Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2366,6 +2366,10 @@
   "cannot use __auto_type with initializer list in C">;
 def err_auto_bitfield : Error<
   "cannot pass bit-field as __auto_type initializer in C">;
+def warn_auto_creates_copy : Warning<
+  "initialization of %0 creates a value copy of the retuned reference;"
+  " did you mean '&'?">,
+  InGroup<DiagGroup<"auto-by-value-copy">>;
 
 // C++1y decltype(auto) type
 def err_decltype_auto_invalid : Error<
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -12263,8 +12263,14 @@
     }
     Init = Res.get();
 
+    QualType OrigTy = VDecl->getType();
     if (DeduceVariableDeclarationType(VDecl, DirectInit, Init))
       return;
+
+    if (!DirectInit && isa<CallExpr>(Init) && OrigTy->isUndeducedAutoType() &&
+        OrigTy.isLocalConstQualified() && Init->getValueKind() == VK_LValue &&
+        isa<RecordType>(Init->getType()))
+      Diag(VDecl->getLocation(), diag::warn_auto_creates_copy) << VDecl;
   }
 
   // dllimport cannot be used on variable definitions.
Index: clang/test/SemaCXX/warn-auto-value-copy.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/warn-auto-value-copy.cpp
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s
+
+struct Big {
+  Big(Big const &) noexcept;
+  unsigned r[50];
+};
+
+Big &RFn();
+Big &(*RPtr)();
+
+const Big &CRFn();
+const Big CVFn();
+
+Big VFn();
+int &SR();
+
+void f() {
+  auto const copy1 = RFn();  // expected-warning{{a value copy of}}
+  auto const copy2 = RPtr(); // expected-warning{{a value copy of}}
+  auto const copy3 = CRFn(); // expected-warning{{a value copy of}}
+
+  auto const paren1(RFn());  // paren-init
+  auto const brace1{RPtr()}; // brace-init
+
+  auto const ok1 = VFn();  // by-value
+  auto const ok2 = CVFn(); // by-value
+  auto const ok3 = SR();   // non-class
+}