diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 03ed6accf700c..1800f584c7e10 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -4253,6 +4253,18 @@ def HLSLGroupSharedAddressSpace : TypeAttr {
   let Documentation = [HLSLGroupSharedAddressSpaceDocs];
 }
 
+def HLSLParamModifier : TypeAttr {
+  let Spellings = [CustomKeyword<"in">, CustomKeyword<"inout">, CustomKeyword<"out">];
+  let Accessors = [Accessor<"isIn", [CustomKeyword<"in">]>,
+                   Accessor<"isInOut", [CustomKeyword<"inout">]>,
+                   Accessor<"isOut", [CustomKeyword<"out">]>,
+                   Accessor<"isAnyOut", [CustomKeyword<"out">, CustomKeyword<"inout">]>,
+                   Accessor<"isAnyIn", [CustomKeyword<"in">, CustomKeyword<"inout">]>];
+  let Subjects = SubjectList<[ParmVar]>;
+  let Documentation = [HLSLParamQualifierDocs];
+  let Args = [DefaultBoolArgument<"MergedSpelling", /*default*/0, /*fake*/1>];
+}
+
 def RandomizeLayout : InheritableAttr {
   let Spellings = [GCC<"randomize_layout">];
   let Subjects = SubjectList<[Record]>;
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index be74535e28d8a..af4a6e027502e 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -7061,6 +7061,25 @@ The full documentation is available here: https://learn.microsoft.com/en-us/wind
   }];
 }
 
+def HLSLParamQualifierDocs : Documentation {
+  let Category = DocCatVariable;
+  let Content = [{
+HLSL function parameters are passed by value. Parameter declarations support
+three qualifiers to denote parameter passing behavior. The three qualifiers are
+`in`, `out` and `inout`.
+
+Parameters annotated with `in` or with no annotation are passed by value from
+the caller to the callee.
+
+Parameters annotated with `out` are written to the argument after the callee
+returns (Note: arguments values passed into `out` parameters _are not_ copied
+into the callee).
+
+Parameters annotated with `inout` are copied into the callee via a temporary,
+and copied back to the argument after the callee returns.
+  }];
+}
+
 def AnnotateTypeDocs : Documentation {
   let Category = DocCatType;
   let Heading = "annotate_type";
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 020ff6387c280..d164177251e4d 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -12004,6 +12004,7 @@ def err_hlsl_numthreads_argument_oor : Error<"argument '%select{X|Y|Z}0' to numt
 def err_hlsl_numthreads_invalid : Error<"total number of threads cannot exceed %0">;
 def err_hlsl_missing_numthreads : Error<"missing numthreads attribute for %0 shader entry">;
 def err_hlsl_attribute_param_mismatch : Error<"%0 attribute parameters do not match the previous declaration">;
+def err_hlsl_duplicate_parameter_modifier : Error<"duplicate parameter modifier %0">;
 def err_hlsl_missing_semantic_annotation : Error<
   "semantic annotations must be present for all parameters of an entry "
   "function or patch constant function">;
@@ -12019,6 +12020,9 @@ def err_hlsl_pointers_unsupported : Error<
 def err_hlsl_operator_unsupported : Error<
   "the '%select{&|*|->}0' operator is unsupported in HLSL">;
 
+def err_hlsl_param_qualifier_mismatch :
+  Error<"conflicting parameter qualifier %0 on parameter %1">;
+
 // Layout randomization diagnostics.
 def err_non_designated_init_used : Error<
   "a randomized struct can only be initialized with a designated initializer">;
diff --git a/clang/include/clang/Basic/TokenKinds.def b/clang/include/clang/Basic/TokenKinds.def
index 3ab420821d82b..5f9915d210221 100644
--- a/clang/include/clang/Basic/TokenKinds.def
+++ b/clang/include/clang/Basic/TokenKinds.def
@@ -628,6 +628,9 @@ KEYWORD(__noinline__                , KEYCUDA)
 KEYWORD(cbuffer                     , KEYHLSL)
 KEYWORD(tbuffer                     , KEYHLSL)
 KEYWORD(groupshared                 , KEYHLSL)
+KEYWORD(in                          , KEYHLSL)
+KEYWORD(inout                       , KEYHLSL)
+KEYWORD(out                         , KEYHLSL)
 
