coroutines: Fix ICE on invalid (PR93458).

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index e8a6a4033f6..ca86c7e6950 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -169,7 +169,8 @@  get_or_insert_coroutine_info (tree fn_decl)
 coroutine_info *
 get_coroutine_info (tree fn_decl)
 {
-  gcc_checking_assert (coroutine_info_table != NULL);
+  if (coroutine_info_table == NULL)
+    return NULL;
 
   coroutine_info **slot = coroutine_info_table->find_slot_with_hash
     (fn_decl, coroutine_info_hasher::hash (fn_decl), NO_INSERT);
@@ -255,11 +256,16 @@  static GTY(()) tree void_coro_handle_type;
 static tree
 find_coro_traits_template_decl (location_t kw)
 {
+  static bool error_emitted = false;
   tree traits_decl = lookup_qualified_name (std_node, coro_traits_identifier,
 					    0, true);
-  if (traits_decl == NULL_TREE || traits_decl == error_mark_node)
+  /* If we are missing fundmental information, such as the traits, then don't
+     emit this for every keyword in a TU.  */
+  if (!error_emitted &&
+      (traits_decl == NULL_TREE || traits_decl == error_mark_node))
     {
       error_at (kw, "cannot find %<coroutine traits%> template");
+      error_emitted = true;
       return NULL_TREE;
     }
   else
@@ -370,34 +376,45 @@  coro_promise_type_found_p (tree fndecl, location_t loc)
 {
   gcc_assert (fndecl != NULL_TREE);
 
-  /* Save the coroutine data on the side to avoid the overhead on every
-     function decl.  */
-
-  /* We only need one entry per coroutine in a TU, the assumption here is that
-     there are typically not 1000s.  */
   if (!coro_initialized)
     {
-      gcc_checking_assert (coroutine_info_table == NULL);
-      /* A table to hold the state, per coroutine decl.  */
-      coroutine_info_table =
-	hash_table<coroutine_info_hasher>::create_ggc (11);
-      /* Set up the identifiers we will use.  */
-      gcc_checking_assert (coro_traits_identifier == NULL);
+      /* Trees we only need to create once.
+	 Set up the identifiers we will use.  */
       coro_init_identifiers ();
-      /* Trees we only need to create once.  */
+
       /* Coroutine traits template.  */
       coro_traits_templ = find_coro_traits_template_decl (loc);
-      gcc_checking_assert (coro_traits_templ != NULL);
+      if (coro_traits_templ == NULL_TREE
+	  || coro_traits_templ == error_mark_node)
+	return false;
+
       /*  coroutine_handle<> template.  */
       coro_handle_templ = find_coro_handle_template_decl (loc);
-      gcc_checking_assert (coro_handle_templ != NULL);
+      if (coro_handle_templ == NULL_TREE
+	  || coro_handle_templ == error_mark_node)
+	return false;
+
       /*  We can also instantiate the void coroutine_handle<>  */
       void_coro_handle_type =
 	instantiate_coro_handle_for_promise_type (loc, NULL_TREE);
-      gcc_checking_assert (void_coro_handle_type != NULL);
+      if (void_coro_handle_type == NULL_TREE
+	  || void_coro_handle_type == error_mark_node)
+	return false;
+
+      /* A table to hold the state, per coroutine decl.  */
+      gcc_checking_assert (coroutine_info_table == NULL);
+      coroutine_info_table =
+	hash_table<coroutine_info_hasher>::create_ggc (11);
+
+      if (coroutine_info_table == NULL)
+	return false;
+
       coro_initialized = true;
     }
 
+  /* Save the coroutine data on the side to avoid the overhead on every
+     function decl tree.  */
+
   coroutine_info *coro_info = get_or_insert_coroutine_info (fndecl);
   /* Without this, we cannot really proceed.  */
   gcc_checking_assert (coro_info);
@@ -407,6 +424,19 @@  coro_promise_type_found_p (tree fndecl, location_t loc)
     {
       /* Get the coroutine traits template class instance for the function
 	 signature we have - coroutine_traits <R, ...>  */
+      if (!CLASS_TYPE_P (TREE_TYPE (TREE_TYPE (fndecl))))
+	{
+	  static bool coro_type_error_emitted = false;
+	  /* It makes more sense to show the function header for this, even
+	     though we will have encountered it when processing a keyword.
+	     Only emit the error once, not for every keyword we encounter.  */
+	  if (!coro_type_error_emitted)
+	    error_at (DECL_SOURCE_LOCATION (fndecl), "a coroutine must have a"
+		      " class or struct return type");
+	  coro_type_error_emitted = true;
+	  return false;
+	}
+
       tree templ_class = instantiate_coro_traits (fndecl, loc);
 
       /* Find the promise type for that.  */
@@ -422,7 +452,7 @@  coro_promise_type_found_p (tree fndecl, location_t loc)
       /* Try to find the handle type for the promise.  */
       tree handle_type =
 	instantiate_coro_handle_for_promise_type (loc, coro_info->promise_type);
-      if (handle_type == NULL_TREE)
+      if (handle_type == NULL_TREE || handle_type == error_mark_node)
 	return false;
 
       /* Complete this, we're going to use it.  */
@@ -597,11 +627,17 @@  coro_function_valid_p (tree fndecl)
 {
   location_t f_loc = DECL_SOURCE_LOCATION (fndecl);
 
+  /* For cases where fundamental information cannot be found, e.g. the
+     coroutine traits are missing, we need to punt early.  */
+  if (!coro_promise_type_found_p (fndecl, f_loc))
+    return false;
+
   /* Since we think the function is a coroutine, that implies we parsed
      a keyword that triggered this.  Keywords check promise validity for
      their context and thus the promise type should be known at this point.  */
-  gcc_checking_assert (get_coroutine_handle_type (fndecl) != NULL_TREE
-		       && get_coroutine_promise_type (fndecl) != NULL_TREE);
+  if (get_coroutine_handle_type (fndecl) == NULL_TREE
+      || get_coroutine_promise_type (fndecl) == NULL_TREE)
+    return false;
 
   if (current_function_returns_value || current_function_returns_null)
     {
diff --git a/gcc/testsuite/g++.dg/coroutines/pr93458-1-missing-traits.C b/gcc/testsuite/g++.dg/coroutines/pr93458-1-missing-traits.C
new file mode 100644
index 00000000000..46adccded98
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr93458-1-missing-traits.C
@@ -0,0 +1,10 @@ 
+//  { dg-additional-options "-fsyntax-only -fexceptions -w" }
+
+// Diagose missing traits (fail to include <coroutine>).
+
+int
+bad_coroutine (void)
+{
+  co_yield 5; // { dg-error {cannot find 'coroutine traits' template} }
+  co_return;
+}
diff --git a/gcc/testsuite/g++.dg/coroutines/pr93458-2-bad-coro-type.C b/gcc/testsuite/g++.dg/coroutines/pr93458-2-bad-coro-type.C
new file mode 100644
index 00000000000..aab9b34b69a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/coroutines/pr93458-2-bad-coro-type.C
@@ -0,0 +1,12 @@ 
+//  { dg-additional-options "-fsyntax-only -fexceptions -w" }
+
+// Diagose bad coroutine function type.
+
+#include "coro.h"
+
+int
+bad_coroutine (void) // { dg-error {a coroutine must have a class or struct return type} }
+{
+  co_yield 5;
+  co_return;
+}