Implement C++20 P1301 [[nodiscard("should have a reason")]].

2019-10-17  JeanHeyd Meneide  <phdofthehouse@gmail.com>

gcc/
        * escaped_string.h (escaped_string): New header.
        * tree.c (escaped_string): Remove escaped_string class.

gcc/c-family
        * c-lex.c (c_common_has_attribute): Update nodiscard value.

gcc/cp/
        * tree.c (handle_nodiscard_attribute) Added C++2a nodiscard
	string message.
        (std_attribute_table) Increase nodiscard argument handling
	max_length from 0 to 1.
        * parser.c (cp_parser_check_std_attribute): Add requirement
	that nodiscard only be seen once in attribute-list.
        (cp_parser_std_attribute): Check that empty parenthesis lists are
        not specified for attributes that have max_length > 0 (e.g.
	[[attr()]]).
        * cvt.c (maybe_warn_nodiscard): Add nodiscard message to
	output, if applicable.
	(convert_to_void): Allow constructors to be nodiscard-able (P1771).

gcc/testsuite/g++.dg/cpp0x
        * gen-attrs-67.C: Test new error message for empty-parenthesis-list.

gcc/testsuite/g++.dg/cpp2a
        * nodiscard-construct.C: New test.
        * nodiscard-once.C: New test.
        * nodiscard-reason-nonstring.C: New test.
        * nodiscard-reason-only-one.C: New test.
        * nodiscard-reason.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

From-SVN: r277200

4 files changed, 62 insertions, 14 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index ba533d5f..30accc5 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,19 @@
+2019-10-17  JeanHeyd Meneide  <phdofthehouse@gmail.com>
+
+	Implement p1301 [[nodiscard("should have a reason")]] + p1771 DR
+	* tree.c (handle_nodiscard_attribute): Handle C++2a nodiscard
+	string message.
+	(std_attribute_table) Increase nodiscard argument handling
+	max_length from 0 to 1.
+	* parser.c (cp_parser_check_std_attribute): Add requirement
+	that nodiscard only be seen once in attribute-list.
+	(cp_parser_std_attribute): Check that empty parenthesis lists are
+	not specified for attributes that have max_length > 0 (e.g.
+	[[attr()]]).
+	* cvt.c (maybe_warn_nodiscard): Add nodiscard message to
+	output, if applicable.
+	(convert_to_void): Allow constructors to be nodiscard-able (P1771).
+
 2019-10-18  Nathan Sidwell  <nathan@acm.org>
 
 	* cp-tree.h (struct lang_type): Remove was_anonymous.
diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index 364af72..d41aeb8 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -35,6 +35,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "convert.h"
 #include "stringpool.h"
 #include "attribs.h"
+#include "escaped_string.h"
 
 static tree convert_to_pointer_force (tree, tree, tsubst_flags_t);
 static tree build_type_conversion (tree, tree);
@@ -1026,22 +1027,39 @@ maybe_warn_nodiscard (tree expr, impl_conv_void implicit)
 
   tree rettype = TREE_TYPE (type);
   tree fn = cp_get_fndecl_from_callee (callee);
+  tree attr;
   if (implicit != ICV_CAST && fn
-      && lookup_attribute ("nodiscard", DECL_ATTRIBUTES (fn)))
+      && (attr = lookup_attribute ("nodiscard", DECL_ATTRIBUTES (fn))))
     {
+      escaped_string msg;
+      tree args = TREE_VALUE (attr);
+      if (args)
+	msg.escape (TREE_STRING_POINTER (TREE_VALUE (args)));
+      const char* format = (msg ?
+	G_("ignoring return value of %qD, "
+	   "declared with attribute %<nodiscard%>: %<%s%>") :
+	G_("ignoring return value of %qD, "
+	   "declared with attribute %<nodiscard%>%s"));
+      const char* raw_msg = msg ? msg : "";
       auto_diagnostic_group d;
-      if (warning_at (loc, OPT_Wunused_result,
-		      "ignoring return value of %qD, "
-		      "declared with attribute nodiscard", fn))
+      if (warning_at (loc, OPT_Wunused_result, format, fn, raw_msg))
 	inform (DECL_SOURCE_LOCATION (fn), "declared here");
     }
   else if (implicit != ICV_CAST
-	   && lookup_attribute ("nodiscard", TYPE_ATTRIBUTES (rettype)))
+	   && (attr = lookup_attribute ("nodiscard", TYPE_ATTRIBUTES (rettype))))
     {
+      escaped_string msg;
+      tree args = TREE_VALUE (attr);
+      if (args)
+	msg.escape (TREE_STRING_POINTER (TREE_VALUE (args)));
+      const char* format = msg ?
+	G_("ignoring returned value of type %qT, "
+	   "declared with attribute %<nodiscard%>: %<%s%>") :
+	G_("ignoring returned value of type %qT, "
+	   "declared with attribute %<nodiscard%>%s");
+      const char* raw_msg = msg ? msg : "";
       auto_diagnostic_group d;
-      if (warning_at (loc, OPT_Wunused_result,
-		      "ignoring returned value of type %qT, "
-		      "declared with attribute nodiscard", rettype))
+      if (warning_at (loc, OPT_Wunused_result, format, rettype, raw_msg))
 	{
 	  if (fn)
 	    inform (DECL_SOURCE_LOCATION (fn),
@@ -1180,7 +1198,7 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 	 instantiations be affected by an ABI property that is, or at
 	 least ought to be transparent to the language.  */
       if (tree fn = cp_get_callee_fndecl_nofold (expr))
-	if (DECL_CONSTRUCTOR_P (fn) || DECL_DESTRUCTOR_P (fn))
+	if (DECL_DESTRUCTOR_P (fn))
 	  return expr;
 
