c++: diagnose usage of co_await and co_yield in default args [PR115906]

This is a partial fix for PR115906.  Per [expr.await] 2s3, "An
await-expression shall not appear in a default argument
([dcl.fct.default])".  This patch introduces the diagnostic in that
case, and in the case of a co_yield (as co_yield is defined in terms of
co_await, so prerequisites of co_await hold).

PR c++/115906 - [coroutines] missing diagnostic and ICE when co_await used as default argument in function declaration

gcc/cp/ChangeLog:

	PR c++/115906
	* parser.cc (cp_parser_unary_expression): Reject await
	expressions if use of local variables is currently forbidden.
	(cp_parser_yield_expression): Reject yield expressions if use of
	local variables is currently forbidden.

gcc/testsuite/ChangeLog:

	PR c++/115906
	* g++.dg/coroutines/pr115906-yield.C: New test.
	* g++.dg/coroutines/pr115906.C: New test.
	* g++.dg/coroutines/co-await-syntax-02-outside-fn.C: Don't rely
	on default arguments.
	* g++.dg/coroutines/co-yield-syntax-01-outside-fn.C: Ditto.

5 files changed, 80 insertions, 3 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index e46cdfd..eb102de 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -9242,6 +9242,14 @@ cp_parser_unary_expression (cp_parser *parser, cp_id_kind * pidk,
 	    if (expr == error_mark_node)
 	      return error_mark_node;
 
+	    /* ... but, we cannot use co_await in default arguments.  */
+	    if (parser->local_variables_forbidden_p & LOCAL_VARS_FORBIDDEN)
+	      {
+		error_at (kw_loc,
+			  "%<co_await%> cannot be used in default arguments");
+		return error_mark_node;
+	      }
+
 	    /* Handle [expr.await].  */
 	    return cp_expr (finish_co_await_expr (kw_loc, expr));
 	  }
@@ -29651,6 +29659,15 @@ cp_parser_yield_expression (cp_parser* parser)
   else
     expr = cp_parser_assignment_expression (parser);
 
+  /* Similar to co_await, we cannot use co_yield in default arguments (as
+     co_awaits underlie co_yield).  */
+  if (parser->local_variables_forbidden_p & LOCAL_VARS_FORBIDDEN)
+    {
+      error_at (kw_loc,
+		"%<co_yield%> cannot be used in default arguments");
+      return error_mark_node;
+    }
+
   if (expr == error_mark_node)
     return expr;
 
diff --git a/gcc/testsuite/g++.dg/coroutines/co-await-syntax-02-outside-fn.C b/gcc/testsuite/g++.dg/coroutines/co-await-syntax-02-outside-fn.C
index 4ce5c2e..132128f 100644
--- a/gcc/testsuite/g++.dg/coroutines/co-await-syntax-02-outside-fn.C
+++ b/gcc/testsuite/g++.dg/coroutines/co-await-syntax-02-outside-fn.C
@@ -2,4 +2,4 @@
 
 #include "coro.h"
 
-auto f (int x = co_await coro::suspend_always{}); // { dg-error {'co_await' cannot be used outside a function} }
+auto x = co_await coro::suspend_always{}; // { dg-error {'co_await' cannot be used outside a function} }
diff --git a/gcc/testsuite/g++.dg/coroutines/co-yield-syntax-01-outside-fn.C b/gcc/testsuite/g++.dg/coroutines/co-yield-syntax-01-outside-fn.C
index 30db0e9..51c3046 100644
--- a/gcc/testsuite/g++.dg/coroutines/co-yield-syntax-01-outside-fn.C
+++ b/gcc/testsuite/g++.dg/coroutines/co-yield-syntax-01-outside-fn.C
@@ -2,5 +2,4 @@
 
 #include "coro.h"
 
-auto f (int x = co_yield 5); // { dg-error {'co_yield' cannot be used outside a function} }
-
+auto x = co_yield 5; // { dg-error {'co_yield' cannot be used outside a function} }
diff --git a/gcc/testsuite/g++.dg/coroutines/pr115906-yield.C b/gcc/testsuite/g++.dg/coroutines/pr115906-yield.C
new file mode 100644
index 0000000..f8b6ded
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr115906-yield.C
@@ -0,0 +1,29 @@
+#include <coroutine>
+
+struct Promise;
+
+struct Handle : std::coroutine_handle<Promise> {
+    using promise_type = Promise;
+};
+
+struct Promise {
+    Handle get_return_object() noexcept {
+        return {Handle::from_promise(*this)};
+    }
+    std::suspend_never initial_suspend() const noexcept { return {}; }
+    std::suspend_never final_suspend() const noexcept { return {}; }
+    void return_void() const noexcept {}
+    void unhandled_exception() const noexcept {}
+    std::suspend_never yield_value(int) { return {}; }
+};
+
+Handle Coro() {
+    [] (int x = co_yield 1){}; // { dg-error ".co_yield. cannot be used in default arguments" }
+    co_return;
+}
+
+int main() {
+    Coro();
+
+    return 0;
+}
diff --git a/gcc/testsuite/g++.dg/coroutines/pr115906.C b/gcc/testsuite/g++.dg/coroutines/pr115906.C
new file mode 100644
index 0000000..28f408f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr115906.C
@@ -0,0 +1,32 @@
+#include <coroutine>
+
+struct Promise;
+
+struct Handle : std::coroutine_handle<Promise> {
+    using promise_type = Promise;
+};
+
+struct Promise {
+    Handle get_return_object() noexcept {
+        return {Handle::from_promise(*this)};
+    }
+    std::suspend_never initial_suspend() const noexcept { return {}; }
+    std::suspend_never final_suspend() const noexcept { return {}; }
+    void return_void() const noexcept {}
+    void unhandled_exception() const noexcept {}
+};
+
+Handle Coro() {
+    struct Awaiter : std::suspend_never {
+        int await_resume() { return 0; }
+    };
+
+    [] (int x = co_await Awaiter{}){}; // { dg-error ".co_await. cannot be used in default arguments" }
+    co_return;
+}
+
+int main() {
+    Coro();
+
+    return 0;
+}