Index: cfe/trunk/include/clang/AST/Decl.h
===================================================================
--- cfe/trunk/include/clang/AST/Decl.h
+++ cfe/trunk/include/clang/AST/Decl.h
@@ -2019,7 +2019,10 @@
   /// These functions have special behavior under C++1y [expr.new]:
   ///    An implementation is allowed to omit a call to a replaceable global
   ///    allocation function. [...]
-  bool isReplaceableGlobalAllocationFunction() const;
+  ///
+  /// If this function is an aligned allocation/deallocation function, return
+  /// true through IsAligned.
+  bool isReplaceableGlobalAllocationFunction(bool *IsAligned = nullptr) const;
 
   /// Compute the language linkage.
   LanguageLinkage getLanguageLinkage() const;
Index: cfe/trunk/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticGroups.td
+++ cfe/trunk/include/clang/Basic/DiagnosticGroups.td
@@ -311,6 +311,7 @@
 def NonVirtualDtor : DiagGroup<"non-virtual-dtor">;
 def : DiagGroup<"effc++", [NonVirtualDtor]>;
 def OveralignedType : DiagGroup<"over-aligned">;
+def AlignedAllocationUnavailable : DiagGroup<"aligned-allocation-unavailable">;
 def OldStyleCast : DiagGroup<"old-style-cast">;
 def : DiagGroup<"old-style-definition">;
 def OutOfLineDeclaration : DiagGroup<"out-of-line-declaration">;
Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6407,6 +6407,12 @@
   "type %0 requires %1 bytes of alignment and the default allocator only "
   "guarantees %2 bytes">,
   InGroup<OveralignedType>, DefaultIgnore;
+def warn_aligned_allocation_unavailable :Warning<
+  "aligned %select{allocation|deallocation}0 function of type '%1' possibly "
+  "unavailable on %2">, InGroup<AlignedAllocationUnavailable>, DefaultError;
+def note_silence_unligned_allocation_unavailable : Note<
+  "if you supply your own aligned allocation functions, use "
+  "-Wno-aligned-allocation-unavailable to silence this diagnostic">;
 
 def err_conditional_void_nonvoid : Error<
   "%select{left|right}1 operand to ? is void, but %select{right|left}1 operand "
Index: cfe/trunk/include/clang/Basic/LangOptions.def
===================================================================
--- cfe/trunk/include/clang/Basic/LangOptions.def
+++ cfe/trunk/include/clang/Basic/LangOptions.def
@@ -199,6 +199,7 @@
 
 LANGOPT(SizedDeallocation , 1, 0, "sized deallocation")
 LANGOPT(AlignedAllocation , 1, 0, "aligned allocation")
+LANGOPT(AlignedAllocationUnavailable, 1, 0, "aligned allocation functions are unavailable")
 LANGOPT(NewAlignOverride  , 32, 0, "maximum alignment guaranteed by '::operator new(size_t)'")
 LANGOPT(ConceptsTS , 1, 0, "enable C++ Extensions for Concepts")
 BENIGN_LANGOPT(ModulesCodegen , 1, 0, "Modules code generation")
Index: cfe/trunk/include/clang/Driver/CC1Options.td
===================================================================
--- cfe/trunk/include/clang/Driver/CC1Options.td
+++ cfe/trunk/include/clang/Driver/CC1Options.td
@@ -568,6 +568,9 @@
 def fno_pch_timestamp : Flag<["-"], "fno-pch-timestamp">,
   HelpText<"Disable inclusion of timestamp in precompiled headers">;
   
+def aligned_alloc_unavailable : Flag<["-"], "faligned-alloc-unavailable">,
+  HelpText<"Aligned allocation/deallocation functions are unavailable">;
+
 //===----------------------------------------------------------------------===//
 // Language Options
 //===----------------------------------------------------------------------===//
Index: cfe/trunk/lib/AST/Decl.cpp
===================================================================
--- cfe/trunk/lib/AST/Decl.cpp
+++ cfe/trunk/lib/AST/Decl.cpp
@@ -2630,7 +2630,7 @@
   return (proto->getParamType(1).getCanonicalType() == Context.VoidPtrTy);
 }
 
-bool FunctionDecl::isReplaceableGlobalAllocationFunction() const {
+bool FunctionDecl::isReplaceableGlobalAllocationFunction(bool *IsAligned) const {
   if (getDeclName().getNameKind() != DeclarationName::CXXOperatorName)
     return false;
   if (getDeclName().getCXXOverloadedOperator() != OO_New &&
@@ -2676,8 +2676,11 @@
 
