cp-tree.h: Clarify exception spec node comment.

	* cp-tree.h: Clarify exception spec node comment.
	* except.c (nothrow_spec_p): Simplify by checking node-equality.

From-SVN: r244576

gcc/cp/ChangeLog

 2017-01-18  Nathan Sidwell  <nathan@acm.org>
 
+	* cp-tree.h: Clarify exception spec node comment.
+	* except.c (nothrow_spec_p): Simplify by checking node-equality.
+
 	PR c++/79091
 	* mangle.c (write_exception_spec): Check nothrow explicitly.
 	(write_encoding): Don't increment processing_template_decl around

gcc/cp/cp-tree.h

@@ -1212,7 +1212,8 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
 #define lang_name_c			cp_global_trees[CPTI_LANG_NAME_C]
 #define lang_name_cplusplus		cp_global_trees[CPTI_LANG_NAME_CPLUSPLUS]
 
-/* Exception specifier used for throw().  */
+/* Exception specifiers used for throw(), noexcept(true) and
+   noexcept(false).  We rely on these being uncloned.  */
 #define empty_except_spec		cp_global_trees[CPTI_EMPTY_EXCEPT_SPEC]
 #define noexcept_true_spec		cp_global_trees[CPTI_NOEXCEPT_TRUE_SPEC]
 #define noexcept_false_spec		cp_global_trees[CPTI_NOEXCEPT_FALSE_SPEC]

gcc/cp/except.c

@@ -1143,15 +1143,17 @@ bool
 nothrow_spec_p (const_tree spec)
 {
   gcc_assert (!DEFERRED_NOEXCEPT_SPEC_P (spec));
-  if (spec == NULL_TREE
-      || TREE_VALUE (spec) != NULL_TREE
-      || spec == noexcept_false_spec)
-    return false;
-  if (TREE_PURPOSE (spec) == NULL_TREE
+
+  if (spec == empty_except_spec
       || spec == noexcept_true_spec)
     return true;
-  gcc_assert (processing_template_decl
-	      || TREE_PURPOSE (spec) == error_mark_node);
+
+  gcc_assert (!spec
+	      || TREE_VALUE (spec)
+	      || spec == noexcept_false_spec
+	      || TREE_PURPOSE (spec) == error_mark_node
+	      || processing_template_decl);
+
   return false;
 }