✨ Add query to look up values in environment

include/stdx/env.hpp

@@ -93,6 +93,20 @@ using append_env_t = boost::mp11::mp_unique_if<
 
 template <_env::autowrap... Args>
 using make_env_t = extend_env_t<env<>, Args...>;
+
+template <envlike Env, typename Q> constexpr auto query(Q q) -> decltype(auto) {
+    return q(Env{});
+}
+
+template <envlike Env, typename Q, typename Default>
+constexpr auto query(Q q, [[maybe_unused]] Default &&d) -> decltype(auto) {
+    if constexpr (requires { q(Env{}); }) {
+        return q(Env{});
+    } else {
+        return std::forward<Default>(d);
+    }
+}
+
 } // namespace v1
 } // namespace stdx
 

test/env.cpp

@@ -18,10 +18,29 @@ namespace {
 } custom;
 } // namespace
 
-TEST_CASE("lookup query with default", "[env]") {
+TEST_CASE("lookup query with internal default", "[env]") {
     STATIC_REQUIRE(custom(stdx::env<>{}) == 42);
 }
 
+TEST_CASE("lookup query with default (free function)", "[env]") {
+    STATIC_REQUIRE(stdx::query<stdx::env<>>(custom) == 42);
+}
+
+namespace {
+[[maybe_unused]] constexpr inline struct custom_no_default_t {
+    template <typename T>
+    [[nodiscard]] CONSTEVAL auto operator()(T &&t) const
+        noexcept(noexcept(std::forward<T>(t).query(std::declval<custom_t>())))
+            -> decltype(std::forward<T>(t).query(*this)) {
+        return std::forward<T>(t).query(*this);
+    }
+} custom_no_default;
+} // namespace
+
+TEST_CASE("lookup query with external default", "[env]") {
+    STATIC_REQUIRE(stdx::query<stdx::env<>>(custom_no_default, 42) == 42);
+}
+
 TEST_CASE("make an environment", "[env]") {
     using E = stdx::make_env_t<custom, 17>;
     STATIC_REQUIRE(custom(E{}) == 17);