ast: fix anon fn with nested anon fn argument (#15415)

vlib/v/ast/table.v

@@ -219,7 +219,7 @@ pub fn (t &Table) fn_type_signature(f &Fn) string {
 		} else {
 			sig += arg_type_sym.str().to_lower().replace_each(['.', '__', '&', '', '[', 'arr_',
 				'chan ', 'chan_', 'map[', 'map_of_', ']', '_to_', '<', '_T_', ',', '_', ' ', '',
-				'>', ''])
+				'>', '', '(', '_', ')', '_'])
 		}
 		if i < f.params.len - 1 {
 			sig += '_'

vlib/v/tests/anon_fn_with_nested_anon_fn_args_test.v

@@ -0,0 +1,12 @@
+module main
+
+fn test_anon_fn_with_nested_anon_fn_args() {
+	mut xa := fn (x fn (int) string, y int) string {
+		return x(y)
+	}
+	a := xa(fn (i int) string {
+		return 'a' + i.str()
+	}, 8)
+	println(a)
+	assert a == 'a8'
+}