 // OpenMP Type Traits
 UNARY_EXPR_OR_TYPE_TRAIT(__builtin_omp_required_simd_align, OpenMPRequiredSimdAlign, KEYALL)
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 5d786fd5fb9a2..e75a8bdb1fc72 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -3737,6 +3737,9 @@ class Sema final {
                                               int X, int Y, int Z);
   HLSLShaderAttr *mergeHLSLShaderAttr(Decl *D, const AttributeCommonInfo &AL,
                                       HLSLShaderAttr::ShaderType ShaderType);
+  HLSLParamModifierAttr *
+  mergeHLSLParamModifierAttr(Decl *D, const AttributeCommonInfo &AL,
+                             HLSLParamModifierAttr::Spelling Spelling);
 
   void mergeDeclAttributes(NamedDecl *New, Decl *Old,
                            AvailabilityMergeKind AMK = AMK_Redeclaration);
diff --git a/clang/lib/AST/TypePrinter.cpp b/clang/lib/AST/TypePrinter.cpp
index e4f5f40cd6259..f694124292736 100644
--- a/clang/lib/AST/TypePrinter.cpp
+++ b/clang/lib/AST/TypePrinter.cpp
@@ -1894,6 +1894,10 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
   case attr::ArmMveStrictPolymorphism:
     OS << "__clang_arm_mve_strict_polymorphism";
     break;
+
+  // Nothing to print for this attribute.
+  case attr::HLSLParamModifier:
+    break;
   }
   OS << "))";
 }
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 8cb5b09fd3b0f..ece3698967e2f 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -4514,6 +4514,9 @@ void Parser::ParseDeclarationSpecifiers(
       break;
 
     case tok::kw_groupshared:
+    case tok::kw_in:
+    case tok::kw_inout:
+    case tok::kw_out:
       // NOTE: ParseHLSLQualifiers will consume the qualifier token.
       ParseHLSLQualifiers(DS.getAttributes());
       continue;
@@ -5550,7 +5553,6 @@ bool Parser::isTypeSpecifierQualifier() {
   case tok::kw___read_write:
   case tok::kw___write_only:
   case tok::kw___funcref:
-  case tok::kw_groupshared:
     return true;
 
   case tok::kw_private:
@@ -5559,6 +5561,13 @@ bool Parser::isTypeSpecifierQualifier() {
   // C11 _Atomic
   case tok::kw__Atomic:
     return true;
+
+  // HLSL type qualifiers
+  case tok::kw_groupshared:
+  case tok::kw_in:
+  case tok::kw_inout:
+  case tok::kw_out:
+    return getLangOpts().HLSL;
   }
 }
 
@@ -6058,6 +6067,9 @@ void Parser::ParseTypeQualifierListOpt(
       break;
 
     case tok::kw_groupshared:
+    case tok::kw_in:
+    case tok::kw_inout:
+    case tok::kw_out:
       // NOTE: ParseHLSLQualifiers will consume the qualifier token.
       ParseHLSLQualifiers(DS.getAttributes());
       continue;
diff --git a/clang/lib/Parse/ParseTentative.cpp b/clang/lib/Parse/ParseTentative.cpp
index 8b653b1c4f8ea..242741c15b5ff 100644
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -1529,6 +1529,9 @@ Parser::isCXXDeclarationSpecifier(ImplicitTypenameContext AllowImplicitTypename,
 
     // HLSL address space qualifiers
   case tok::kw_groupshared:
+  case tok::kw_in:
+  case tok::kw_inout:
+  case tok::kw_out:
 
     // GNU
   case tok::kw_restrict:
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 23dd8ae15c165..77ff4e1df8e9c 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -3373,6 +3373,26 @@ static void mergeParamDeclAttributes(ParmVarDecl *newDecl,
            diag::note_carries_dependency_missing_first_decl) << 1/*Param*/;
   }
 
