diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -1599,5 +1599,7 @@
 def err_expected_semantic_identifier : Error<
   "expected HLSL Semantic identifier">;
 def err_unknown_hlsl_semantic : Error<"unknown HLSL semantic %0">;
+def ext_hlsl_access_specifiers : ExtWarn<
+  "access specifiers are a clang HLSL extension">;
 
 } // end of Parser diagnostics
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -2170,8 +2170,11 @@
 
   // Parse an (optional) access specifier.
   AccessSpecifier Access = getAccessSpecifierIfPresent();
-  if (Access != AS_none)
+  if (Access != AS_none) {
     ConsumeToken();
+    if (getLangOpts().HLSL)
+      Diag(Tok.getLocation(), diag::ext_hlsl_access_specifiers);
+  }
 
   CheckMisplacedCXX11Attribute(Attributes, StartLoc);
 
@@ -3270,6 +3273,8 @@
     LLVM_FALLTHROUGH;
   case tok::kw_public:
   case tok::kw_protected: {
+    if (getLangOpts().HLSL)
+      Diag(Tok.getLocation(), diag::ext_hlsl_access_specifiers);
     AccessSpecifier NewAS = getAccessSpecifierIfPresent();
     assert(NewAS != AS_none);
     // Current token is a C++ access specifier.
@@ -3509,8 +3514,9 @@
   // C++ 11p3: Members of a class defined with the keyword class are private
   // by default. Members of a class defined with the keywords struct or union
   // are public by default.
+  // HLSL: In HLSL members of a class are public by default.
   AccessSpecifier CurAS;
-  if (TagType == DeclSpec::TST_class)
+  if (TagType == DeclSpec::TST_class && !getLangOpts().HLSL)
     CurAS = AS_private;
   else
     CurAS = AS_public;
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -2501,6 +2501,11 @@
                          bool Virtual, AccessSpecifier Access,
                          TypeSourceInfo *TInfo,
                          SourceLocation EllipsisLoc) {
+  // In HLSL, unspecified class access is public rather than private.
+  if (getLangOpts().HLSL && Class->getTagKind() == TTK_Class &&
+      Access == AS_none)
+    Access = AS_public;
+
   QualType BaseType = TInfo->getType();
   if (BaseType->containsErrors()) {
     // Already emitted a diagnostic when parsing the error type.
diff --git a/clang/test/ParserHLSL/access_specifiers.hlsl b/clang/test/ParserHLSL/access_specifiers.hlsl
new file mode 100644
--- /dev/null
+++ b/clang/test/ParserHLSL/access_specifiers.hlsl
@@ -0,0 +1,48 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -x hlsl -ast-dump -o - %s -verify
+
+class Doggo {
+  // expected-note@+1 {{member is declared here}}
+  int legs;
+// expected-warning@+1 {{access specifiers are a clang HLSL extension}}
+private:
+  // expected-note@+2 {{declared private here}}
+  // expected-note@+1 {{declared private here}}
+  int tail;
+};
+
+// expected-warning@+1 {{access specifiers are a clang HLSL extension}}
+class Shiba : public Doggo {
+  int undercoat;
+};
+
+class Akita : Doggo {
+  int floof;
+};
+
+// expected-note@+2 {{constrained by private inheritance here}}
+// expected-warning@+1 {{access specifiers are a clang HLSL extension}}
+class Chow : private Doggo {
+  int megafloof;
+};
+
+void main() {
+  Shiba Shibe;
+  Shibe.undercoat = 0xFFFF;
+  Shibe.legs = 4;
+
+  // expected-error@+1 {{'tail' is a private member of 'Doggo'}}
+  Shibe.tail = 1;
+
+  Akita Aki;
+  Aki.floof = 0xFFFF;
+  Aki.legs = 4;
+
+  // expected-error@+1 {{'tail' is a private member of 'Doggo'}}
+  Aki.tail = 1;
+
+  Chow Ch;
+  Ch.megafloof = 0xFFFF;
+
+  // expected-error@+1 {{'legs' is a private member of 'Doggo'}}
+  Ch.legs = 4;
+}