coroutines: Implement n4849 changes to exception handling.

The standard now calls up a revised mechanism to handle exceptions
where exceptions thrown by the await_resume () method of the
initial suspend expression are considered in the same manner as
exceptions thrown by the user-authored function body.

This implements [dcl.fct.def.coroutine] / 5.3.

gcc/cp/ChangeLog:

2020-03-26  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (co_await_expander): If we are expanding the
	initial await expression, set a boolean flag to show that we
	have now reached the initial await_resume() method call.
	(expand_co_awaits): Handle the 'initial await resume called' flag.
	(build_actor_fn): Insert the initial await expression into the
	start of the user-authored function-body. Handle the 'initial await
	resume called' flag.
	(morph_fn_to_coro): Initialise the 'initial await resume called'
	flag.  Modify the unhandled exception catch clause to recognise
	exceptions that occur before the initial await_resume() and re-
	throw them.

gcc/testsuite/ChangeLog:

2020-03-26  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/torture/exceptions-test-01-n4849-a.C: New test.

1 files changed, 213 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/coroutines/torture/exceptions-test-01-n4849-a.C b/gcc/testsuite/g++.dg/coroutines/torture/exceptions-test-01-n4849-a.C
new file mode 100644
index 0000000..e96b4ed
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/torture/exceptions-test-01-n4849-a.C
@@ -0,0 +1,213 @@
+//  { dg-do run }
+
+// Test exceptions in the initial await expression, per n4849.
+
+#include "../coro.h"
+#include <exception>
+
+int gX = 0;
+
+struct coro1 {
+  struct promise_type;
+  using handle_type = coro::coroutine_handle<coro1::promise_type>;
+  handle_type handle;
+
+  coro1 () : handle(0) {}
+  coro1 (handle_type _handle)
+    : handle(_handle) {
+        PRINT("Created coro1 object from handle");
+  }
+  coro1 (const coro1 &) = delete; // no copying
+  coro1 (coro1 &&s) : handle(s.handle) {
+    s.handle = nullptr;
+    PRINT("coro1 mv ctor ");
+  }
+  coro1 &operator = (coro1 &&s) {
+    handle = s.handle;
+    s.handle = nullptr;
+    PRINT("coro1 op=  ");
+    return *this;
+  }
+  ~coro1() {
+    PRINT("Destroyed coro1");
+    if ( handle )
+      handle.destroy();
+  }
+
+  struct suspend_never_prt {
+    bool await_ready() const noexcept { return true; }
+    void await_suspend(handle_type) const noexcept { PRINT ("susp-never-susp"); }
+    void await_resume() const noexcept { PRINT ("susp-never-resume");}
+  };
+
+  struct  suspend_always_prt {
+    bool await_ready() const noexcept { return false; }
+    void await_suspend(handle_type) const noexcept { PRINT ("susp-always-susp"); }
+    void await_resume() const noexcept { PRINT ("susp-always-resume"); }
+    ~suspend_always_prt() { PRINT ("susp-always-DTOR"); }
+  };
+
+  /* Constructing this with:
+      * a value of '1' will cause the initial suspend await_suspend()
+      call to throw.
+      * a value of '2' will cause the await resume to throw.  */
+  struct  suspend_always_susp_throws_prt {
+    int thrower;
+    suspend_always_susp_throws_prt (int _t) : thrower(_t) {}
+    bool await_ready() const noexcept { return false; }
+
+    void await_suspend(handle_type) const
+      { PRINT ("suspend_always_susp_throws_prt:await_suspend");
+        if (thrower == 1)
+          throw (42);
+      }
+    void await_resume() const
+      { PRINT ("suspend_always_susp_throws_prt:await_resume");
+        if (thrower == 2)
+          throw (6174);
+      }