+  // HLSL parameter declarations for inout and out must match between
+  // declarations. In HLSL inout and out are ambiguous at the call site, but
+  // have different calling behavior, so you cannot overload a method based on a
+  // difference between inout and out annotations.
+  if (S.getLangOpts().HLSL) {
+    const auto *NDAttr = newDecl->getAttr<HLSLParamModifierAttr>();
+    const auto *ODAttr = oldDecl->getAttr<HLSLParamModifierAttr>();
+    // We don't need to cover the case where one declaration doesn't have an
+    // attribute. The only possible case there is if one declaration has an `in`
+    // attribute and the other declaration has no attribute. This case is
+    // allowed since parameters are `in` by default.
+    if (NDAttr && ODAttr &&
+        NDAttr->getSpellingListIndex() != ODAttr->getSpellingListIndex()) {
+      S.Diag(newDecl->getLocation(), diag::err_hlsl_param_qualifier_mismatch)
+          << NDAttr << newDecl;
+      S.Diag(oldDecl->getLocation(), diag::note_previous_declaration_as)
+          << ODAttr;
+    }
+  }
+
   if (!oldDecl->hasAttrs())
     return;
 
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 8966a301de1db..87c78d742d0ff 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -7311,6 +7311,36 @@ static void handleHLSLResourceBindingAttr(Sema &S, Decl *D,
     D->addAttr(NewAttr);
 }
 
+static void handleHLSLParamModifierAttr(Sema &S, Decl *D,
+                                        const ParsedAttr &AL) {
+  HLSLParamModifierAttr *NewAttr = S.mergeHLSLParamModifierAttr(
+      D, AL,
+      static_cast<HLSLParamModifierAttr::Spelling>(AL.getSemanticSpelling()));
+  if (NewAttr)
+    D->addAttr(NewAttr);
+}
+
+HLSLParamModifierAttr *
+Sema::mergeHLSLParamModifierAttr(Decl *D, const AttributeCommonInfo &AL,
+                                 HLSLParamModifierAttr::Spelling Spelling) {
+  // We can only merge an `in` attribute with an `out` attribute. All other
+  // combinations of duplicated attributes are ill-formed.
+  if (HLSLParamModifierAttr *PA = D->getAttr<HLSLParamModifierAttr>()) {
+    if ((PA->isIn() && Spelling == HLSLParamModifierAttr::Keyword_out) ||
+        (PA->isOut() && Spelling == HLSLParamModifierAttr::Keyword_in)) {
+      D->dropAttr<HLSLParamModifierAttr>();
+      SourceRange AdjustedRange = {PA->getLocation(), AL.getRange().getEnd()};
+      return HLSLParamModifierAttr::Create(
+          Context, /*MergedSpelling=*/true, AdjustedRange,
+          HLSLParamModifierAttr::Keyword_inout);
+    }
+    Diag(AL.getLoc(), diag::err_hlsl_duplicate_parameter_modifier) << AL;
+    Diag(PA->getLocation(), diag::note_conflicting_attribute);
+    return nullptr;
+  }
+  return HLSLParamModifierAttr::Create(Context, AL);
+}
+
 static void handleMSInheritanceAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (!S.LangOpts.CPlusPlus) {
     S.Diag(AL.getLoc(), diag::err_attribute_not_supported_in_lang)
@@ -9461,6 +9491,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_HLSLResourceBinding:
     handleHLSLResourceBindingAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_HLSLParamModifier:
+    handleHLSLParamModifierAttr(S, D, AL);
+    break;
 
   case ParsedAttr::AT_AbiTag:
     handleAbiTagAttr(S, D, AL);
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 08f4ba00fc9f7..d768bb72e07c0 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -663,6 +663,14 @@ static bool isRelevantAttr(Sema &S, const Decl *D, const Attr *A) {
   return true;
 }
 
+static void instantiateDependentHLSLParamModifierAttr(
+    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
+    const HLSLParamModifierAttr *Attr, Decl *New) {
+  ParmVarDecl *P = cast<ParmVarDecl>(New);
+  P->addAttr(Attr->clone(S.getASTContext()));
+  P->setType(S.getASTContext().getLValueReferenceType(P->getType()));
+}
+
 void Sema::InstantiateAttrsForDecl(
     const MultiLevelTemplateArgumentList &TemplateArgs, const Decl *Tmpl,
     Decl *New, LateInstantiatedAttrVec *LateAttrs,
@@ -784,6 +792,12 @@ void Sema::InstantiateAttrs(const MultiLevelTemplateArgumentList &TemplateArgs,
                                                *AMDGPUFlatWorkGroupSize, New);
     }
 
