diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 703f5970cff..e2373b20064 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,9 @@
 2018-03-12  Jason Merrill  <jason@redhat.com>
 
+	PR c++/84355 - ICE with deduction for member class template.
+	* pt.c (tsubst) [TEMPLATE_TYPE_PARM]: Always substitute into
+	CLASS_PLACEHOLDER_TEMPLATE.
+
 	PR c++/84802 - ICE capturing uninstantiated class.
 	* lambda.c (build_capture_proxy): Call complete_type.
 
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index a92b36a6031..4640ca08ce0 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -14125,8 +14125,7 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 			= tsubst_constraint (constr, args, complain, in_decl);
 		    else if (tree pl = CLASS_PLACEHOLDER_TEMPLATE (t))
 		      {
-			if (DECL_TEMPLATE_TEMPLATE_PARM_P (pl))
-			  pl = tsubst (pl, args, complain, in_decl);
+			pl = tsubst_copy (pl, args, complain, in_decl);
 			CLASS_PLACEHOLDER_TEMPLATE (r) = pl;
 		      }
 		  }
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction50.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction50.C
new file mode 100644
index 00000000000..e8cdd8c710f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction50.C
@@ -0,0 +1,22 @@
+// PR c++/84355
+// { dg-additional-options -std=c++17 }
+
+template <class, class> struct same;
+template <class T> struct same<T,T> {};
+
+template<typename T> struct A
+{
+  template<class U> struct B
+  {
+    B(U);
+  };
+
+  A() {
+    B b(0);
+    same<decltype(b),B<int>>{};
+  }
+};
+
+struct C {};
+
+A<C> a;