+    ~suspend_always_susp_throws_prt() { PRINT ("suspend_always_susp_throws_prt-DTOR"); }
+  };
+
+  struct promise_type {
+  int throw_control = 0;
+  int value;
+  promise_type(int k)  : throw_control(k), value(-373)
+  {  PRINTF ("Created Promise with %d\n", k);}
+
+  ~promise_type() { PRINT ("Destroyed Promise"); }
+
+  auto get_return_object () {
+    PRINT ("get_return_object: handle from promise");
+    return handle_type::from_promise (*this);
+  }
+  // This provides the tests for what catches exceptions thrown at
+  // different points in the initial await expression.
+  auto initial_suspend () {
+    PRINT ("get initial_suspend (always)");
+    return suspend_always_susp_throws_prt(throw_control);
+  }
+  auto final_suspend () {
+    PRINT ("get final_suspend (always)");
+    return suspend_always_prt{};
+  }
+  void return_value (int v) {
+    PRINTF ("return_value () %d\n",v);
+    value = v;
+  }
+  auto yield_value (int v) {
+    PRINTF ("yield_value () %d and suspend always\n",v);
+    value = v;
+    return suspend_always_prt{};
+  }
+  int get_value (void) { return value; }
+
+  void unhandled_exception() {
+    PRINT ("unhandled_exception: caught one!");
+    gX = -11;
+    // returning from here should end up in final_suspend.
+    }
+  };
+};
+
+// This doesn't have to do much - we only need to exercise the initial
+// await expression.
+
+struct coro1
+n4849_ia_thrower (int k)
+{
+  int caught = 0;
+  PRINT ("f: about to return 22");
+  co_return 22;
+}
+
+int main ()
+{
+  {
+  /* Case 0 - nothing should throw.  */
+  struct coro1 x0;
+  try {
+    x0 = n4849_ia_thrower (0);
+  } catch (...) {
+    PRINT ("main: case 0 ctor threw?");
+    abort ();
+  }
+  /* Resume the initial suspend expression.  */
+  PRINT ("main: got coro, resuming..");
+  x0.handle.resume();
+  int y = x0.handle.promise().get_value();
+  if ( y != 22 )
+    {
+      PRINT ("main: case 0 got the wrong answer.");
+      abort ();
+    }
+  if (!x0.handle.done())
+    {
+      PRINT ("main: case 0 not done.");
+      abort ();
+    }
+  if (gX != 0)
+    {
+      PRINT ("main: case 0 body threw?");
+      abort ();
+    }
+  }
+
+  {
+  /* Case 1 - initial suspend should throw and thus be caught by the 
+     ramp's caller.  */
+  struct coro1 x1;
+  try {
+    x1 = n4849_ia_thrower (1);
+  } catch (int message) {
+    PRINTF ("main: caught an int %d\n", message);
+    if (message != 42)
+      {
+        PRINT ("main: unexpected value?");
+        abort ();
+      }
+  } catch (...) {
+    PRINT ("main: case 1 ctor threw something else?");
+    abort ();
+  }
+  if (gX != 0)
+    {
+      PRINT ("main: case 0 body threw (how?)");
+      abort ();
+    }
+  }
+
+  {
+  /* Case 2 - the await_resume from the initial await expression throws
+     this should be caught by the regular function body wrapper.  */
+  struct coro1 x2;
+  try {
+    x2 = n4849_ia_thrower (2);
+  } catch (...) {
+    PRINT ("main: case 2 ctor threw?");
+    abort ();
+  }
+  // We now resume - and expect the await_resume to throw which should
+  // be caught by unhandled_exception().
+  PRINT ("main: got coro, resuming..");
+  x2.handle.resume();
+  int y = x2.handle.promise().get_value();
+  if ( y != -373 )
+    {
+      PRINT ("main: case 2 got the wrong answer.");
+      abort ();
+    }
+  if (!x2.handle.done())
+    {
+      PRINT ("main: case 2 not done.");
+      abort ();
+    }
+  if (gX != -11)
+    {
+      PRINT ("main: n4849_is_thrower await_resume exception not caught");
+      abort ();
+    }
+  }
+  PRINT ("main: returning");
+  return 0;
+}