coroutines: Improve error recovery [PR94817, PR94829, PR95087].

When we have completely missing key information (e.g. the
coroutine_traits) or a partially transformed function body, we
need to try and balance returning useful information about
failures with the possibility that some part of the diagnostics
machinery or following code will not be able to handle the
state.

The PRs (and revised testcase) point to cases where that processing
has failed.

This revises the process to avoid special handling for the
ramp, and falls back on the same code used for regular function
fails.

There are test-cases (in addition to the ones for the PRs) that now
cover all early exit points [where the transforms are considered
to have failed in a manner that does not allow compilation to
continue].

Diagnosing bad uses of 'return' in coroutines is somewhat
tricky, since the user can use the keyword before we know
that the function is a coroutine (where such returns are not
permitted).  At present, we are just doing a check for any
use of 'return' and erroring on that.  However, we can't then
pass the function body on, since it will contain unlowered
coroutine trees.

This avoids the issue by dropping the entire function body
under that circumstance.

	Backport c7100843831147a034fe37d231c54ac53ceace45 and a1bb808504643e6c3c0df0fdd68a941ed2a64c7f0

gcc/cp/ChangeLog:

	PR c++/94817
	PR c++/94829
	PR c++/95087
	* coroutines.cc (morph_fn_to_coro): Set unformed outline
	functions to error_mark_node.  For early error returns suppress
	warnings about missing ramp return values.  Fix reinstatement
	of the function body on pre-existing initial error.  If we see
	an early fatal error, drop the erroneous function body.
	* decl.c (finish_function): Use the normal error path for fails
	in the ramp function, do not try to compile the helpers if the
	transform fails.

gcc/testsuite/ChangeLog:

	PR c++/94817
	PR c++/94829
	PR c++/95087
	* g++.dg/coroutines/coro-missing-final-suspend.C: New test.
	* g++.dg/coroutines/coro-missing-initial-suspend.C: New test.
	* g++.dg/coroutines/coro-missing-promise-yield.C: Check for
	continuation of compilation.
	* g++.dg/coroutines/coro-missing-promise.C: Likewise.
	* g++.dg/coroutines/coro-missing-ret-value.C: Likewise
	* g++.dg/coroutines/coro-missing-ret-void.C: Likewise
	* g++.dg/coroutines/coro-missing-ueh-3.C: Likewise
	* g++.dg/coroutines/pr94817.C: New test.
	* g++.dg/coroutines/pr94829.C: New test.
	* g++.dg/coroutines/co-return-syntax-08-bad-return.C:
	Adjust the testcase to do the compile (rather than an
	-fsyntax-only parse).
	* g++.dg/coroutines/coro1-ret-int-yield-int.h
	(MISSING_INITIAL_SUSPEND, MISSING_FINAL_SUSPEND): New.

gcc/cp/coroutines.cc

@@ -3577,14 +3577,24 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
 {
   gcc_checking_assert (orig && TREE_CODE (orig) == FUNCTION_DECL);
 
+  *resumer = error_mark_node;
+  *destroyer = error_mark_node;
   if (!coro_function_valid_p (orig))
-    return false;
+    {
+      /* For early errors, we do not want a diagnostic about the missing
+	 ramp return value, since the user cannot fix this - a 'return' is
+	 not allowed in a coroutine.  */
+      TREE_NO_WARNING (orig) = true;
+      /* Discard the body, we can't process it further.  */
+      pop_stmt_list (DECL_SAVED_TREE (orig));
+      DECL_SAVED_TREE (orig) = push_stmt_list ();
+      return false;
+    }
 
   /* We can't validly get here with an empty statement list, since there's no
      way for the FE to decide it's a coroutine in the absence of any code.  */
   tree fnbody = pop_stmt_list (DECL_SAVED_TREE (orig));
-  if (fnbody == NULL_TREE)
-    return false;
+  gcc_checking_assert (fnbody != NULL_TREE);
 
   /* We don't have the locus of the opening brace - it's filled in later (and
      there doesn't really seem to be any easy way to get at it).
@@ -3598,7 +3608,9 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
   if (body_start == NULL_TREE || body_start == error_mark_node)
     {
       DECL_SAVED_TREE (orig) = push_stmt_list ();
-      append_to_statement_list (DECL_SAVED_TREE (orig), &fnbody);
+      append_to_statement_list (fnbody, &DECL_SAVED_TREE (orig));
+      /* Suppress warnings about the missing return value.  */
+      TREE_NO_WARNING (orig) = true;
       return false;
     }
 
