Index: lib/Sema/Sema.cpp
===================================================================
--- lib/Sema/Sema.cpp
+++ lib/Sema/Sema.cpp
@@ -542,6 +542,9 @@
     // __attribute__((weakref)) is basically a definition.
     if (ND->hasAttr<WeakRefAttr>()) continue;
 
+    if (isa<CXXDeductionGuideDecl>(ND))
+      continue;
+
     if (FunctionDecl *FD = dyn_cast<FunctionDecl>(ND)) {
       if (FD->isDefined())
         continue;
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
@@ -1,4 +1,13 @@
-// RUN: %clang_cc1 -std=c++1z -verify %s
+// RUN: %clang_cc1 -std=c++1z -verify %s -DTEST1
+// RUN: %clang_cc1 -std=c++1z -verify %s -DTEST2
+
+// This file has to be split into two tests because
+// the former test contains expected diagnostics,
+// whereas the second test should not produce any
+// diagnostics (expected-no-diagnostics cannot
+// be used with other expected- diagnostics checks).
+
+#if defined(TEST1)
 
 namespace std {
   using size_t = decltype(sizeof(0));
@@ -280,3 +289,17 @@
     scoped_lock l = {};
   }
 }
+
+#elif defined(TEST2)
+
+// expected-no-diagnostics
+namespace undefined_warnings {
+  template <typename T>
+  struct TemplDObj {
+    explicit TemplDObj(T func) noexcept {}
+  };
+
+  auto test = TemplDObj([] {});
+}
+
+#endif