[PATCH 5/3] C++ bits to improve detection of attribute conflicts (PR 81544)

[Martin Sebor <msebor at gmail dot com>]

To make review easier I broke out the C++ changes for the attributes
work into a patch of their own.  I also found the API I had asked
about, to look up a declaration based on one that's about to be
added/merged.

This patch depends on the foundation bits posted here:

  https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00599.html

Thanks
Martin

PS The decls_match changes were required only because the x86_64
target's targetm.target_option.function_versions() fails with
a hard error when it finds a conflict in the target attribute
while comparing declarations.  That doesn't seem quite right
for a predicate-type of an API but I didn't see a better way
around it than to introduce a new argument and avoid calling
into the back end when comparing attributes.

PPS I found a surprising number of bugs in the C++ front end
while working on this project: 81873, 81762, 81761, and 81609
so I split up the tests I had initially added under c-c++-common
between C and C++ and left in c-c++-common only the subset that
passes in both languages.

PR c/81544 - attribute noreturn and warn_unused_result on the same function accepted

gcc/cp/ChangeLog:

	PR c/81544
	* cp-tree.h (decls_match): Add default argument.
	* decl.c (decls_match): Avoid calling into the target back end
	and triggering an error.
	* decl2.c (cplus_decl_attributes): Look up existing declaration and
	pass it to decl_attributes.
	* tree.c (cxx_attribute_table): Initialize new member of struct
	attribute_spec.

gcc/testsuite/ChangeLog:

	PR c/81544
	* g++.dg/Wattributes-2.C: New test.

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 115cdaf..f2bac2a 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6080,7 +6080,7 @@ extern void finish_scope			(void);
 extern void push_switch				(tree);
 extern void pop_switch				(void);
 extern tree make_lambda_name			(void);
-extern int decls_match				(tree, tree);
+extern int decls_match				(tree, tree, bool = true);
 extern tree duplicate_decls			(tree, tree, bool);
 extern tree declare_local_label			(tree);
 extern tree define_label			(location_t, tree);
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index aab2019..c9c8cd1 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -993,7 +993,7 @@ push_local_name (tree decl)
    `const int&'.  */
 
 int
-decls_match (tree newdecl, tree olddecl)
+decls_match (tree newdecl, tree olddecl, bool record_versions /* = true */)
 {
   int types_match;
 
@@ -1088,6 +1088,7 @@ decls_match (tree newdecl, tree olddecl)
       if (types_match
 	  && !DECL_EXTERN_C_P (newdecl)
 	  && !DECL_EXTERN_C_P (olddecl)
+	  && record_versions
 	  && targetm.target_option.function_versions (newdecl, olddecl))
 	{
 	  /* Mark functions as versions if necessary.  Modify the mangled decl
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index 2a52f8c..82df5e3 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -1404,7 +1404,31 @@ cplus_decl_attributes (tree *decl, tree attributes, int flags)
 		       attributes, flags);
     }
   else
-    decl_attributes (decl, attributes, flags);
+    {
+      tree last_decl = (DECL_P (*decl) && DECL_NAME (*decl)
+			? lookup_name (DECL_NAME (*decl)) : NULL_TREE);
+
+      if (last_decl && TREE_CODE (last_decl) == OVERLOAD)
+	for (ovl_iterator iter (last_decl, true); ; ++iter)
+	  {
+	    if (!iter)
+	      {
+		last_decl = NULL_TREE;
+		break;
+	      }
+
+	    if (TREE_CODE (*iter) == OVERLOAD)
+	      continue;
+
+	    if (decls_match (*decl, *iter, /*record_decls=*/false))
+	      {
+		last_decl = *iter;
+		break;
+	      }
+	  }
+
+      decl_attributes (decl, attributes, flags, last_decl);
+    }
 
   if (TREE_CODE (*decl) == TYPE_DECL)
     SET_IDENTIFIER_TYPE_VALUE (DECL_NAME (*decl), TREE_TYPE (*decl));
diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 8f18665..6cbf4b2 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -4314,10 +4314,10 @@ const struct attribute_spec cxx_attribute_table[] =
   /* { name, min_len, max_len, decl_req, type_req, fn_type_req, handler,
        affects_type_identity } */
   { "init_priority",  1, 1, true,  false, false,
-    handle_init_priority_attribute, false },
+    handle_init_priority_attribute, false, NULL },
   { "abi_tag", 1, -1, false, false, false,
-    handle_abi_tag_attribute, true },
-  { NULL,	      0, 0, false, false, false, NULL, false }
+    handle_abi_tag_attribute, true, NULL },
+  { NULL, 0, 0, false, false, false, NULL, false, NULL }
 };
 
 /* Table of C++ standard attributes.  */
@@ -4326,10 +4326,10 @@ const struct attribute_spec std_attribute_table[] =
   /* { name, min_len, max_len, decl_req, type_req, fn_type_req, handler,
        affects_type_identity } */
   { "maybe_unused", 0, 0, false, false, false,
-    handle_unused_attribute, false },
+    handle_unused_attribute, false, NULL },
   { "nodiscard", 0, 0, false, false, false,
-    handle_nodiscard_attribute, false },
-  { NULL,	      0, 0, false, false, false, NULL, false }
+    handle_nodiscard_attribute, false, NULL },
+  { NULL, 0, 0, false, false, false, NULL, false, NULL }
 };
 
 /* Handle an "init_priority" attribute; arguments as in
diff --git a/gcc/testsuite/g++.dg/Wattributes-2.C b/gcc/testsuite/g++.dg/Wattributes-2.C
new file mode 100644
index 0000000..1470b16
--- /dev/null
+++ b/gcc/testsuite/g++.dg/Wattributes-2.C
@@ -0,0 +1,35 @@
+// Test to verify that attributes on distinct overloads of a function
+//  with the same name are properly looked up and applied.
+// { dg-do compile }
+// { dg-options "-Wall" }
+
+int
+foo (int);
+
+int __attribute__ ((noreturn))
+foo (int, int);
+
+int __attribute__ ((warn_unused_result))
+foo (int, int, int);
+
+int call_foo_1 ()
+{
+  foo (1);
+}                       // { dg-warning "\\\[-Wreturn-type]" }
+
+int call_foo_2 ()
+{
+  foo (1, 2);
+}
+
+int call_foo_3 ()
+{
+  foo (1, 2, 3);        // { dg-warning "\\\[-Wunused-result]" }
+}                       // { dg-warning "\\\[-Wreturn-type]" }
+
+int call_foo_4 ()
+{
+  // Make sure an error doesn't trigger bogus warnings or an ICE.
+  foo (1, 2, 3, 4);     // { dg-error "no matching function" }
+  return 0;
+}