@@ -3670,13 +3682,21 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
      the coroutine promise.  */
   tree initial_await = build_init_or_final_await (fn_start, false);
   if (initial_await == error_mark_node)
-    return false;
+    {
+      /* Suppress warnings about the missing return value.  */
+      TREE_NO_WARNING (orig) = true;
+      return false;
+    }
   /* The type of the frame var for this is the type of its temp proxy.  */
   tree initial_suspend_type = TREE_TYPE (TREE_OPERAND (initial_await, 1));
 
   tree final_await = build_init_or_final_await (fn_start, true);
   if (final_await == error_mark_node)
-    return false;
+    {
+      /* Suppress warnings about the missing return value.  */
+      TREE_NO_WARNING (orig) = true;
+      return false;
+    }
 
   /* The type of the frame var for this is the type of its temp proxy.  */
   tree final_suspend_type = TREE_TYPE (TREE_OPERAND (final_await, 1));
@@ -4260,6 +4280,8 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
     {
       BIND_EXPR_BODY (ramp_bind) = pop_stmt_list (ramp_body);
       DECL_SAVED_TREE (orig) = newbody;
+      /* Suppress warnings about the missing return value.  */
+      TREE_NO_WARNING (orig) = true;
       return false;
     }
 

gcc/cp/decl.c

@@ -16888,6 +16888,7 @@ finish_function (bool inline_p)
   bool coro_p = flag_coroutines
 		&& !processing_template_decl
 		&& DECL_COROUTINE_P (fndecl);
+  bool coro_emit_helpers = false;
 
   /* When we get some parse errors, we can end up without a
      current_function_decl, so cope.  */
@@ -16916,18 +16917,16 @@ finish_function (bool inline_p)
 
   if (coro_p)
     {
-      if (!morph_fn_to_coro (fndecl, &resumer, &destroyer))
-	{
-	  DECL_SAVED_TREE (fndecl) = pop_stmt_list (DECL_SAVED_TREE (fndecl));
-	  poplevel (1, 0, 1);
-	  DECL_SAVED_TREE (fndecl) = error_mark_node;
-	  return fndecl;
-	}
+      /* Only try to emit the coroutine outlined helper functions if the
+	 transforms succeeded.  Otherwise, treat errors in the same way as
+	 a regular function.  */
+      coro_emit_helpers = morph_fn_to_coro (fndecl, &resumer, &destroyer);
 
       /* We should handle coroutine IFNs in middle end lowering.  */
       cfun->coroutine_component = true;
 
-      if (use_eh_spec_block (fndecl))
+      /* Do not try to process the ramp's EH unless outlining succeeded.  */
+      if (coro_emit_helpers && use_eh_spec_block (fndecl))
 	finish_eh_spec_block (TYPE_RAISES_EXCEPTIONS
 			      (TREE_TYPE (fndecl)),
 			      current_eh_spec_block);
@@ -17176,8 +17175,9 @@ finish_function (bool inline_p)
       && !DECL_OMP_DECLARE_REDUCTION_P (fndecl))
     cp_genericize (fndecl);
 
-  /* Emit the resumer and destroyer functions now.  */
-  if (coro_p)
+  /* Emit the resumer and destroyer functions now, providing that we have
+     not encountered some fatal error.  */
+  if (coro_emit_helpers)
     {
       emit_coro_helper (resumer);
       emit_coro_helper (destroyer);

gcc/testsuite/g++.dg/coroutines/co-return-syntax-08-bad-return.C

-//  { dg-additional-options "-fsyntax-only -w" }
+//  { dg-additional-options "-w" }
 
 #include "coro.h"
 

gcc/testsuite/g++.dg/coroutines/coro-missing-final-suspend.C

+//  { dg-additional-options "-fsyntax-only -w" }
+#include "coro.h"
+
+// Check diagnostic return from missing promise initial suspend entry.
+
+#define MISSING_FINAL_SUSPEND
+#include "coro1-ret-int-yield-int.h"
+
+coro1
+my_coro () // { dg-error {no member named 'final_suspend' in} }
+{
+  co_return 0;
+}
+
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};

