Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -1050,3 +1050,5 @@
 
 // A group for cross translation unit static analysis related warnings.
 def CrossTU : DiagGroup<"ctu">;
+
+def CTADMaybeUnsupported : DiagGroup<"ctad-maybe-unsupported">;
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2129,6 +2129,12 @@
   "class template argument deduction is incompatible with C++ standards "
   "before C++17%select{|; for compatibility, use explicit type name %1}0">,
   InGroup<CXXPre17Compat>, DefaultIgnore;
+def warn_ctad_maybe_unsupported : Warning<
+  "%0 may not intend to support class template argument deduction">,
+  InGroup<CTADMaybeUnsupported>, DefaultIgnore;
+def note_suppress_ctad_maybe_unsupported : Note<
+  "add a deduction guide to suppress this warning">;
+
 
 // C++14 deduced return types
 def err_auto_fn_deduction_failure : Error<
Index: lib/Sema/SemaInit.cpp
===================================================================
--- lib/Sema/SemaInit.cpp
+++ lib/Sema/SemaInit.cpp
@@ -9264,9 +9264,14 @@
   OverloadCandidateSet Candidates(Kind.getLocation(),
                                   OverloadCandidateSet::CSK_Normal);
   OverloadCandidateSet::iterator Best;
+
+  bool HasAnyDeductionGuide = false;
+
   auto tryToResolveOverload =
       [&](bool OnlyListConstructors) -> OverloadingResult {
     Candidates.clear(OverloadCandidateSet::CSK_Normal);
+    HasAnyDeductionGuide = false;
+
     for (auto I = Guides.begin(), E = Guides.end(); I != E; ++I) {
       NamedDecl *D = (*I)->getUnderlyingDecl();
       if (D->isInvalidDecl())
@@ -9278,6 +9283,9 @@
       if (!GD)
         continue;
 
+      if (!GD->isImplicit())
+        HasAnyDeductionGuide = true;
+
       // C++ [over.match.ctor]p1: (non-list copy-initialization from non-class)
       //   For copy-initialization, the candidate functions are all the
       //   converting constructors (12.3.1) of that class.
@@ -9430,5 +9438,15 @@
   Diag(TSInfo->getTypeLoc().getBeginLoc(),
        diag::warn_cxx14_compat_class_template_argument_deduction)
       << TSInfo->getTypeLoc().getSourceRange() << 1 << DeducedType;
+
+  // Warn if CTAD was used on a type that does not have any user-defined
+  // deduction guides.
+  if (!HasAnyDeductionGuide) {
+    Diag(TSInfo->getTypeLoc().getBeginLoc(),
+         diag::warn_ctad_maybe_unsupported)
+        << TemplateName;
+    Diag(Template->getLocation(), diag::note_suppress_ctad_maybe_unsupported);
+  }
+
   return DeducedType;
 }
Index: test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
===================================================================
--- test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
+++ test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
@@ -409,6 +409,86 @@
 
 }
 
+#pragma clang diagnostic push
+#pragma clang diagnostic warning "-Wctad-maybe-unsupported"
+namespace test_implicit_ctad_warning {
+
+template <class T>
+struct Tag {};
+
+template <class T>
+struct NoExplicit { // expected-note {{add a deduction guide to suppress this warning}}
+  NoExplicit(T) {}
+  NoExplicit(T, int) {}
+};
+
+// expected-warning@+1 {{'NoExplicit' may not intend to support class template argument deduction}}
+NoExplicit ne(42);
+
+template <class U>
+struct HasExplicit {
+  HasExplicit(U) {}
+  HasExplicit(U, int) {}
+};
+template <class U> HasExplicit(U, int) -> HasExplicit<Tag<U>>;
+
+HasExplicit he(42);
+
+// Motivating examples from (taken from Stephan Lavavej's 2018 Cppcon talk)
+template <class T, class U>
+struct AmateurPair { // expected-note {{add a deduction guide to suppress this warning}}
+  T first;
+  U second;
+  explicit AmateurPair(const T &t, const U &u) {}
+};
+// expected-warning@+1 {{'AmateurPair' may not intend to support class template argument deduction}}
+AmateurPair p1(42, "hello world"); // deduces to Pair<int, char[12]>
+
+template <class T, class U>
+struct AmateurPair2 { // expected-note {{add a deduction guide to suppress this warning}}
+  T first;
+  U second;
+  explicit AmateurPair2(T t, U u) {}
+};
+// expected-warning@+1 {{'AmateurPair2' may not intend to support class template argument deduction}}
+AmateurPair2 p2(42, "hello world"); // deduces to Pair2<int, const char*>
+
+template <class T, class U>
+struct ProPair {
+  T first; U second;
+    explicit ProPair(T const& t, U  const& u)  {}
+};
+template<class T1, class T2>
+ProPair(T1, T2) -> ProPair<T1, T2>;
+ProPair p3(42, "hello world"); // deduces to ProPair<int, const char*>
+static_assert(__is_same(decltype(p3), ProPair<int, const char*>));
+
+// Test that user-defined explicit guides suppress the warning even if they
+// aren't used as candidates.
+template <class T>
+struct TestExplicitCtor {
+  TestExplicitCtor(T) {}
+};
+template <class T>
+explicit TestExplicitCtor(TestExplicitCtor<T> const&) -> TestExplicitCtor<void>;
+TestExplicitCtor<int> ce1{42};
+TestExplicitCtor ce2 = ce1;
+static_assert(__is_same(decltype(ce2), TestExplicitCtor<int>), "");
+
+struct allow_ctad_t {
+  allow_ctad_t() = delete;
+};
+
+template <class T>
+struct TestSuppression {
+  TestSuppression(T) {}
+};
+TestSuppression(allow_ctad_t)->TestSuppression<void>;
+TestSuppression ta("abc");
+static_assert(__is_same(decltype(ta), TestSuppression<const char *>), "");
+}
+#pragma clang diagnostic pop
+
 #else
 
 // expected-no-diagnostics