* decl.c (start_enum): std::byte aliases anything.

From-SVN: r246212

gcc/cp/ChangeLog

 2017-03-16  Jason Merrill  <jason@redhat.com>
 
+	* decl.c (start_enum): std::byte aliases anything.
+
 	PR c++/79797
 	* constexpr.c (lookup_placeholder): Tweak.
 

gcc/cp/decl.c

@@ -14079,6 +14079,12 @@ start_enum (tree name, tree enumtype, tree underlying_type,
 	{
 	  enumtype = cxx_make_type (ENUMERAL_TYPE);
 	  enumtype = pushtag (name, enumtype, /*tag_scope=*/ts_current);
+
+	  /* std::byte aliases anything.  */
+	  if (enumtype != error_mark_node
+	      && TYPE_CONTEXT (enumtype) == std_node
+	      && !strcmp ("byte", TYPE_NAME_STRING (enumtype)))
+	    TYPE_ALIAS_SET (enumtype) = 0;
 	}
       else
 	  enumtype = xref_tag (enum_type, name, /*tag_scope=*/ts_current,

gcc/testsuite/g++.dg/cpp1z/byte1.C

+// Test for std::byte aliasing properties.
+// { dg-options "-std=c++1z -O3" }
+
+#include <cstddef>
+
+using byte = std::byte;
+
+enum class notbyte: unsigned char {} *np;
+
+int main()
+{
+  int x;
+
+  /* Stores through byte* can alias int, so the compiler can't optimize
+     "x != 0".  */
+  byte *p = (byte*)&x;
+  x = 42;
+  for (int i = 0; i < 4; ++i)
+    p[i] = byte(0);
+  if (x != 0)
+    __builtin_abort();
+
+  /* Stores through notbyte* mustn't alias int, so at -O3 the compiler should
+     optimize "x != 42" to false.  */
+  notbyte *np = (notbyte*)&x; 
+  x = 42;
+  for (int i = 0; i < 4; ++i)
+    np[i] = notbyte(0);
+  if (x != 42)
+    __builtin_abort();
+}