fix

vlib/v/gen/c/assign.v

@@ -589,7 +589,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				}
 				fn_name := c_fn_name(g.get_ternary_name(ident.name))
 
-				if val_type.has_flag(.option) && val is ast.SelectorExpr {
+				if val_type.has_flag(.option) && val in [ast.CastExpr, ast.SelectorExpr] {
 					ret_styp := g.typ(g.unwrap_generic(val_type))
 					g.write('${ret_styp} ${fn_name}')
 				} else {

vlib/v/tests/options/option_fntype_assign_test.v

@@ -0,0 +1,16 @@
+type DataFn = fn (name string) string
+
+fn which_lang(name string) string {
+	return name
+}
+
+fn find_func(name string) ?DataFn {
+	a := ?DataFn(which_lang)
+	return a
+}
+
+fn test_main() {
+	if a := find_func('foo') {
+		assert a('bar') == 'bar'
+	}
+}