Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -175,6 +175,7 @@
 
 def InvalidIOSDeploymentTarget : DiagGroup<"invalid-ios-deployment-target">;
 
+def CTAD : DiagGroup<"ctad">;
 def CXX17CompatMangling : DiagGroup<"c++17-compat-mangling">;
 def : DiagGroup<"c++1z-compat-mangling", [CXX17CompatMangling]>;
 // Name of this warning in GCC.
@@ -185,7 +186,8 @@
 def CXXPre14CompatPedantic : DiagGroup<"c++98-c++11-compat-pedantic",
                                        [CXXPre14Compat,
                                         CXXPre14CompatBinaryLiteral]>;
-def CXXPre17Compat : DiagGroup<"c++98-c++11-c++14-compat">;
+def CXXPre17Compat : DiagGroup<"c++98-c++11-c++14-compat",
+                                       [CTAD]>;
 def CXXPre17CompatPedantic : DiagGroup<"c++98-c++11-c++14-compat-pedantic",
                                        [CXXPre17Compat]>;
 def CXXPre2aCompat : DiagGroup<"c++98-c++11-c++14-c++17-compat">;
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2159,7 +2159,7 @@
 def warn_cxx14_compat_class_template_argument_deduction : Warning<
   "class template argument deduction is incompatible with C++ standards "
   "before C++17%select{|; for compatibility, use explicit type name %1}0">,
-  InGroup<CXXPre17Compat>, DefaultIgnore;
+  InGroup<CTAD>, DefaultIgnore;
 
 // C++14 deduced return types
 def err_auto_fn_deduction_failure : Error<
Index: test/SemaCXX/cxx14-compat-ctad.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/cxx14-compat-ctad.cpp
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++17 -Wctad -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++2a -Wctad -verify %s
+
+template<typename T = int> struct X {};
+X x; // expected-warning {{class template argument deduction is incompatible with C++ standards before C++17; for compatibility, use explicit type name 'X<int>'}}
+
+template<template<typename> class> struct Y {};
+Y<X> yx; // ok, not class template argument deduction
+
+template<typename T> void f(T t) {
+  X x = t; // expected-warning {{class template argument deduction is incompatible with C++ standards before C++17}}
+}
+
+template<typename T> void g(T t) {
+  typename T::X x = t; // expected-warning {{class template argument deduction is incompatible with C++ standards before C++17; for compatibility, use explicit type name 'typename A::X<A>' (aka 'A::X<A>')}}
+}
+
+struct A { template<typename T> struct X { X(T); }; };
+void h(A a) { g(a); } // expected-note {{in instantiation of function template specialization 'g<A>'}}
+
+template<class T> struct V { V(const T&) {} };
+
+V(int) -> V<int*>;  // ok, deduction guide is not a use of class template argument deduction
+
+void f2() { V v('a'); } // expected-warning {{class template argument deduction is incompatible with C++ standards before C++17; for compatibility, use explicit type name 'V<char>'}}
+void g2() { V v(0); } // expected-warning {{class template argument deduction is incompatible with C++ standards before C++17; for compatibility, use explicit type name 'V<int *>'}}
+
+void h2() {
+    auto lam = [](){};
+    V v(lam); // expected-warning {{class template argument deduction is incompatible with C++ standards before C++17}}
+}