c++: simplify handling implicit INDIRECT_REF and co_await in convert_to_void

convert_to_void has, so far, when converting a co_await expression to
void altered the await_resume expression of a co_await so that it is
also converted to void.  This meant that the type of the await_resume
expression, which is also supposed to be the type of the whole co_await
expression, was not the same as the type of the CO_AWAIT_EXPR tree.

While this has not caused problems so far, it is unexpected, I think.

Also, convert_to_void had a special case when an INDIRECT_REF wrapped a
CALL_EXPR.  In this case, we also diagnosed maybe_warn_nodiscard.  This
was a duplication of logic related to converting call expressions to
void.

Instead, we can generalize a bit, and rather discard the expression that
was implicitly dereferenced instead.

This patch changes the diagnostic of:

  void f(struct S* x) { static_cast<volatile S&>(*x); }

... from:

  warning: indirection will not access object of incomplete type
           'volatile S' in statement

... to:

  warning: implicit dereference will not access object of type
           ‘volatile S’ in statement

... but should have no impact in other cases.

gcc/cp/ChangeLog:

	* coroutines.cc (co_await_get_resume_call): Return a tree
	directly, rather than a tree pointer.
	* cp-tree.h (co_await_get_resume_call): Adjust signature
	accordingly.
	* cvt.cc (convert_to_void): Do not alter CO_AWAIT_EXPRs when
	discarding them.  Simplify handling implicit INDIRECT_REFs.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/nodiscard-1.C: New test.

4 files changed, 132 insertions, 53 deletions

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index 50904e0..8e4c55a 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -730,14 +730,14 @@ coro_get_destroy_function (tree decl)
 
 /* Given a CO_AWAIT_EXPR AWAIT_EXPR, return its resume call.  */
 
-tree*
+tree
 co_await_get_resume_call (tree await_expr)
 {
   gcc_checking_assert (TREE_CODE (await_expr) == CO_AWAIT_EXPR);
   tree vec = TREE_OPERAND (await_expr, 3);
   if (!vec)
     return nullptr;
-  return &TREE_VEC_ELT (vec, 2);
+  return TREE_VEC_ELT (vec, 2);
 }
 
 
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 7c438ec..39c065e 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8782,7 +8782,7 @@ extern tree coro_get_actor_function		(tree);
 extern tree coro_get_destroy_function		(tree);
 extern tree coro_get_ramp_function		(tree);
 
-extern tree* co_await_get_resume_call		(tree await_expr);
+extern tree co_await_get_resume_call		(tree await_expr);
 
 
 /* contracts.cc */
diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index df02b8f..526937d 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -1272,88 +1272,96 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 	  complete_type (type);
 	int is_complete = COMPLETE_TYPE_P (type);
 
