Don't destroy value in expected<void,E>

Fixes #43

tests/issues.cpp

@@ -100,4 +100,9 @@ struct non_copyable {
 
 TEST_CASE("Issue 42", "[issues.42]") {
 	tl::expected<non_copyable,int>{}.map([](non_copyable) {});
+}
+
+TEST_CASE("Issue 43", "[issues.43]") {
+	auto result = tl::expected<void, std::string>{};
+	result = tl::make_unexpected(std::string{ "foo" });
 }

tl/expected.hpp

@@ -732,7 +732,7 @@ struct expected_operations_base : expected_storage_base<T, E> {
       if (rhs.m_has_val) {
         get() = std::forward<Rhs>(rhs).get();
       } else {
-        get().~T();
+		destroy_val();
         construct_error(std::forward<Rhs>(rhs).geterr());
       }
     } else {
@@ -763,6 +763,10 @@ struct expected_operations_base : expected_storage_base<T, E> {
     return std::move(this->m_unexpect);
   }
 #endif
+
+  constexpr void destroy_val() {
+	get().~T();
+  }
 };
 
 // This base class provides some handy member functions which can be used in
@@ -814,6 +818,10 @@ struct expected_operations_base<void, E> : expected_storage_base<void, E> {
     return std::move(this->m_unexpect);
   }
 #endif
+
+  constexpr void destroy_val() {
+	  //no-op
+  }
 };
 
 // This class manages conditionally having a trivial copy constructor
@@ -1655,7 +1663,7 @@ public:
     if (!has_value()) {
       err() = rhs;
     } else {
-      val().~T();
+      this->destroy_val();
       ::new (errptr()) unexpected<E>(rhs);
       this->m_has_val = false;
     }
@@ -1670,7 +1678,7 @@ public:
     if (!has_value()) {
       err() = std::move(rhs);
     } else {
-      val().~T();
+      this->destroy_val();
       ::new (errptr()) unexpected<E>(std::move(rhs));
       this->m_has_val = false;
     }