patch 8.2.0206: calling Vim9 function using default argument fails

Problem:    Calling Vim9 function using default argument fails.
Solution:   Give an appropriate error. (closes #5572)

src/testdir/test_vim9_script.vim

@@ -131,6 +131,34 @@ def Test_call_varargs()
   assert_equal('one,two,three', MyVarargs('one', 'two', 'three'))
 enddef
 
+"def Test_call_func_defined_later()
+"  call assert_equal('one', DefineLater('one'))
+"  call assert_fails('call NotDefined("one")', 'E99:')
+"enddef
+
+func DefineLater(arg)
+  return a:arg
+endfunc
+
+def MyDefaultArgs(name = 'string'): string
+  return name
+enddef
+
+func Test_call_default_args_from_func()
+  " TODO: implement using default value for optional argument
+  "call assert_equal('string', MyDefaultArgs())
+  call assert_fails('call MyDefaultArgs()', 'optional arguments not implemented yet')
+  call assert_equal('one', MyDefaultArgs('one'))
+  call assert_fails('call MyDefaultArgs("one", "two")', 'E118:')
+endfunc
+
+def Test_call_default_args()
+  " TODO: implement using default value for optional argument
+  "assert_equal('string', MyDefaultArgs())
+  assert_equal('one', MyDefaultArgs('one'))
+  assert_fails('call MyDefaultArgs("one", "two")', 'E118:')
+enddef
+
 def Test_return_type_wrong()
   " TODO: why is ! needed for Mac and FreeBSD?
   CheckScriptFailure(['def Func(): number', 'return "a"', 'enddef'], 'expected number but got string')

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    206,
 /**/
     205,
 /**/

src/vim9compile.c

@@ -1024,9 +1024,11 @@ generate_UCALL(cctx_T *cctx, char_u *name, int argcount)
     isn->isn_arg.ufunc.cuf_argcount = argcount;
 
     stack->ga_len -= argcount; // drop the arguments
-
-    // drop the funcref/partial, get back the return value
-    ((type_T **)stack->ga_data)[stack->ga_len - 1] = &t_any;
+    if (ga_grow(stack, 1) == FAIL)
+	return FAIL;
+    // add return value
+    ((type_T **)stack->ga_data)[stack->ga_len] = &t_any;
+    ++stack->ga_len;
 
     return OK;
 }

src/vim9execute.c

@@ -362,6 +362,7 @@ call_def_function(
     int		idx;
     int		ret = FAIL;
     dfunc_T	*dfunc;
+    int		optcount = ufunc_argcount(ufunc) - argc;
 
 // Get pointer to item in the stack.
 #define STACK_TV(idx) (((typval_T *)ectx.ec_stack.ga_data) + idx)
@@ -392,6 +393,12 @@ call_def_function(
     ectx.ec_frame = ectx.ec_stack.ga_len;
     initial_frame_ptr = ectx.ec_frame;
 
+// TODO: Put omitted argument default values on the stack.
+    if (optcount > 0)
+    {
+	emsg("optional arguments not implemented yet");
+	return FAIL;
+    }
     // dummy frame entries
     for (idx = 0; idx < STACK_FRAME_SIZE; ++idx)
     {