-	/* Can't load the value if we don't know the type.  */
-	if (is_volatile && !is_complete)
+	/* Don't load the value if this is an implicit dereference, or if
+	   the type needs to be handled by ctors/dtors.  */
+	if (is_reference)
           {
-            if (complain & tf_warning)
+            if (is_volatile && (complain & tf_warning)
+		/* A co_await expression, in its await_resume expression, also
+		   contains an implicit dereference.  As a result, we don't
+		   need to warn about them here.  */
+		&& TREE_CODE (TREE_OPERAND (expr, 0)) != CO_AWAIT_EXPR)
 	      switch (implicit)
 		{
 	      	  case ICV_CAST:
 		    warning_at (loc, 0, "conversion to void will not access "
-				"object of incomplete type %qT", type);
+				"object of type %qT", type);
 		    break;
 		  case ICV_SECOND_OF_COND:
-		    warning_at (loc, 0, "indirection will not access object of "
-				"incomplete type %qT in second operand "
-				"of conditional expression", type);
+		    warning_at (loc, 0, "implicit dereference will not access "
+				"object of type %qT in second operand of "
+				"conditional expression", type);
 		    break;
 		  case ICV_THIRD_OF_COND:
-		    warning_at (loc, 0, "indirection will not access object of "
-				"incomplete type %qT in third operand "
-				"of conditional expression", type);
+		    warning_at (loc, 0, "implicit dereference will not access "
+				"object of type %qT in third operand of "
+				"conditional expression", type);
 		    break;
 		  case ICV_RIGHT_OF_COMMA:
-		    warning_at (loc, 0, "indirection will not access object of "
-				"incomplete type %qT in right operand of "
+		    warning_at (loc, 0, "implicit dereference will not access "
+				"object of type %qT in right operand of "
 				"comma operator", type);
 		    break;
 		  case ICV_LEFT_OF_COMMA:
-		    warning_at (loc, 0, "indirection will not access object of "
-				"incomplete type %qT in left operand of "
-				"comma operator", type);
+		    warning_at (loc, 0, "implicit dereference will not access "
+				"object of type %qT in left operand of comma "
+				"operator", type);
 		    break;
 		  case ICV_STATEMENT:
-		    warning_at (loc, 0, "indirection will not access object of "
-				"incomplete type %qT in statement", type);
+		    warning_at (loc, 0, "implicit dereference will not access "
+				"object of type %qT in statement",  type);
 		     break;
 		  case ICV_THIRD_IN_FOR:
-		    warning_at (loc, 0, "indirection will not access object of "
-				"incomplete type %qT in for increment "
-				"expression", type);
+		    warning_at (loc, 0, "implicit dereference will not access "
+				"object of type %qT in for increment expression",
+				type);
 		    break;
 		  default:
 		    gcc_unreachable ();
 		}
+
+	    /* Since this was an implicit dereference, we should also act as if
+	       it was never there.  */
+	    return convert_to_void (TREE_OPERAND (expr, 0), implicit, complain);
           }
-	/* Don't load the value if this is an implicit dereference, or if
-	   the type needs to be handled by ctors/dtors.  */
-	else if (is_volatile && is_reference)
+	/* Can't load the value if we don't know the type.  */
+	else if (is_volatile && !is_complete)
           {
             if (complain & tf_warning)
 	      switch (implicit)
 		{
 	      	  case ICV_CAST:
 		    warning_at (loc, 0, "conversion to void will not access "
-				"object of type %qT", type);
+				"object of incomplete type %qT", type);
 		    break;
 		  case ICV_SECOND_OF_COND:
-		    warning_at (loc, 0, "implicit dereference will not access "
-				"object of type %qT in second operand of "
-				"conditional expression", type);
+		    warning_at (loc, 0, "indirection will not access object of "
+				"incomplete type %qT in second operand "
+				"of conditional expression", type);
 		    break;
 		  case ICV_THIRD_OF_COND:
-		    warning_at (loc, 0, "implicit dereference will not access "
-				"object of type %qT in third operand of "
-				"conditional expression", type);
+		    warning_at (loc, 0, "indirection will not access object of "
+				"incomplete type %qT in third operand "
+				"of conditional expression", type);
 		    break;
 		  case ICV_RIGHT_OF_COMMA:
-		    warning_at (loc, 0, "implicit dereference will not access "
-				"object of type %qT in right operand of "
+		    warning_at (loc, 0, "indirection will not access object of "
+				"incomplete type %qT in right operand of "
 				"comma operator", type);
 		    break;
 		  case ICV_LEFT_OF_COMMA:
-		    warning_at (loc, 0, "implicit dereference will not access "
-				"object of type %qT in left operand of comma "
-				"operator", type);
+		    warning_at (loc, 0, "indirection will not access object of "
+				"incomplete type %qT in left operand of "
+				"comma operator", type);
 		    break;
 		  case ICV_STATEMENT:
-		    warning_at (loc, 0, "implicit dereference will not access "
-				"object of type %qT in statement",  type);
+		    warning_at (loc, 0, "indirection will not access object of "
+				"incomplete type %qT in statement", type);
 		     break;
 		  case ICV_THIRD_IN_FOR:
-		    warning_at (loc, 0, "implicit dereference will not access "
-				"object of type %qT in for increment expression",
-				type);
+		    warning_at (loc, 0, "indirection will not access object of "
+				"incomplete type %qT in for increment "
+				"expression", type);
 		    break;
 		  default:
 		    gcc_unreachable ();
@@ -1403,7 +1411,7 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 		    gcc_unreachable ();
 		}
 	  }
-	if (is_reference || !is_volatile || !is_complete || TREE_ADDRESSABLE (type))
+	if (!is_volatile || !is_complete || TREE_ADDRESSABLE (type))
           {
             /* Emit a warning (if enabled) when the "effect-less" INDIRECT_REF
                operation is stripped off. Note that we don't warn about
@@ -1418,9 +1426,6 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
                 && !is_reference)
               warning_at (loc, OPT_Wunused_value, "value computed is not used");
             expr = TREE_OPERAND (expr, 0);
-	    if (TREE_CODE (expr) == CALL_EXPR
-		&& (complain & tf_warning))
-	      maybe_warn_nodiscard (expr, implicit);
           }
 