+    if (const auto *ParamAttr = dyn_cast<HLSLParamModifierAttr>(TmplAttr)) {
+      instantiateDependentHLSLParamModifierAttr(*this, TemplateArgs, ParamAttr,
+                                                New);
+      continue;
+    }
+
     // Existing DLL attribute on the instantiation takes precedence.
     if (TmplAttr->getKind() == attr::DLLExport ||
         TmplAttr->getKind() == attr::DLLImport) {
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 560feafa1857c..56d133f20a293 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -8661,6 +8661,17 @@ static void HandleLifetimeBoundAttr(TypeProcessingState &State,
   }
 }
 
+static void HandleHLSLParamModifierAttr(QualType &CurType,
+                                        const ParsedAttr &Attr, Sema &S) {
+  // Don't apply this attribute to template dependent types. It is applied on
+  // substitution during template instantiation.
+  if (CurType->isDependentType())
+    return;
+  if (Attr.getSemanticSpelling() == HLSLParamModifierAttr::Keyword_inout ||
+      Attr.getSemanticSpelling() == HLSLParamModifierAttr::Keyword_out)
+    CurType = S.getASTContext().getLValueReferenceType(CurType);
+}
+
 static void processTypeAttrs(TypeProcessingState &state, QualType &type,
                              TypeAttrLocation TAL,
                              const ParsedAttributesView &attrs,
@@ -8837,6 +8848,12 @@ static void processTypeAttrs(TypeProcessingState &state, QualType &type,
       break;
     }
 