   // In C++17, the next parameter can be a 'std::align_val_t' for aligned
   // new/delete.
-  if (Ctx.getLangOpts().AlignedAllocation && !Ty.isNull() && Ty->isAlignValT())
+  if (Ctx.getLangOpts().AlignedAllocation && !Ty.isNull() && Ty->isAlignValT()) {
+    if (IsAligned)
+      *IsAligned = true;
     Consume();
+  }
 
   // Finally, if this is not a sized delete, the final parameter can
   // be a 'const std::nothrow_t&'.
Index: cfe/trunk/lib/Driver/ToolChains/Darwin.h
===================================================================
--- cfe/trunk/lib/Driver/ToolChains/Darwin.h
+++ cfe/trunk/lib/Driver/ToolChains/Darwin.h
@@ -384,6 +384,14 @@
     return TargetVersion < VersionTuple(V0, V1, V2);
   }
 
+  /// Return true if c++17 aligned allocation/deallocation functions are not
+  /// implemented in the c++ standard library of the deployment target we are
+  /// targeting.
+  bool isAlignedAllocationUnavailable() const;
+
+  void addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
+                             llvm::opt::ArgStringList &CC1Args) const override;
+
   StringRef getPlatformFamily() const;
   static StringRef getSDKName(StringRef isysroot);
   StringRef getOSLibraryNameSuffix() const;
Index: cfe/trunk/lib/Driver/ToolChains/Darwin.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChains/Darwin.cpp
+++ cfe/trunk/lib/Driver/ToolChains/Darwin.cpp
@@ -1667,6 +1667,27 @@
   AddLinkRuntimeLib(Args, CmdArgs, CompilerRT, false, true);
 }
 
+bool Darwin::isAlignedAllocationUnavailable() const {
+  switch (TargetPlatform) {
+  case MacOS: // Earlier than 10.13.
+    return TargetVersion < VersionTuple(10U, 13U, 0U);
+  case IPhoneOS:
+  case IPhoneOSSimulator:
+  case TvOS:
+  case TvOSSimulator: // Earlier than 11.0.
+    return TargetVersion < VersionTuple(11U, 0U, 0U);
+  case WatchOS:
+  case WatchOSSimulator: // Earlier than 4.0.
+    return TargetVersion < VersionTuple(4U, 0U, 0U);
+  }
+}
+
+void Darwin::addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
+                                   llvm::opt::ArgStringList &CC1Args) const {
+  if (isAlignedAllocationUnavailable())
+    CC1Args.push_back("-faligned-alloc-unavailable");
+}
+
 DerivedArgList *
 Darwin::TranslateArgs(const DerivedArgList &Args, StringRef BoundArch,
                       Action::OffloadKind DeviceOffloadKind) const {
Index: cfe/trunk/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- cfe/trunk/lib/Frontend/CompilerInvocation.cpp
+++ cfe/trunk/lib/Frontend/CompilerInvocation.cpp
@@ -2106,6 +2106,8 @@
   Opts.AlignedAllocation =
       Args.hasFlag(OPT_faligned_allocation, OPT_fno_aligned_allocation,
                    Opts.AlignedAllocation);
+  Opts.AlignedAllocationUnavailable =
+      Opts.AlignedAllocation && Args.hasArg(OPT_aligned_alloc_unavailable);
   Opts.NewAlignOverride =
       getLastArgIntValue(Args, OPT_fnew_alignment_EQ, 0, Diags);
   if (Opts.NewAlignOverride && !llvm::isPowerOf2_32(Opts.NewAlignOverride)) {
Index: cfe/trunk/lib/Sema/SemaExprCXX.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaExprCXX.cpp
+++ cfe/trunk/lib/Sema/SemaExprCXX.cpp
@@ -1646,6 +1646,27 @@
   return false;
 }
 