gcc/testsuite/g++.dg/coroutines/coro-missing-initial-suspend.C

+//  { dg-additional-options "-fsyntax-only -w" }
+#include "coro.h"
+
+// Check diagnostic return from missing promise initial suspend entry.
+
+#define MISSING_INITIAL_SUSPEND
+#include "coro1-ret-int-yield-int.h"
+
+coro1
+my_coro () // { dg-error {no member named 'initial_suspend' in} }
+{
+  co_return 0;
+}
+
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};

gcc/testsuite/g++.dg/coroutines/coro-missing-promise-yield.C

@@ -27,6 +27,12 @@ bar ()
   co_return 0;
 }
 
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};
+
 int main (int ac, char *av[]) {
   MissingPromiseYield x = bar ();
   return 0;

gcc/testsuite/g++.dg/coroutines/coro-missing-promise.C

@@ -16,3 +16,9 @@ bar ()
   co_yield 22; // { dg-error {unable to find the promise type for this coroutine} }
   co_return 0;
 }
+
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};

gcc/testsuite/g++.dg/coroutines/coro-missing-ret-value.C

@@ -28,6 +28,12 @@ bar ()
   co_return 6174; // { dg-error {no member named 'return_value' in} }
 }
 
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};
+
 int main (int ac, char *av[]) {
   MissingRetValue x = bar ();
   return 0;

gcc/testsuite/g++.dg/coroutines/coro-missing-ret-void.C

@@ -28,6 +28,12 @@ bar ()
   co_return; // { dg-error "no member named .return_void. in" }
 }
 
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};
+
 int main (int ac, char *av[]) {
   MissingRetVoid x = bar ();
   return 0;

gcc/testsuite/g++.dg/coroutines/coro-missing-ueh-3.C

@@ -12,6 +12,12 @@ bar ()
   co_return;
 }
 
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};
+
 int main (int ac, char *av[]) {
   MissingUEH x = bar ();
   return 0;

gcc/testsuite/g++.dg/coroutines/coro1-ret-int-yield-int.h

@@ -99,14 +99,20 @@ struct coro1 {
     return handle_type::from_promise (*this);
   }
 
+#ifdef MISSING_INITIAL_SUSPEND
+#else
   auto initial_suspend () {
     PRINT ("get initial_suspend (always)");
     return suspend_always_prt{};
   }
+#endif
+#ifdef MISSING_FINAL_SUSPEND
+#else
   auto final_suspend () {
     PRINT ("get final_suspend (always)");
     return suspend_always_prt{};
   }
+#endif
 
 #ifdef USE_AWAIT_TRANSFORM
 

gcc/testsuite/g++.dg/coroutines/pr94817.C

+
+void no_coroutine_traits() {
+  co_await 4; // { dg-error {coroutines require a traits template\; cannot find 'std::coroutine_traits'} }
+}
+
+// check we have not messed up continuation of the compilation.
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};

gcc/testsuite/g++.dg/coroutines/pr94829.C

+
+namespace std::experimental {
+template <typename R, typename... T> struct coroutine_traits {
+  using promise_type = typename R::promise_type;
+};
+
+template <class Promise = void> struct coroutine_handle;
+template <> struct coroutine_handle<void> {
+  static coroutine_handle from_address(void *) noexcept;
+  coroutine_handle() = default;
+  template <class PromiseType>
+  coroutine_handle(coroutine_handle<PromiseType>) noexcept;
+};
+template <class Promise> struct coroutine_handle : coroutine_handle<void> {
+  coroutine_handle() = default;
+  static coroutine_handle from_address(void *) noexcept;
+};
+}
+
+struct suspend_always {
+  bool await_ready() noexcept;
+  void await_suspend(std::experimental::coroutine_handle<>) noexcept;
+  void await_resume() noexcept;
+};
+
+struct Task {
+  struct promise_type {
+    Task get_return_object();
+    void return_void() {}
+    suspend_always initial_suspend() noexcept;
+    suspend_always final_suspend() noexcept;
+    void unhandled_exception() noexcept;
+  };
+};
+
+template <typename _AwrT> auto SyncAwait(_AwrT &&A) {
+  if (!A.await_ready()) {
+    auto AwaitAsync = [&]() -> Task {
+      try { (void)(co_await A); } catch (...) {} // { dg-error {coroutines require a traits template; cannot find 'std::coroutine_traits'} }
+    };
+    Task t = AwaitAsync();
+  }
+  return A.await_resume();
+}
+
+void f() {
+  suspend_always test;
+  SyncAwait(test);
+}