1 files changed, 80 insertions, 0 deletions
diff --git a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
index c310c651408..29adc8f5603 100644
--- a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
+++ b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
@@ -409,6 +409,86 @@ B b(0, {});
 
 }
 
+#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