+    case ParsedAttr::AT_HLSLParamModifier: {
+      HandleHLSLParamModifierAttr(type, attr, state.getSema());
+      attr.setUsedAsTypeAttr();
+      break;
+    }
+
     MS_TYPE_ATTRS_CASELIST:
       if (!handleMSPointerTypeQualifierAttr(state, attr, type))
         attr.setUsedAsTypeAttr();
diff --git a/clang/test/ParserHLSL/hlsl_groupshared.cpp b/clang/test/ParserHLSL/hlsl_groupshared.cpp
new file mode 100644
index 0000000000000..eaa195a411471
--- /dev/null
+++ b/clang/test/ParserHLSL/hlsl_groupshared.cpp
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 %s -verify
+extern groupshared float f; // expected-error{{unknown type name 'groupshared'}}
+
+extern float groupshared f2; // expected-error{{expected ';' after top level declarator}}
+
+namespace {
+float groupshared [[]] f3 = 12; // expected-error{{expected ';' after top level declarator}}
+}
+
+// expected-error@#fgc{{expected ';' after top level declarator}}
+// expected-error@#fgc{{a type specifier is required for all declarations}}
+float groupshared const f4 = 12; // #fgc
diff --git a/clang/test/ParserHLSL/hlsl_parameter_modifiers.cpp b/clang/test/ParserHLSL/hlsl_parameter_modifiers.cpp
new file mode 100644
index 0000000000000..cb553fbfa4d32
--- /dev/null
+++ b/clang/test/ParserHLSL/hlsl_parameter_modifiers.cpp
@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 %s -verify
+
+// expected-error@#fn{{unknown type name 'in'}}
+// expected-error@#fn{{expected ')'}}
+// expected-note@#fn{{to match this '('}}
+void fn(in out float f); // #fn
+
+// expected-error@#fn2{{unknown type name 'in'}}
+// expected-error@#fn2{{expected ')'}}
+// expected-note@#fn2{{to match this '('}}
+void fn2(in in float f); // #fn2
+
+// expected-error@#fn3{{unknown type name 'out'}}
+// expected-error@#fn3{{expected ')'}}
+// expected-note@#fn3{{to match this '('}}
+void fn3(out out float f); // #fn3
+
+// expected-error@#fn4{{unknown type name 'inout'}}
+// expected-error@#fn4{{expected ')'}}
+// expected-note@#fn4{{to match this '('}}
+void fn4(inout in out float f); // #fn4
+
+// expected-error@#fn5{{unknown type name 'inout'}}
+// expected-error@#fn5{{expected ')'}}
+// expected-note@#fn5{{to match this '('}}
+void fn5(inout in float f); // #fn5
+
+// expected-error@#fn6{{unknown type name 'inout'}}
+// expected-error@#fn6{{expected ')'}}
+// expected-note@#fn6{{to match this '('}}
+void fn6(inout out float f); // #fn6
+
+void implicitFn(float f);
+
+// expected-error@#inFn{{unknown type name 'in'}}
+void inFn(in float f); // #inFn
+
+// expected-error@#inoutFn{{unknown type name 'inout'}}
+void inoutFn(inout float f); // #inoutFn
+
+// expected-error@#outFn{{unknown type name 'out'}}
+void outFn(out float f); // #outFn
+
+// expected-error@#fn7{{unknown type name 'inout'}}
+// expected-error@#fn7{{declaration of 'T' shadows template parameter}}
+// expected-error@#fn7{{expected ')'}}
+// expected-note@#fn7{{to match this '('}}
+template <typename T> // expected-note{{template parameter is declared here}}
+void fn7(inout T f); // #fn7
+
diff --git a/clang/test/SemaHLSL/parameter_modifiers.hlsl b/clang/test/SemaHLSL/parameter_modifiers.hlsl
new file mode 100644
index 0000000000000..c728a41b650ee
--- /dev/null
+++ b/clang/test/SemaHLSL/parameter_modifiers.hlsl
@@ -0,0 +1,94 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.6-library %s -verify
+void fn(in out float f); // #fn
+
+// expected-error@#fn2{{duplicate parameter modifier 'in'}}
+// expected-note@#fn2{{conflicting attribute is here}}
+void fn2(in in float f); // #fn2
+
+// expected-error@#fn3{{duplicate parameter modifier 'out'}}
+// expected-note@#fn3{{conflicting attribute is here}}
+void fn3(out out float f); // #fn3
+
+// expected-error@#fn4{{duplicate parameter modifier 'in'}}
+// expected-error@#fn4{{duplicate parameter modifier 'out'}}
+// expected-note@#fn4{{conflicting attribute is here}}
+// expected-note@#fn4{{conflicting attribute is here}}
+void fn4(inout in out float f); // #fn4
+
+// expected-error@#fn5{{duplicate parameter modifier 'in'}}
+// expected-note@#fn5{{conflicting attribute is here}}
+void fn5(inout in float f); // #fn5
+
+// expected-error@#fn6{{duplicate parameter modifier 'out'}}
+// expected-note@#fn6{{conflicting attribute is here}}
+void fn6(inout out float f); // #fn6
+
+// expected-error@#fn-def{{conflicting parameter qualifier 'out' on parameter 'f'}}
+// expected-note@#fn{{previously declared as 'inout' here}}
+void fn(out float f) { // #fn-def
+  f = 2;
+}
+
+// Overload resolution failure.
+void fn(in float f); // #fn-in
+
+void failOverloadResolution() {
+  float f = 1.0;
+  fn(f); // expected-error{{call to 'fn' is ambiguous}}
