Fix noexcept merging with system headers.

	* typeck.c (merge_types): Limit matching attribute shortcut to
	the default case.

From-SVN: r259042

gcc/cp/ChangeLog

+2018-04-03  Jason Merrill  <jason@redhat.com>
+
+	* typeck.c (merge_types): Limit matching attribute shortcut to
+	the default case.
+
 2018-04-03  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/85147

gcc/cp/typeck.c

@@ -899,14 +899,14 @@ merge_types (tree t1, tree t2)
       return t1;
 
     default:;
+      if (attribute_list_equal (TYPE_ATTRIBUTES (t1), attributes))
+	return t1;
+      else if (attribute_list_equal (TYPE_ATTRIBUTES (t2), attributes))
+	return t2;
+      break;
     }
 
-  if (attribute_list_equal (TYPE_ATTRIBUTES (t1), attributes))
-    return t1;
-  else if (attribute_list_equal (TYPE_ATTRIBUTES (t2), attributes))
-    return t2;
-  else
-    return cp_build_type_attribute_variant (t1, attributes);
+  return cp_build_type_attribute_variant (t1, attributes);
 }
 
 /* Return the ARRAY_TYPE type without its domain.  */

gcc/testsuite/g++.dg/cpp1z/noexcept-type19.C

+// { dg-do compile { target c++11 } }
+
+#include "noexcept-type19.h"
+
+extern "C" void *malloc (size_t);
+
+template<class T> void f(T*);
+
+void *g(size_t);
+
+int main()
+{
+  f<decltype(malloc)>(g);
+}

gcc/testsuite/g++.dg/cpp1z/noexcept-type19.h

+#pragma GCC system_header
+
+typedef decltype(sizeof(0)) size_t;
+extern "C" void *malloc (size_t) throw();