patch 8.2.2580: Vim9: checking vararg type may be wrong

Problem:    Vim9: checking vararg type is wrong when function is auto-loaded.
Solution:   Use the member type. (closes #7933)
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 2743668..32312b0 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -3154,6 +3154,10 @@
        return 'test'
      enddef
      g:some#name = 'name'
+
+     def some#varargs(a1: string, ...l: list<string>): string
+       return a1 .. l[0] .. l[1]
+     enddef
   END
 
   mkdir('Xdir/autoload', 'p')
@@ -3166,6 +3170,8 @@
   g:some#other = 'other'
   assert_equal('other', g:some#other)
 
+  assert_equal('abc', some#varargs('a', 'b', 'c'))
+
   # upper case script name works
   lines =<< trim END
      vim9script
diff --git a/src/version.c b/src/version.c
index 4a4d65a..f3c79c8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2580,
+/**/
     2579,
 /**/
     2578,
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 2e7e204..d4435c4 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -807,9 +807,12 @@
 	    // types are correct.
 	    for (i = 0; i < argcount; ++i)
 	    {
-		type_T *type = i < ufunc->uf_args.ga_len
-				  ? ufunc->uf_arg_types[i] : ufunc->uf_va_type;
+		type_T *type = NULL;
 
+		if (i < ufunc->uf_args.ga_len)
+		    type = ufunc->uf_arg_types[i];
+		else if (ufunc->uf_va_type != NULL)
+		    type = ufunc->uf_va_type->tt_member;
 		if (type != NULL && check_typval_arg_type(type,
 						      &argv[i], i + 1) == FAIL)
 		    return FAIL;