+// Emit a diagnostic if an aligned allocation/deallocation function that is not
+// implemented in the standard library is selected.
+static void diagnoseUnavailableAlignedAllocation(const FunctionDecl &FD,
+                                                 SourceLocation Loc, bool IsDelete,
+                                                 Sema &S) {
+  if (!S.getLangOpts().AlignedAllocationUnavailable)
+    return;
+
+  // Return if there is a definition.
+  if (FD.isDefined())
+    return;
+
+  bool IsAligned = false;
+  if (FD.isReplaceableGlobalAllocationFunction(&IsAligned) && IsAligned) {
+    S.Diag(Loc, diag::warn_aligned_allocation_unavailable)
+         << IsDelete << FD.getType().getAsString()
+         << S.getASTContext().getTargetInfo().getTriple().str();
+    S.Diag(Loc, diag::note_silence_unligned_allocation_unavailable);
+  }
+}
+
 ExprResult
 Sema::BuildCXXNew(SourceRange Range, bool UseGlobal,
                   SourceLocation PlacementLParen,
@@ -2023,11 +2044,13 @@
     if (DiagnoseUseOfDecl(OperatorNew, StartLoc))
       return ExprError();
     MarkFunctionReferenced(StartLoc, OperatorNew);
+    diagnoseUnavailableAlignedAllocation(*OperatorNew, StartLoc, false, *this);
   }
   if (OperatorDelete) {
     if (DiagnoseUseOfDecl(OperatorDelete, StartLoc))
       return ExprError();
     MarkFunctionReferenced(StartLoc, OperatorDelete);
+    diagnoseUnavailableAlignedAllocation(*OperatorDelete, StartLoc, true, *this);
   }
 
   // C++0x [expr.new]p17:
@@ -3243,6 +3266,9 @@
                       PDiag(diag::err_access_dtor) << PointeeElem);
       }
     }
+
+    diagnoseUnavailableAlignedAllocation(*OperatorDelete, StartLoc, true,
+                                         *this);
   }
 
