op == '.' && (*(p + 1) == '.' || current_sctx.sc_version < 2);
-	if (op != '+' && op != '-' && !concat)
+	if ((op != '+' && op != '-' && !concat) || p[1] == '=')
 	    break;
 
 	evaluate = evalarg == NULL ? 0 : (evalarg->eval_flags & EVAL_EVALUATE);

src/ex_docmd.c

@@ -3332,9 +3332,13 @@ find_ex_command(
 
 		// When followed by "=" or "+=" then it is an assignment.
 		++emsg_silent;
-		if (skip_expr(&after, NULL) == OK
-				  && (*after == '='
-				      || (*after != NUL && after[1] == '=')))
+		if (skip_expr(&after, NULL) == OK)
+		    after = skipwhite(after);
+		else
+		    after = (char_u *)"";
+		if (*after == '=' || (*after != NUL && after[1] == '=')
+					 || (after[0] == '.' && after[1] == '.'
+							   && after[2] == '='))
 		    eap->cmdidx = CMD_var;
 		else
 		    eap->cmdidx = CMD_eval;

src/testdir/test_vim9_assign.vim

@@ -866,18 +866,17 @@ def Test_assign_dict_unknown_type()
   END
   CheckScriptSuccess(lines)
 
-  # doesn't work yet
-  #lines =<< trim END
-  #    vim9script
-  #    var mylist = [[]]
-  #    mylist[0] += [#{one: 'one'}]
-  #    def Func()
-  #      var dd = mylist[0][0]
-  #      assert_equal('one', dd.one)
-  #    enddef
-  #    Func()
-  #END
-  #CheckScriptSuccess(lines)
+  lines =<< trim END
+      vim9script
+      var mylist = [[]]
+      mylist[0] += [#{one: 'one'}]
+      def Func()
+        var dd = mylist[0][0]
+        assert_equal('one', dd.one)
+      enddef
+      Func()
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def Test_assign_lambda()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2066,
 /**/
     2065,
 /**/