Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -352,7 +352,6 @@
 def NullPointerArithmetic : DiagGroup<"null-pointer-arithmetic">;
 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: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -6419,12 +6419,12 @@
   "type %0 requires %1 bytes of alignment and the default allocator only "
   "guarantees %2 bytes">,
   InGroup<OveralignedType>, DefaultIgnore;
-def warn_aligned_allocation_unavailable :Warning<
+def err_aligned_allocation_unavailable : Error<
   "aligned %select{allocation|deallocation}0 function of type '%1' is only "
-  "available on %2 %3 or newer">, InGroup<AlignedAllocationUnavailable>, DefaultError;
+  "available on %2 %3 or newer">;
 def note_silence_unligned_allocation_unavailable : Note<
   "if you supply your own aligned allocation functions, use "
-  "-Wno-aligned-allocation-unavailable to silence this diagnostic">;
+  "-faligned-allocation 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: lib/Driver/ToolChains/Darwin.cpp
===================================================================
--- lib/Driver/ToolChains/Darwin.cpp
+++ lib/Driver/ToolChains/Darwin.cpp
@@ -2018,7 +2018,12 @@
 void Darwin::addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
                                    llvm::opt::ArgStringList &CC1Args,
                                    Action::OffloadKind DeviceOffloadKind) const {
-  if (isAlignedAllocationUnavailable())
+  // Pass "-faligned-alloc-unavailable" only when the user hasn't manually
+  // enabled or disabled aligned allocations and hasn't specified a custom
+  // standard library installation.
+  if (!DriverArgs.hasArgNoClaim(options::OPT_faligned_allocation,
+                                options::OPT_fno_aligned_allocation) &&
+      isAlignedAllocationUnavailable())
     CC1Args.push_back("-faligned-alloc-unavailable");
 }
 
Index: lib/Frontend/InitPreprocessor.cpp
===================================================================
--- lib/Frontend/InitPreprocessor.cpp
+++ lib/Frontend/InitPreprocessor.cpp
@@ -551,7 +551,7 @@
     Builder.defineMacro("__cpp_nontype_template_args", "201411");
     Builder.defineMacro("__cpp_fold_expressions", "201603");
   }
-  if (LangOpts.AlignedAllocation)
+  if (LangOpts.AlignedAllocation && !LangOpts.AlignedAllocationUnavailable)
     Builder.defineMacro("__cpp_aligned_new", "201606");
 
   // TS features.
Index: lib/Sema/SemaExprCXX.cpp
===================================================================
--- lib/Sema/SemaExprCXX.cpp
+++ lib/Sema/SemaExprCXX.cpp
@@ -1697,9 +1697,9 @@
     StringRef OSName = AvailabilityAttr::getPlatformNameSourceSpelling(
         S.getASTContext().getTargetInfo().getPlatformName());
 
-    S.Diag(Loc, diag::warn_aligned_allocation_unavailable)
-         << IsDelete << FD.getType().getAsString() << OSName
-         << alignedAllocMinVersion(T.getOS()).getAsString();
+    S.Diag(Loc, diag::err_aligned_allocation_unavailable)
+        << IsDelete << FD.getType().getAsString() << OSName
+        << alignedAllocMinVersion(T.getOS()).getAsString();
     S.Diag(Loc, diag::note_silence_unligned_allocation_unavailable);
   }
 }
Index: test/Driver/unavailable_aligned_allocation.cpp
===================================================================
--- test/Driver/unavailable_aligned_allocation.cpp
+++ test/Driver/unavailable_aligned_allocation.cpp
@@ -51,4 +51,13 @@
 // RUN: -c -### %s 2>&1 \
 // RUN:   | FileCheck %s -check-prefix=AVAILABLE
 //
+// Check that passing -faligned-allocation or -fno-aligned-allocation stops the
+// driver from passing -faligned-alloc-unavailable to cc1.
+//
+// RUN: %clang -target x86_64-apple-macosx10.12 -faligned-allocation -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+//
+// RUN: %clang -target x86_64-apple-macosx10.12 -fno-aligned-allocation -c -### %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=AVAILABLE
+
 // AVAILABLE-NOT: "-faligned-alloc-unavailable"
Index: test/Lexer/aligned-allocation.cpp
===================================================================
--- /dev/null
+++ test/Lexer/aligned-allocation.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.12.0 -fexceptions -std=c++17 -verify %s \
+// RUN:   -DEXPECT_DEFINED
+//
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.12.0 -fexceptions -std=c++17 -verify %s \
+// RUN:   -faligned-alloc-unavailable
+//
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.12.0 -fexceptions -std=c++17 -verify %s \
+// RUN:   -faligned-allocation -faligned-alloc-unavailable
+
+// Test that __cpp_aligned_new is not defined when CC1 is passed
+// -faligned-alloc-unavailable by the Darwin driver, even when aligned
+// allocation is actually enabled.
+
+// expected-no-diagnostics
+#ifdef EXPECT_DEFINED
+# ifndef __cpp_aligned_new
+#   error "__cpp_aligned_new" should be defined
+# endif
+#else
+# ifdef __cpp_aligned_new
+#   error "__cpp_aligned_new" should not be defined
+# endif
+#endif