+  // expected-note@#fn-def{{candidate function}}
+  // expected-note@#fn-in{{candidate function}}
+}
+
+void implicitFn(float f);
+void inFn(in float f);
+void inoutFn(inout float f); // #inoutFn
+void outFn(out float f); // #outFn
+
+void callFns() {
+  // Call with literal arguments.
+  implicitFn(1); // Ok.
+  inFn(1); // Ok.
+  inoutFn(1); // expected-error{{no matching function for call to 'inoutFn'}}
+  // expected-note@#inoutFn{{candidate function not viable: no known conversion from 'int' to 'float &' for 1st argument}}
+  outFn(1); // expected-error{{no matching function for call to 'outFn}}
+  // expected-note@#outFn{{candidate function not viable: no known conversion from 'int' to 'float &' for 1st argument}}
+  
+  // Call with variables.
+  float f;
+  implicitFn(f); // Ok.
+  inFn(f); // Ok.
+  inoutFn(f); // Ok.
+  outFn(f); // Ok.
+}
+
+// No errors on these scenarios.
+
+// Alternating `inout` and `in out` spellings between declaration and
+// definitions is fine since they have the same semantic meaning.
+void fn7(inout float f);
+void fn7(in out float f) {}
+
+void fn8(in out float f);
+void fn8(inout float f) {}
+
+// These two declare two different functions (although calling them will be
+// ambiguous). This is equivalent to declaring a function that takes a
+// reference and a function that takes a value of the same type.
+void fn9(in float f);
+void fn9(out float f);
+
+// The `in` attribute is effectively optional. If no attribute is present it is
+// the same as `in`, so these declarations match the functions.
+void fn10(in float f);
+void fn10(float f) {}
+
+void fn11(float f);
+void fn11(in float f) {}
+
+template <typename T>
+void fn12(inout T f);
+
+void fn13() {
+  float f;
+  fn12<float>(f);
+}
diff --git a/clang/test/SemaHLSL/parameter_modifiers_ast.hlsl b/clang/test/SemaHLSL/parameter_modifiers_ast.hlsl
new file mode 100644
index 0000000000000..50b162bdfc26c
--- /dev/null
+++ b/clang/test/SemaHLSL/parameter_modifiers_ast.hlsl
@@ -0,0 +1,65 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.6-library %s -ast-dump | FileCheck %s
+
+// CHECK: FunctionDecl {{.*}} fn 'void (float)'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'float'
+// CHECK-NOT: HLSLParamModifierAttr
+void fn(float f);
+
+// CHECK: FunctionDecl {{.*}}6 fn2 'void (float)'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'float'
+// CHECK-NEXT: HLSLParamModifierAttr {{.*}} in
+// CHECK-NOT: HLSLParamModifierAttr
+void fn2(in float f);
+
+// CHECK: FunctionDecl {{.*}} fn3 'void (float &)'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'float &'
+// CHECK-NEXT: HLSLParamModifierAttr {{.*}} out
+// CHECK-NOT: HLSLParamModifierAttr
+void fn3(out float f);
+
+// CHECK: FunctionDecl {{.*}} fn4 'void (float &)'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'float &'
+// CHECK-NEXT: HLSLParamModifierAttr {{.*}} inout
+// CHECK-NOT: HLSLParamModifierAttr
+void fn4(inout float f);
+
+// CHECK: FunctionDecl {{.*}} fn5 'void (float &)'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'float &'
+// CHECK-NEXT: HLSLParamModifierAttr {{.*}} inout MergedSpelling
+// CHECK-NOT: HLSLParamModifierAttr
+void fn5(out in float f);
+
+// CHECK: FunctionDecl {{.*}} fn6 'void (float &)'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'float &'
+// CHECK-NEXT: HLSLParamModifierAttr {{.*}} inout MergedSpelling
+// CHECK-NOT: HLSLParamModifierAttr
+void fn6(in out float f);
+
+// CHECK-NEXT: FunctionTemplateDecl [[Template:0x[0-9a-fA-F]+]] {{.*}} fn7
+// CHECK-NEXT: TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+// CHECK-NEXT: FunctionDecl {{.*}} fn7 'void (T)'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'T'
+// CHECK-NEXT: HLSLParamModifierAttr {{.*}} inout
+// CHECK-NEXT: FunctionDecl [[Instantiation:0x[0-9a-fA-F]+]] {{.*}} used fn7 'void (float &)' implicit_instantiation
+// CHECK-NEXT: TemplateArgument type 'float'
+// CHECK-NEXT:  BuiltinType {{.*}} 'float'
+// CHECK-NEXT: ParmVarDecl {{.*}} f 'float &'
+// CHECK-NEXT: HLSLParamModifierAttr {{.*}} inout
+
+template <typename T>
+void fn7(inout T f);
+
+// CHECK: FunctionDecl {{.*}} fn8 'void ()'
+// CHECK-NEXT: CompoundStmt
+// CHECK-NEXT: DeclStmt
+// CHECK-NEXT: VarDecl {{.*}} used f 'float'
+// CHECK-NEXT: CallExpr {{.*}} 'void'
+// CHECK-NEXT: ImplicitCastExpr {{.*}} 'void (*)(float &)' <FunctionToPointerDecay>
+// CHECK-NEXT: DeclRefExpr {{.*}} 'void (float &)' lvalue
+// CHECK-SAME: Function [[Instantiation]] 'fn7' 'void (float &)'
+// CHECK-SAME: (FunctionTemplate [[Template]] 'fn7')
+// CHECK-NEXT: DeclRefExpr {{.*}} 'float' lvalue Var {{.*}} 'f' 'float'
+void fn8() {
+  float f;
+  fn7<float>(f);
+}