util: Make Expected::value() throw

This is not expected to be needed in this codebase, but brings the
implementation closer to std::expected::value().

Also, add noexcept, where std::expected has them. This will make
operator-> and operator* terminate, when has_value() is false.

src/test/util_expected_tests.cpp

@@ -53,6 +53,15 @@ BOOST_AUTO_TEST_CASE(expected_value_or)
     BOOST_CHECK_EQUAL(const_val.value_or("fallback"), "fallback");
 }
 
+BOOST_AUTO_TEST_CASE(expected_value_throws)
+{
+    const Expected<int, std::string> e{Unexpected{"fail"}};
+    BOOST_CHECK_THROW(e.value(), BadExpectedAccess);
+
+    const Expected<void, std::string> void_e{Unexpected{"fail"}};
+    BOOST_CHECK_THROW(void_e.value(), BadExpectedAccess);
+}
+
 BOOST_AUTO_TEST_CASE(expected_error)
 {
     Expected<void, std::string> e{};

src/util/expected.h

@@ -8,6 +8,7 @@
 #include <attributes.h>
 
 #include <cassert>
+#include <exception>
 #include <type_traits>
 #include <utility>
 #include <variant>
@@ -30,6 +31,10 @@ private:
     E m_error;
 };
 
+struct BadExpectedAccess : std::exception {
+    const char* what() const noexcept override { return "Bad util::Expected access"; }
+};
+
 /// The util::Expected class provides a standard way for low-level functions to
 /// return either error values or result values.
 ///
@@ -55,12 +60,16 @@ public:
 
     constexpr const ValueType& value() const LIFETIMEBOUND
     {
-        assert(has_value());
+        if (!has_value()) {
+            throw BadExpectedAccess{};
+        }
         return std::get<0>(m_data);
     }
     constexpr ValueType& value() LIFETIMEBOUND
     {
-        assert(has_value());
+        if (!has_value()) {
+            throw BadExpectedAccess{};
+        }
         return std::get<0>(m_data);
     }
 
@@ -86,11 +95,11 @@ public:
         return std::get<1>(m_data);
     }
 
-    constexpr ValueType& operator*() LIFETIMEBOUND { return value(); }
+    constexpr ValueType& operator*() noexcept LIFETIMEBOUND { return value(); }
-    constexpr const ValueType& operator*() const LIFETIMEBOUND { return value(); }
+    constexpr const ValueType& operator*() const noexcept LIFETIMEBOUND { return value(); }
 
-    constexpr ValueType* operator->() LIFETIMEBOUND { return &value(); }
+    constexpr ValueType* operator->() noexcept LIFETIMEBOUND { return &value(); }
-    constexpr const ValueType* operator->() const LIFETIMEBOUND { return &value(); }
+    constexpr const ValueType* operator->() const noexcept LIFETIMEBOUND { return &value(); }
 };
 
 } // namespace util