 	break;
@@ -1503,12 +1508,9 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
       break;
 
     case CO_AWAIT_EXPR:
-      {
-	auto awr = co_await_get_resume_call (expr);
-	if (awr && *awr)
-	  *awr = convert_to_void (*awr, implicit, complain);
-	break;
-      }
+      if (auto awr = co_await_get_resume_call (expr))
+	convert_to_void (awr, implicit, complain);
+      break;
 
     default:;
     }
diff --git a/gcc/testsuite/g++.dg/coroutines/nodiscard-1.C b/gcc/testsuite/g++.dg/coroutines/nodiscard-1.C
new file mode 100644
index 0000000..b4c6729
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/nodiscard-1.C
@@ -0,0 +1,77 @@
+#include <coroutine>
+/* Make sure that we correctly warn for unused co_await results.  */
+
+struct task
+{
+  struct promise_type
+  {
+    std::suspend_never initial_suspend () noexcept;
+    std::suspend_never final_suspend () noexcept;
+    void return_void ();
+    void unhandled_exception ();
+    task get_return_object ();
+  };
+};
+
+template<typename T>
+struct nodiscardable : std::suspend_never
+{ [[nodiscard]] T await_resume (); };
+
+template<typename T>
+struct discardable : std::suspend_never
+{ T await_resume (); };
+
+task
+thing ()
+{
+  co_await nodiscardable<int&>{}; /* { dg-warning "attribute 'nodiscard'" }  */
+  co_await nodiscardable<int>{}; /* { dg-warning "attribute 'nodiscard'" }  */
+
+  (void) co_await nodiscardable<int&>{};
+  (void) co_await nodiscardable<int>{};
+
+  co_await discardable<int&>{};
+  co_await discardable<volatile int&>{};
+  /* { dg-warning "implicit dereference will not access" "" { target *-*-* } {.-1} }  */
+}
+
+template<typename>
+task
+other_thing ()
+{
+  co_await nodiscardable<int&>{}; /* { dg-warning "attribute 'nodiscard'" }  */
+  co_await nodiscardable<int>{}; /* { dg-warning "attribute 'nodiscard'" }  */
+
+  (void) co_await nodiscardable<int&>{};
+  (void) co_await nodiscardable<int>{};
+
+  co_await discardable<int&>{};
+  co_await discardable<volatile int&>{};
+  /* { dg-warning "implicit dereference will not access" "" { target *-*-* } {.-1} }  */
+}
+
+void
+other_thing_caller ()
+{
+  other_thing <int> ();
+}
+
+task
+yet_another_thing (auto)
+{
+  co_await nodiscardable<int&>{}; /* { dg-warning "attribute 'nodiscard'" }  */
+  co_await nodiscardable<int>{}; /* { dg-warning "attribute 'nodiscard'" }  */
+
+  (void) co_await nodiscardable<int&>{};
+  (void) co_await nodiscardable<int>{};
+
+  co_await discardable<int&>{};
+  co_await discardable<volatile int&>{};
+  /* { dg-warning "implicit dereference will not access" "" { target *-*-* } {.-1} }  */
+}
+
+void
+yet_another_thing_caller ()
+{
+  yet_another_thing (1);
+}