       maybe_warn_nodiscard (expr, implicit);
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index b29ed0c..450b144 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -26551,6 +26551,11 @@ cp_parser_std_attribute (cp_parser *parser, tree attr_ns)
       arguments = error_mark_node;
     else
       {
+	if (vec->is_empty())
+	  /* e.g. [[attr()]].  */
+	  error_at (token->location, "parentheses must be omitted if "
+		    "%qE attribute argument list is empty",
+		    attr_id);
 	arguments = build_tree_list_vec (vec);
 	release_tree_vector (vec);
       }
@@ -26565,9 +26570,9 @@ cp_parser_std_attribute (cp_parser *parser, tree attr_ns)
 }
 
 /* Check that the attribute ATTRIBUTE appears at most once in the
-   attribute-list ATTRIBUTES.  This is enforced for noreturn (7.6.3)
-   and deprecated (7.6.5).  Note that carries_dependency (7.6.4)
-   isn't implemented yet in GCC.  */
+   attribute-list ATTRIBUTES.  This is enforced for noreturn (7.6.3),
+   nodiscard, and deprecated (7.6.5).  Note that
+   carries_dependency (7.6.4) isn't implemented yet in GCC.  */
 
 static void
 cp_parser_check_std_attribute (tree attributes, tree attribute)
@@ -26583,6 +26588,10 @@ cp_parser_check_std_attribute (tree attributes, tree attribute)
 	       && lookup_attribute ("deprecated", attributes))
 	error ("attribute %<deprecated%> can appear at most once "
 	       "in an attribute-list");
+      else if (is_attribute_p ("nodiscard", name)
+	       && lookup_attribute ("nodiscard", attributes))
+	error ("attribute %<nodiscard%> can appear at most once "
+	       "in an attribute-list");
     }
 }
 
diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 7f71891..a004bb1 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -4369,9 +4369,14 @@ zero_init_p (const_tree t)
    warn_unused_result attribute.  */
 
 static tree
-handle_nodiscard_attribute (tree *node, tree name, tree /*args*/,
+handle_nodiscard_attribute (tree *node, tree name, tree args,
 			    int /*flags*/, bool *no_add_attrs)
 {
+  if (args && TREE_CODE (TREE_VALUE (args)) != STRING_CST)
+    {
+      error ("%qE attribute argument must be a string constant", name);
+      *no_add_attrs = true;
+    }
   if (TREE_CODE (*node) == FUNCTION_DECL)
     {
       if (VOID_TYPE_P (TREE_TYPE (TREE_TYPE (*node)))
@@ -4461,7 +4466,7 @@ const struct attribute_spec std_attribute_table[] =
        affects_type_identity, handler, exclude } */
   { "maybe_unused", 0, 0, false, false, false, false,
     handle_unused_attribute, NULL },
-  { "nodiscard", 0, 0, false, false, false, false,
+  { "nodiscard", 0, 1, false, false, false, false,
     handle_nodiscard_attribute, NULL },
   { "no_unique_address", 0, 0, true, false, false, false,
     handle_no_unique_addr_attribute, NULL },