patch 8.2.1339: Vim9: assigning to global dict variable doesn't work

Problem:    Vim9: assigning to global dict variable doesn't work.
Solution:   Guess variable type based in index type. (issue #6591)

Author: brammool

src/testdir/test_vim9_script.vim

@@ -274,6 +274,30 @@ def Test_assignment_dict()
     FillDict()
   END
   call CheckScriptFailure(lines, 'E1103:')
+
+  # assignment to global dict
+  lines =<< trim END
+    vim9script
+    g:test = {}
+    def FillDict(): dict<any>
+      g:test['a'] = 43
+      return g:test
+    enddef
+    assert_equal(#{a: 43}, FillDict())
+  END
+  call CheckScriptSuccess(lines)
+
+  # assignment to buffer dict
+  lines =<< trim END
+    vim9script
+    b:test = {}
+    def FillDict(): dict<any>
+      b:test['a'] = 43
+      return b:test
+    enddef
+    assert_equal(#{a: 43}, FillDict())
+  END
+  call CheckScriptSuccess(lines)
 enddef
 
 def Test_assignment_local()

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1339,
 /**/
     1338,
 /**/

src/vim9compile.c

@@ -5486,11 +5486,9 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	    {
 		has_index = TRUE;
 		if (type->tt_member == NULL)
-		{
-		    semsg(_("E1088: cannot use an index on %s"), name);
-		    goto theend;
-		}
-		member_type = type->tt_member;
+		    member_type = &t_any;
+		else
+		    member_type = type->tt_member;
 	    }
 	    else
 	    {
@@ -5719,6 +5717,18 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		emsg(_(e_missbrac));
 		goto theend;
 	    }
+	    if (type == &t_any)
+	    {
+		type_T	    *idx_type = ((type_T **)stack->ga_data)[
+							    stack->ga_len - 1];
+		// Index on variable of unknown type: guess the type from the
+		// index type: number is dict, otherwise dict.
+		// TODO: should do the assignment at runtime
+		if (idx_type->tt_type == VAR_NUMBER)
+		    type = &t_list_any;
+		else
+		    type = &t_dict_any;
+	    }
 	    if (type->tt_type == VAR_DICT
 		    && may_generate_2STRING(-1, cctx) == FAIL)
 		goto theend;