coroutines: Handle initial awaiters with non-void returns [PR 100127].

The way in which a C++20 coroutine is specified discards any value
that might be returned from the initial or final await expressions.

This ICE was caused by an initial await expression with an
await_resume () returning a reference, the function rewrite code
was not set up to expect this.

Fixed by looking through any indirection present and by explicitly
discarding the value, if any, returned by await_resume().

It does not seem useful to make a diagnostic for this, since
the user could define a generic awaiter that usefully returns
values when used in a different position from the initial (or
final) await expressions.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

	PR c++/100127

gcc/cp/ChangeLog:

	* coroutines.cc (coro_rewrite_function_body): Handle initial
	await expressions that try to produce a reference value.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr100127.C: New test.

gcc/cp/coroutines.cc

@@ -4211,9 +4211,15 @@ coro_rewrite_function_body (location_t fn_start, tree fnbody, tree orig,
 	{
 	  /* Build a compound expression that sets the
 	     initial-await-resume-called variable true and then calls the
-	     initial suspend expression await resume.  */
+	     initial suspend expression await resume.
+	     In the case that the user decides to make the initial await
+	     await_resume() return a value, we need to discard it and, it is
+	     a reference type, look past the indirection.  */
+	  if (INDIRECT_REF_P (initial_await))
+	    initial_await = TREE_OPERAND (initial_await, 0);
 	  tree vec = TREE_OPERAND (initial_await, 3);
 	  tree aw_r = TREE_VEC_ELT (vec, 2);
+	  aw_r = convert_to_void (aw_r, ICV_STATEMENT, tf_warning_or_error);
 	  tree update = build2 (MODIFY_EXPR, boolean_type_node, i_a_r_c,
 				boolean_true_node);
 	  aw_r = cp_build_compound_expr (update, aw_r, tf_warning_or_error);

gcc/testsuite/g++.dg/coroutines/pr100127.C

@@ -0,0 +1,65 @@
+#ifdef __clang__
+#include <experimental/coroutine>
+namespace std {
+  using namespace std::experimental;
+}
+#else
+#include <coroutine>
+#endif
+#include <optional>
+
+struct future
+{
+    using value_type = int;
+    struct promise_type;
+    using handle_type = std::coroutine_handle<promise_type>;
+
+    handle_type _coroutine;
+
+    future(handle_type h) : _coroutine{h} {}
+
+    ~future() noexcept{
+        if (_coroutine) {
+            _coroutine.destroy();
+        }
+    }
+
+    value_type get() {
+        auto ptr = _coroutine.promise()._value;
+        return *ptr;
+    }
+
+    struct promise_type {
+        std::optional<value_type> _value = std::nullopt;
+
+        future get_return_object() {
+            return future{handle_type::from_promise(*this)};
+        }
+        void return_value(value_type val) {
+            _value = static_cast<value_type &&>(val);
+        }
+        auto initial_suspend() noexcept {
+            class awaiter {
+                std::optional<value_type> & value;
+            public:
+                explicit awaiter(std::optional<value_type> & val) noexcept : value{val} {}
+                bool await_ready() noexcept { return value.has_value(); }
+                void await_suspend(handle_type) noexcept { }
+                value_type & await_resume() noexcept { return *value; }
+            };
+
+            return awaiter{_value};
+        }
+        std::suspend_always final_suspend() noexcept {
+            return {};
+        }
+        //void return_void() {}
+        void unhandled_exception() {}
+    };
+};
+
+future create_future()
+{ co_return 2021; }
+
+int main()
+{ auto f = create_future(); }