v/fmt: fix formatting of type Foo = fn (a int)? (#6047)

vlib/v/fmt/fmt.v

@@ -562,7 +562,9 @@ pub fn (mut f Fmt) type_decl(node ast.TypeDecl) {
 			f.write(')')
 			if fn_info.return_type.idx() != table.void_type_idx {
 				ret_str := f.no_cur_mod(f.table.type_to_str(fn_info.return_type))
-				f.write(' ' + ret_str)
+				f.write(' $ret_str')
+			} else if fn_info.return_type.has_flag(.optional) {
+				f.write(' ?')
 			}
 		}
 		ast.SumTypeDecl {

vlib/v/fmt/tests/fntype_return_optional_expected.vv

@@ -0,0 +1,3 @@
+type Foo = fn (a int) ?
+
+type Foo2 = fn (num int) ?int

vlib/v/fmt/tests/fntype_return_optional_input.vv

@@ -0,0 +1,3 @@
+type Foo = fn (a int)?
+
+type Foo2 = fn (num int)?int