patch 8.2.4586: Vim9: no error for using lower case name for "func" argument

Problem:    Vim9: no error for using lower case name for "func" argument.
            (Ernie Rael)
Solution:   Check the name as soon as the type is known.

src/testdir/test_vim9_func.vim

@@ -1479,12 +1479,12 @@ def Test_pass_legacy_lambda_to_def_func()
 
   lines =<< trim END
       vim9script
-      def g:TestFunc(f: func)
+      def g:TestFunc(F: func)
       enddef
       legacy call g:TestFunc({-> 0})
       delfunc g:TestFunc
 
-      def g:TestFunc(f: func(number))
+      def g:TestFunc(F: func(number))
       enddef
       legacy call g:TestFunc({nr -> 0})
       delfunc g:TestFunc
@@ -3788,8 +3788,8 @@ def Test_check_func_arg_types()
         return x + 1
       enddef
 
-      def G(g: func): dict<func>
-        return {f: g}
+      def G(Fg: func): dict<func>
+        return {f: Fg}
       enddef
 
       def H(d: dict<func>): string
@@ -3799,6 +3799,8 @@ def Test_check_func_arg_types()
 
   v9.CheckScriptSuccess(lines + ['echo H(G(F1))'])
   v9.CheckScriptFailure(lines + ['echo H(G(F2))'], 'E1013:')
+
+  v9.CheckScriptFailure(lines + ['def SomeFunc(ff: func)', 'enddef'], 'E704:')
 enddef
 
 def Test_call_func_with_null()

src/userfunc.c

@@ -429,6 +429,12 @@ parse_argument_types(ufunc_T *fp, garray_T *argtypes, int varargs)
 		if (type == NULL)
 		    return FAIL;
 		fp->uf_arg_types[i] = type;
+		if (i < fp->uf_args.ga_len
+			&& (type->tt_type == VAR_FUNC
+			    || type->tt_type == VAR_PARTIAL)
+			&& var_wrong_func_name(
+				    ((char_u **)fp->uf_args.ga_data)[i], TRUE))
+		    return FAIL;
 	    }
 	}
     }

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4586,
 /**/
     4585,
 /**/