Update checker.v

Need check `sum_type` and `function` also.
And `function` type try to get the correct mod name.

vlib/v/checker/checker.v

@@ -5293,11 +5293,25 @@ fn (mut c Checker) ensure_type_exists(typ ast.Type, pos token.Pos) bool {
 		return false
 	}
 	sym := c.table.sym(typ)
-	if !c.is_builtin_mod && sym.kind in [.struct, .alias] && !sym.is_pub && sym.mod != c.mod
-		&& c.comptime.comptime_for_field_var == '' && !c.inside_recheck {
-		c.error('${sym.kind} `${sym.name}` was declared as private to module `${sym.mod}`, so it can not be used inside module `${c.mod}`',
-			pos)
-		return false
+	if !c.is_builtin_mod && sym.kind in [.struct, .alias, .sum_type, .function]! && !sym.is_pub
+		&& sym.mod != c.mod && c.comptime.comptime_for_field_var == '' && !c.inside_recheck {
+		if sym.kind == .function {
+			fn_info := sym.info as ast.FnType
+			// hack: recover fn mod from func name
+			mut fn_mod := fn_info.func.name.all_before_last('.')
+			if fn_mod == fn_info.func.name {
+				fn_mod = 'builtin'
+			}
+			if fn_mod != '' && fn_mod != c.mod && fn_info.func.name != '' && !fn_info.is_anon {
+				c.error('function type `${fn_info.func.name}` was declared as private to module `${fn_mod}`, so it can not be used inside module `${c.mod}`',
+					pos)
+				return false
+			}
+		} else {
+			c.error('${sym.kind} `${sym.name}` was declared as private to module `${sym.mod}`, so it can not be used inside module `${c.mod}`',
+				pos)
+			return false
+		}
 	}
 	match sym.kind {
 		.placeholder {