test: instantiate the reference specializations in the hardened build

tests/beman/expected/expected_hardened.test.cpp

@@ -9,6 +9,8 @@
 #include <catch2/catch_test_macros.hpp>
 
 #include <string>
+#include <type_traits>
+#include <utility>
 
 using namespace beman::expected;
 
@@ -89,6 +91,132 @@ TEST_CASE("hardened: error() on error-state expected<void,int>", "[hardened]") {
     CHECK(std::move(ce2).error() == 13);
 }
 
+// ---------------------------------------------------------------------------
+// Reference specializations
+//
+// Before this section the hardened build only ever instantiated expected<T, E>
+// and expected<void, E> with a value E, so the precondition checks in the
+// expected<T&, E> specialization were never compiled at all, and neither the
+// primary nor the void specialization was ever instantiated with a reference E.
+// The checks themselves are the same `if (!has_val_) TRAP` shape throughout;
+// what these add is proof that the guarded bodies still compile and return the
+// right thing once T or E is a reference -- in particular that the shallow-const
+// observers (T*, T&, E& from a const expected) are unaffected by hardening.
+//
+// Happy paths only, as above: a violated precondition traps, which cannot be
+// observed from inside the process.
+// ---------------------------------------------------------------------------
+
+// --- expected<T&, E>: operator-> and operator* (one const overload each) ---
+
+TEST_CASE("hardened: observers on value-state expected<T&,E>", "[hardened]") {
+    std::string                 s = "hello";
+    expected<std::string&, int> e(s);
+    CHECK(e->size() == 5);
+    CHECK(*e == "hello");
+    CHECK(&*e == &s);
+
+    // Shallow const: a const expected<T&, E> still hands out a mutable T& / T*.
+    const expected<std::string&, int> ce(s);
+    static_assert(std::is_same_v<decltype(ce.operator->()), std::string*>);
+    static_assert(std::is_same_v<decltype(*ce), std::string&>);
+    CHECK(ce->size() == 5);
+    CHECK(&*ce == &s);
+}
+
+// --- expected<T&, E>: error() ---
+
+TEST_CASE("hardened: error() on error-state expected<T&,E>", "[hardened]") {
+    expected<int&, int> e(unexpect, 42);
+    CHECK(e.error() == 42);
+
+    const expected<int&, int> ce(unexpect, 99);
+    CHECK(ce.error() == 99);
+
+    expected<int&, int> e2(unexpect, 7);
+    CHECK(std::move(e2).error() == 7);
+
+    const expected<int&, int> ce2(unexpect, 13);
+    CHECK(std::move(ce2).error() == 13);
+}
+
+// --- expected<T, E&>: primary template instantiated with a reference E ---
+
+TEST_CASE("hardened: observers on expected<T,E&>", "[hardened]") {
+    expected<std::string, int&> e(std::in_place, "world");
+    CHECK(e->size() == 5);
+    CHECK(*e == "world");
+
+    const expected<std::string, int&> ce(std::in_place, "there");
+    CHECK(ce->size() == 5);
+    CHECK(*ce == "there");
+
+    expected<std::string, int&> e2(std::in_place, "moved");
+    CHECK(*std::move(e2) == "moved");
+}
+
+TEST_CASE("hardened: error() on error-state expected<T,E&>", "[hardened]") {
+    int                 err = 42;
+    expected<int, int&> e(unexpect, err);
+    // Shallow const: error() yields int&, not const int&, even from a const
+    // expected -- `const E&` with E = int& collapses back to int&.
+    static_assert(std::is_same_v<decltype(e.error()), int&>);
+    CHECK(&e.error() == &err);
+
+    const expected<int, int&> ce(unexpect, err);
+    static_assert(std::is_same_v<decltype(ce.error()), int&>);
+    CHECK(&ce.error() == &err);
+
+    CHECK(&std::move(e).error() == &err);
+    CHECK(&std::move(ce).error() == &err);
+}
+
+// --- expected<T&, E&>: T& specialization instantiated with a reference E ---
+
+TEST_CASE("hardened: observers on expected<T&,E&>", "[hardened]") {
+    int                  x = 5;
+    expected<int&, int&> e(x);
+    CHECK(*e == 5);
+    CHECK(&*e == &x);
+
+    const expected<int&, int&> ce(x);
+    CHECK(&*ce == &x);
+}
+
+TEST_CASE("hardened: error() on error-state expected<T&,E&>", "[hardened]") {
+    int                  err = 7;
+    expected<int&, int&> e(unexpect, err);
+    CHECK(&e.error() == &err);
+
+    const expected<int&, int&> ce(unexpect, err);
+    CHECK(&ce.error() == &err);
+
+    CHECK(&std::move(e).error() == &err);
+    CHECK(&std::move(ce).error() == &err);
+}
+
+// --- expected<void, E&>: void specialization with a reference E ---
+
+TEST_CASE("hardened: operator* on value-state expected<void,E&>", "[hardened]") {
+    expected<void, int&> e;
+    *e;
+
+    const expected<void, int&> ce;
+    *ce;
+}
+
+TEST_CASE("hardened: error() on error-state expected<void,E&>", "[hardened]") {
+    int                  err = 42;
+    expected<void, int&> e(unexpect, err);
+    CHECK(&e.error() == &err);
+
+    const expected<void, int&> ce(unexpect, err);
+    CHECK(&ce.error() == &err);
+
+    CHECK(&std::move(e).error() == &err);
+    CHECK(&std::move(ce).error() == &err);
+}
+
 // ---------------------------------------------------------------------------
 // unexpected friend swap: constraint check (beman-only)
 // ---------------------------------------------------------------------------