patch 8.2.1300: Vim9: optional argument type not parsed properly

Problem:    Vim9: optional argument type not parsed properly.
Solution:   Skip over the "?". (issue #6507)

src/evalvars.c

@@ -1013,7 +1013,7 @@ skip_var_one(char_u *arg, int include_type)
 	if (end == arg + 2 && end[-1] == ':')
 	    --end;
 	if (*end == ':')
-	    end = skip_type(skipwhite(end + 1));
+	    end = skip_type(skipwhite(end + 1), FALSE);
     }
     return end;
 }

src/proto/vim9compile.pro

@@ -6,7 +6,7 @@ type_T *typval2type_vimvar(typval_T *tv, garray_T *type_gap);
 int check_typval_type(type_T *expected, typval_T *actual_tv);
 int check_type(type_T *expected, type_T *actual, int give_msg);
 int check_compare_types(exptype_T type, typval_T *tv1, typval_T *tv2);
-char_u *skip_type(char_u *start);
+char_u *skip_type(char_u *start, int optional);
 type_T *parse_type(char_u **arg, garray_T *type_gap);
 char *vartype_name(vartype_T type);
 char *type_name(type_T *type, char **tofree);

src/testdir/test_vim9_func.vim

@@ -350,6 +350,19 @@ def Test_call_funcref()
     let Funcref: func(string) = function('UseNumber')
   END
   CheckScriptFailure(lines, 'E1013: type mismatch, expected func(string) but got func(number)')
+
+  lines =<< trim END
+    vim9script
+    def EchoNr(nr = 34)
+      g:echo = nr
+    enddef
+    let Funcref: func(?number) = function('EchoNr')
+    Funcref()
+    assert_equal(34, g:echo)
+    Funcref(123)
+    assert_equal(123, g:echo)
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 let SomeFunc = function('len')

src/userfunc.c

@@ -123,7 +123,7 @@ one_function_arg(char_u *arg, garray_T *newargs, garray_T *argtypes, int skip)
 		return arg;
 	    }
 	    type = skipwhite(p);
-	    p = skip_type(type);
+	    p = skip_type(type, TRUE);
 	    type = vim_strnsave(type, p - type);
 	}
 	else if (*skipwhite(p) != '=')
@@ -2778,7 +2778,7 @@ def_function(exarg_T *eap, char_u *name_arg)
 	if (*p == ':')
 	{
 	    ret_type = skipwhite(p + 1);
-	    p = skip_type(ret_type);
+	    p = skip_type(ret_type, FALSE);
 	    if (p > ret_type)
 	    {
 		ret_type = vim_strnsave(ret_type, p - ret_type);

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1300,
 /**/
     1299,
 /**/

src/vim9compile.c

@@ -534,8 +534,16 @@ typval2type(typval_T *tv, garray_T *type_gap)
 	if (name != NULL)
 	    // TODO: how about a builtin function?
 	    ufunc = find_func(name, FALSE, NULL);
-	if (ufunc != NULL && ufunc->uf_func_type != NULL)
+	if (ufunc != NULL)
-	    return ufunc->uf_func_type;
+	{
+	    // May need to get the argument types from default values by
+	    // compiling the function.
+	    if (ufunc->uf_def_status == UF_TO_BE_COMPILED
+			    && compile_def_function(ufunc, TRUE, NULL) == FAIL)
+		return NULL;
+	    if (ufunc->uf_func_type != NULL)
+		return ufunc->uf_func_type;
+	}
     }
 
     actual = alloc_type(type_gap);
@@ -1916,12 +1924,15 @@ free_locals(cctx_T *cctx)
 
 /*
  * Skip over a type definition and return a pointer to just after it.
+ * When "optional" is TRUE then a leading "?" is accepted.
  */
     char_u *
-skip_type(char_u *start)
+skip_type(char_u *start, int optional)
 {
     char_u *p = start;
 
+    if (optional && *p == '?')
+	++p;
     while (ASCII_ISALNUM(*p) || *p == '_')
 	++p;
 
@@ -1929,7 +1940,7 @@ skip_type(char_u *start)
     if (*skipwhite(p) == '<')
     {
 	p = skipwhite(p);
-	p = skip_type(skipwhite(p + 1));
+	p = skip_type(skipwhite(p + 1), FALSE);
 	p = skipwhite(p);
 	if (*p == '>')
 	    ++p;
@@ -1945,7 +1956,7 @@ skip_type(char_u *start)
 	    {
 		char_u *sp = p;
 
-		p = skip_type(p);
+		p = skip_type(p, TRUE);
 		if (p == sp)
 		    return p;  // syntax error
 		if (*p == ',')
@@ -1954,7 +1965,7 @@ skip_type(char_u *start)
 	    if (*p == ')')
 	    {
 		if (p[1] == ':')
-		    p = skip_type(skipwhite(p + 2));
+		    p = skip_type(skipwhite(p + 2), FALSE);
 		else
 		    ++p;
 	    }
@@ -1962,7 +1973,7 @@ skip_type(char_u *start)
 	else
 	{
 	    // handle func: return_type
-	    p = skip_type(skipwhite(p + 1));
+	    p = skip_type(skipwhite(p + 1), FALSE);
 	}
     }