diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
--- a/clang/include/clang/Basic/Builtins.def
+++ b/clang/include/clang/Basic/Builtins.def
@@ -546,7 +546,7 @@
 BUILTIN(__builtin_va_end, "vA", "n")
 BUILTIN(__builtin_va_copy, "vAA", "n")
 BUILTIN(__builtin_stdarg_start, "vA.", "nt")
-BUILTIN(__builtin_assume_aligned, "v*vC*z.", "nc")
+BUILTIN(__builtin_assume_aligned, "v*vC*z.", "nct")
 BUILTIN(__builtin_bcmp, "ivC*vC*z", "Fn")
 BUILTIN(__builtin_bcopy, "vv*v*z", "n")
 BUILTIN(__builtin_bzero, "vv*z", "nF")
diff --git a/clang/lib/Basic/Builtins.cpp b/clang/lib/Basic/Builtins.cpp
--- a/clang/lib/Basic/Builtins.cpp
+++ b/clang/lib/Basic/Builtins.cpp
@@ -209,6 +209,7 @@
 
 bool Builtin::Context::canBeRedeclared(unsigned ID) const {
   return ID == Builtin::NotBuiltin || ID == Builtin::BI__va_start ||
+         ID == Builtin::BI__builtin_assume_aligned ||
          (!hasReferenceArgsOrResult(ID) && !hasCustomTypechecking(ID)) ||
          isInStdNamespace(ID);
 }
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -141,6 +141,15 @@
          << Call->getSourceRange();
 }
 
+/// Checks that a call expression's argument count is in the desired range. This
+/// is useful when doing custom type-checking on a variadic function. Returns
+/// true on error.
+static bool checkArgCountRange(Sema &S, CallExpr *Call, unsigned MinArgCount,
+                               unsigned MaxArgCount) {
+  return checkArgCountAtLeast(S, Call, MinArgCount) ||
+         checkArgCountAtMost(S, Call, MaxArgCount);
+}
+
 /// Checks that a call expression's argument count is the desired number.
 /// This is useful when doing custom type-checking.  Returns true on error.
 static bool checkArgCount(Sema &S, CallExpr *Call, unsigned DesiredArgCount) {
@@ -7643,17 +7652,15 @@
 /// Handle __builtin_assume_aligned. This is declared
 /// as (const void*, size_t, ...) and can take one optional constant int arg.
 bool Sema::SemaBuiltinAssumeAligned(CallExpr *TheCall) {
-  if (checkArgCountAtMost(*this, TheCall, 3))
+  if (checkArgCountRange(*this, TheCall, 2, 3))
     return true;
 
   unsigned NumArgs = TheCall->getNumArgs();
   Expr *FirstArg = TheCall->getArg(0);
-  if (auto *CE = dyn_cast<CastExpr>(FirstArg))
-    FirstArg = CE->getSubExprAsWritten();
-
+  
   {
     ExprResult FirstArgResult =
-        DefaultFunctionArrayLvalueConversion(FirstArg, /*Diagnose=*/false);
+        DefaultFunctionArrayLvalueConversion(FirstArg);
     if (FirstArgResult.isInvalid())
       return true;
     TheCall->setArg(0, FirstArgResult.get());
diff --git a/clang/test/Sema/builtin-redecl.cpp b/clang/test/Sema/builtin-redecl.cpp
--- a/clang/test/Sema/builtin-redecl.cpp
+++ b/clang/test/Sema/builtin-redecl.cpp
@@ -2,6 +2,8 @@
 // RUN: %clang_cc1 %s -fsyntax-only -verify -x c
 // RUN: %clang_cc1 %s -fsyntax-only -verify -fms-compatibility
 
+#include <stddef.h>
+
 // Redeclaring library builtins is OK.
 void exit(int);
 
@@ -16,3 +18,9 @@
 // RUN: %clang_cc1 %s -fsyntax-only -verify -x c
 
 void __va_start(__builtin_va_list*, ...);
+
+#ifdef __cplusplus
+void *__builtin_assume_aligned(const void *, size_t, ...) noexcept;
+#else
+void *__builtin_assume_aligned(const void *, size_t, ...);
+#endif