commit 0249b02abba1c309eaf118a0ef158a32f6833cb3
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Apr 2 17:49:06 2018 -0400

    Fix noexcept merging with system headers.
    
            * typeck.c (merge_types): Limit matching attribute shortcut to
            the default case.

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index d454c6c5a29..e33f2c34c7f 100644
--- a/gcc/cp/typeck.c
+++ b/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.  */
diff --git a/gcc/testsuite/g++.dg/cpp1z/noexcept-type19.C b/gcc/testsuite/g++.dg/cpp1z/noexcept-type19.C
new file mode 100644
index 00000000000..571c426aa67
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/noexcept-type19.C
@@ -0,0 +1,14 @@
+// { 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);
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/noexcept-type19.h b/gcc/testsuite/g++.dg/cpp1z/noexcept-type19.h
new file mode 100644
index 00000000000..33a29357e7f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/noexcept-type19.h
@@ -0,0 +1,4 @@
+#pragma GCC system_header
+
+typedef decltype(sizeof(0)) size_t;
+extern "C" void *malloc (size_t) throw();