patch 8.2.3263: Vim9: "..=" does not accept same types as the ".." operator

Problem:    Vim9: "..=" does not accept same types as the ".." operator.
Solution:   Convert value to string like ".." does. (issue #8664)

src/testdir/test_vim9_assign.vim

@@ -239,6 +239,32 @@ def Test_assignment()
   END
 enddef
 
+let g:someNumber = 43
+
+def Test_assign_concat()
+  var lines =<< trim END
+    var s = '-'
+    s ..= 99
+    s ..= true
+    s ..= '-'
+    s ..= v:null
+    s ..= g:someNumber
+    assert_equal('-99true-null43', s)
+  END
+  CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+    var s = '-'
+    s ..= [1, 2]
+  END
+  CheckDefAndScriptFailure2(lines, 'E1105: Cannot convert list to string', 'E734: Wrong variable type for .=', 2)
+  lines =<< trim END
+    var s = '-'
+    s ..= {a: 2}
+  END
+  CheckDefAndScriptFailure2(lines, 'E1105: Cannot convert dict to string', 'E734: Wrong variable type for .=', 2)
+enddef
+
 def Test_assign_register()
   var lines =<< trim END
     @c = 'areg'

src/testdir/test_vim9_disassemble.vim

@@ -1254,7 +1254,7 @@ def Test_disassemble_for_loop_eval()
         'res ..= str\_s*' ..
         '\d\+ LOAD $0\_s*' ..
         '\d\+ LOAD $2\_s*' ..
-        '\d\+ CHECKTYPE string stack\[-1\]\_s*' ..
+        '\d 2STRING_ANY stack\[-1\]\_s*' ..
         '\d\+ CONCAT\_s*' ..
         '\d\+ STORE $0\_s*' ..
         'endfor\_s*' ..

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3263,
 /**/
     3262,
 /**/

src/vim9compile.c

@@ -7086,8 +7086,12 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	    type_T	    *stacktype;
 
 	    if (*op == '.')
-		expected = &t_string;
+	    {
+		if (may_generate_2STRING(-1, FALSE, cctx) == FAIL)
+		    goto theend;
+	    }
 	    else
+	    {
 		expected = lhs.lhs_member_type;
 		stacktype = ((type_T **)stack->ga_data)[stack->ga_len - 1];
 		if (
@@ -7098,6 +7102,7 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		    need_type(stacktype, expected, -1, 0, cctx,
 							 FALSE, FALSE) == FAIL)
 		    goto theend;
+	    }
 
 	    if (*op == '.')
 	    {