   CXXDeleteExpr *Result = new (Context) CXXDeleteExpr(
Index: cfe/trunk/test/Driver/unavailable_aligned_allocation.cpp
===================================================================
--- cfe/trunk/test/Driver/unavailable_aligned_allocation.cpp
+++ cfe/trunk/test/Driver/unavailable_aligned_allocation.cpp
@@ -0,0 +1,54 @@
+// RUN: %clang -target x86_64-apple-macosx10.12 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=UNAVAILABLE
+//
+// RUN: %clang -target arm64-apple-ios10 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=UNAVAILABLE
+//
+// RUN: %clang -target arm64-apple-tvos10 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=UNAVAILABLE
+//
+// RUN: %clang -target thumbv7-apple-watchos3 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=UNAVAILABLE
+//
+// RUN: %clang -target x86_64-apple-macosx10.13 -mios-simulator-version-min=10 \
+// RUN:  -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=UNAVAILABLE
+//
+// RUN: %clang -target x86_64-apple-macosx10.13 -mtvos-simulator-version-min=10 \
+// RUN: -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=UNAVAILABLE
+//
+// RUN: %clang -target x86_64-apple-macosx10.13 -mwatchos-simulator-version-min=3 \
+// RUN: -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=UNAVAILABLE
+//
+// UNAVAILABLE: "-faligned-alloc-unavailable"
+
+// RUN: %clang -target x86_64-apple-macosx10.13 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target arm64-apple-ios11 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target arm64-apple-tvos11 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target armv7k-apple-watchos4 -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target x86_64-unknown-linux-gnu -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target x86_64-apple-macosx10.12 -mios-simulator-version-min=11 \
+// RUN:  -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target x86_64-apple-macosx10.12 -mtvos-simulator-version-min=11 \
+// RUN: -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target x86_64-apple-macosx10.12 -mwatchos-simulator-version-min=4 \
+// RUN: -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// AVAILABLE-NOT: "-faligned-alloc-unavailable"
Index: cfe/trunk/test/SemaCXX/unavailable_aligned_allocation.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/unavailable_aligned_allocation.cpp
+++ cfe/trunk/test/SemaCXX/unavailable_aligned_allocation.cpp
@@ -0,0 +1,109 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.12.0 -fexceptions -faligned-alloc-unavailable -std=c++1z -verify %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.12.0 -fexceptions -std=c++1z -verify -DNO_ERRORS %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.12.0 -fexceptions -faligned-allocation -faligned-alloc-unavailable -std=c++14 -verify %s
+
+namespace std {
+  typedef decltype(sizeof(0)) size_t;
+  enum class align_val_t : std::size_t {};
+  struct nothrow_t {};
+  nothrow_t nothrow;
+}
+
+void *operator new(std::size_t __sz, const std::nothrow_t&) noexcept;
+void *operator new[](std::size_t __sz, const std::nothrow_t&) noexcept;
+
+void *operator new(std::size_t __sz, std::align_val_t, const std::nothrow_t&) noexcept;
+void *operator new[](std::size_t __sz, std::align_val_t, const std::nothrow_t&) noexcept;
+void operator delete(void *, std::align_val_t, const std::nothrow_t&);
+void operator delete[](void *, std::align_val_t, const std::nothrow_t&);
+void operator delete(void*, std::size_t, std::align_val_t) noexcept;
+void operator delete[](void*, std::size_t, std::align_val_t) noexcept;
+
+void *operator new(std::size_t, std::align_val_t, long long);
+
+struct alignas(256) OveralignedS {
+  int x[16];
+};
+
+struct S {
+  int x[16];
+};
+
+void test() {
+  auto *p = new S;
+  delete p;
+  p = new (std::nothrow) S;
+
+  auto *pa = new S[4];
+  delete[] pa;
+  pa = new (std::nothrow) S[4];
+}
+
+void testOveraligned() {
+  auto *p = new OveralignedS;
+  p = new ((std::align_val_t)8) OveralignedS;
+  delete p;
+  p = new (std::nothrow) OveralignedS;
+
+  auto *pa = new OveralignedS[4];
+  pa = new ((std::align_val_t)8) OveralignedS[4];
+  delete[] pa;
+  pa = new (std::nothrow) OveralignedS[4];
+  // No error here since it is not calling a replaceable allocation function.
+  p = new ((std::align_val_t)8, 10LL) OveralignedS;
+}
+
+#ifdef NO_ERRORS
+// expected-no-diagnostics
+#else
+// expected-error@-16 {{aligned allocation function of type 'void *(unsigned long, enum std::align_val_t)' possibly unavailable on}}
+// expected-note@-17 {{if you supply your own aligned allocation functions}}
+// expected-error@-18 {{aligned deallocation function of type 'void (void *, enum std::align_val_t) noexcept' possibly unavailable on}}
+// expected-note@-19 {{if you supply your own aligned allocation functions}}
+
+// expected-error@-20 {{aligned allocation function of type 'void *(unsigned long, enum std::align_val_t)' possibly unavailable on}}
+// expected-note@-21 {{if you supply your own aligned allocation functions}}
+// expected-error@-22 {{aligned deallocation function of type 'void (void *, enum std::align_val_t) noexcept' possibly unavailable on}}
+// expected-note@-23 {{if you supply your own aligned allocation functions}}
+
+// expected-error@-24 {{aligned deallocation function of type 'void (void *, enum std::align_val_t) noexcept' possibly unavailable on}}
+// expected-note@-25 {{if you supply your own aligned allocation functions}}
+
+// expected-error@-26 {{aligned allocation function of type 'void *(std::size_t, std::align_val_t, const std::nothrow_t &) noexcept' possibly unavailable on}}
+// expected-note@-27 {{if you supply your own aligned allocation functions}}
+// expected-error@-28 {{aligned deallocation function of type 'void (void *, std::align_val_t, const std::nothrow_t &) noexcept' possibly unavailable on}}
+// expected-note@-29 {{if you supply your own aligned allocation functions}}
+
+// expected-error@-29 {{aligned allocation function of type 'void *(unsigned long, enum std::align_val_t)' possibly unavailable on}}
+// expected-note@-30 {{if you supply your own aligned allocation functions}}
+// expected-error@-31 {{aligned deallocation function of type 'void (void *, enum std::align_val_t) noexcept' possibly unavailable on}}
+// expected-note@-32 {{if you supply your own aligned allocation functions}}
+
+// expected-error@-33 {{aligned allocation function of type 'void *(unsigned long, enum std::align_val_t)' possibly unavailable on}}
+// expected-note@-34 {{if you supply your own aligned allocation functions}}
+// expected-error@-35 {{aligned deallocation function of type 'void (void *, enum std::align_val_t) noexcept' possibly unavailable on}}
+// expected-note@-36 {{if you supply your own aligned allocation functions}}
+
+// expected-error@-37 {{aligned deallocation function of type 'void (void *, enum std::align_val_t) noexcept' possibly unavailable on}}
+// expected-note@-38 {{if you supply your own aligned allocation functions}}
+
+// expected-error@-39 {{aligned allocation function of type 'void *(std::size_t, std::align_val_t, const std::nothrow_t &) noexcept' possibly unavailable on}}
+// expected-note@-40 {{if you supply your own aligned allocation functions}}
+// expected-error@-41 {{aligned deallocation function of type 'void (void *, std::align_val_t, const std::nothrow_t &) noexcept' possibly unavailable on}}
+// expected-note@-42 {{if you supply your own aligned allocation functions}}
+
+#endif
+
+// No errors if user-defined aligned allocation functions are available.
+void *operator new(std::size_t __sz, std::align_val_t) {
+  static char array[256];
+  return &array;
+}
+
+void operator delete(void *p, std::align_val_t) {
+}
+
+void testOveraligned2() {
+  auto p = new ((std::align_val_t)8) OveralignedS;
+  delete p;
+}