Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2366,6 +2366,9 @@
   "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<
+  "variable %0 is a value copy of call's lvalue return; did you mean '&'?">,
+  InGroup<DiagGroup<"auto-by-value-copy">>, DefaultIgnore;
 
 // 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
@@ -12255,8 +12255,17 @@
     }
     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->getDeclName();
   }
 
   // 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 +1,23 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -Wauto-by-value-copy %s
+
+struct Big {
+  Big (Big const &) noexcept;
+  unsigned r[50];
+};
+
+Big &Fn ();
+Big &(*P) ();
+
+Big VFn ();
+int &SR();
+
+void f () {
+  auto const r = Fn(); // expected-warning{{is a value copy of}}
+  auto const s = P(); // expected-warning{{is a value copy of}}
+
+  auto const t (Fn()); // paren-init
+  auto const u {P()}; // brace-init
+
+  auto const v = VFn(); // by-value
+  auto const w = SR(); // non-class
+}