libtuil: allow non-default-constructible types in generators

references remain forbidden because std::optional does not want to
contain them, and specializing generators to use pointers where we
can't use optionals is simply too much work for a feature we don't
even need. reference wrappers and bindings still work well enough.

Change-Id: I2e6ca74719584ce16e2357c452fdd5c5a9e23d5a

lix/libutil/generator.hh

@@ -39,9 +39,9 @@ struct failure
 template<typename T>
 struct promise_state
 {
-    // result of the most recent coroutine resumption: a value,
-    // a nested coroutine to drain, an error, or our completion
-    std::variant<T, link<T>, failure, finished> value{};
+    // result of the most recent coroutine resumption: a nested
+    // coroutine to drain, a value, an error, or our completion
+    std::variant<link<T>, T, failure, finished> value{};
     // coroutine to resume when this one has finished. set when
     // one generator yields another, such that the entire chain
     // of parents always linearly points to the root generator.
@@ -86,7 +86,7 @@ struct promise : promise_state<T>
         }
     std::suspend_always yield_value(From && from)
     {
-        this->value.template emplace<0>(convert(std::forward<From>(from)));
+        this->value.template emplace<1>(convert(std::forward<From>(from)));
         return {};
     }
 

tests/unit/libutil/generator.cc

@@ -260,4 +260,20 @@ TEST(Generator, iterators)
     }
 }
 
+TEST(Generator, nonDefaultCtor)
+{
+    auto g = []() -> Generator<std::reference_wrapper<int>> {
+        int i = 0;
+        co_yield i;
+        i += 1;
+        co_yield i;
+    }();
+
+    auto i = g.next();
+    ASSERT_EQ(*i, 0);
+    i->get() = 10;
+    i = g.next();
+    ASSERT_EQ(*i, 11);
+}
+
 }