diff --git a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
index 71e852545203e..abd6d7b4cd60f 100644
--- a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -22,6 +22,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTDiagnostic.h"
 #include "clang/AST/Attr.h"
+#include "clang/AST/Expr.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
@@ -528,6 +529,8 @@ void ClangTidyDiagnosticConsumer::forwardDiagnostic(const Diagnostic &Info) {
     case clang::DiagnosticsEngine::ak_addrspace:
       Builder << static_cast<LangAS>(Info.getRawArg(Index));
       break;
+    case clang::DiagnosticsEngine::ak_expr:
+      Builder << reinterpret_cast<const Expr *>(Info.getRawArg(Index));
     }
   }
 }
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f8f4dfbafb4f8..e671183522565 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -280,6 +280,9 @@ Improvements to Clang's diagnostics
 - Clang now better preserves the sugared types of pointers to member.
 - Clang now better preserves the presence of the template keyword with dependent
   prefixes.
+- Clang now respects the current language mode when printing expressions in
+  diagnostics. This fixes a bunch of `bool` being printed as `_Bool`, and also
+  a bunch of HLSL types being printed as their C++ equivalents.
 - When printing types for diagnostics, clang now doesn't suppress the scopes of
   template arguments contained within nested names.
 - The ``-Wshift-bool`` warning has been added to warn about shifting a boolean. (#GH28334)
diff --git a/clang/include/clang/AST/Expr.h b/clang/include/clang/AST/Expr.h
index dedbff5944af8..20f70863a05b3 100644
--- a/clang/include/clang/AST/Expr.h
+++ b/clang/include/clang/AST/Expr.h
@@ -7379,6 +7379,14 @@ class RecoveryExpr final : public Expr,
   friend class ASTStmtWriter;
 };
 
+/// Insertion operator for diagnostics.  This allows sending
+/// Expr into a diagnostic with <<.
+inline const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,
+                                             const Expr *E) {
+  DB.AddTaggedVal(reinterpret_cast<uint64_t>(E), DiagnosticsEngine::ak_expr);
+  return DB;
+}
+
 } // end namespace clang
 
 #endif // LLVM_CLANG_AST_EXPR_H
diff --git a/clang/include/clang/AST/TemplateBase.h b/clang/include/clang/AST/TemplateBase.h
index a800a16fc3e7a..bea624eb04942 100644
--- a/clang/include/clang/AST/TemplateBase.h
+++ b/clang/include/clang/AST/TemplateBase.h
@@ -262,7 +262,7 @@ class TemplateArgument {
   /// This form of template argument only occurs in template argument
   /// lists used for dependent types and for expression; it will not
   /// occur in a non-dependent, canonical template argument list.
-  TemplateArgument(Expr *E, bool IsDefaulted = false) {
+  explicit TemplateArgument(Expr *E, bool IsDefaulted = false) {
     TypeOrValue.Kind = Expression;
     TypeOrValue.IsDefaulted = IsDefaulted;
     TypeOrValue.V = reinterpret_cast<uintptr_t>(E);
diff --git a/clang/include/clang/Basic/Diagnostic.h b/clang/include/clang/Basic/Diagnostic.h
index 848acce3c4f13..19524856a9bb3 100644
--- a/clang/include/clang/Basic/Diagnostic.h
+++ b/clang/include/clang/Basic/Diagnostic.h
@@ -284,7 +284,10 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
     ak_qualtype_pair,
 
     /// Attr *
-    ak_attr
+    ak_attr,
+
+    /// Expr *
+    ak_expr,
   };
 
   /// Represents on argument value, which is a union discriminated
diff --git a/clang/lib/AST/ASTDiagnostic.cpp b/clang/lib/AST/ASTDiagnostic.cpp
index b4e7360e126fb..ccfef9c7ae361 100644
--- a/clang/lib/AST/ASTDiagnostic.cpp
+++ b/clang/lib/AST/ASTDiagnostic.cpp
@@ -508,6 +508,14 @@ void clang::FormatASTNodeDiagnosticArgument(
       NeedQuotes = false;
       break;
     }
+    case DiagnosticsEngine::ak_expr: {
+      const Expr *E = reinterpret_cast<Expr *>(Val);
+      assert(E && "Received null Expr!");
+      E->printPretty(OS, /*Helper=*/nullptr, Context.getPrintingPolicy());
+      // FIXME: Include quotes when printing expressions.
+      NeedQuotes = false;
+      break;
+    }
   }
 
   if (NeedQuotes) {
diff --git a/clang/lib/AST/TemplateBase.cpp b/clang/lib/AST/TemplateBase.cpp
index 0be0a83b7010d..42da4e6ca9964 100644
--- a/clang/lib/AST/TemplateBase.cpp
+++ b/clang/lib/AST/TemplateBase.cpp
@@ -478,7 +478,7 @@ TemplateArgument TemplateArgument::getPackExpansionPattern() const {
     return getAsType()->castAs<PackExpansionType>()->getPattern();
 
   case Expression:
-    return cast<PackExpansionExpr>(getAsExpr())->getPattern();
+    return TemplateArgument(cast<PackExpansionExpr>(getAsExpr())->getPattern());
 
   case TemplateExpansion:
     return TemplateArgument(getAsTemplateOrTemplatePattern());
@@ -654,18 +654,8 @@ static const T &DiagTemplateArg(const T &DB, const TemplateArgument &Arg) {
   case TemplateArgument::TemplateExpansion:
     return DB << Arg.getAsTemplateOrTemplatePattern() << "...";
 
-  case TemplateArgument::Expression: {
-    // This shouldn't actually ever happen, so it's okay that we're
-    // regurgitating an expression here.
-    // FIXME: We're guessing at LangOptions!
-    SmallString<32> Str;
-    llvm::raw_svector_ostream OS(Str);
-    LangOptions LangOpts;
-    LangOpts.CPlusPlus = true;
-    PrintingPolicy Policy(LangOpts);
-    Arg.getAsExpr()->printPretty(OS, nullptr, Policy);
-    return DB << OS.str();
-  }
+  case TemplateArgument::Expression:
+    return DB << Arg.getAsExpr();
 
   case TemplateArgument::Pack: {
     // FIXME: We're guessing at LangOptions!
diff --git a/clang/lib/Basic/Diagnostic.cpp b/clang/lib/Basic/Diagnostic.cpp
index 9e2f134135647..4b4a85aaccf8b 100644
--- a/clang/lib/Basic/Diagnostic.cpp
+++ b/clang/lib/Basic/Diagnostic.cpp
@@ -1247,6 +1247,7 @@ FormatDiagnostic(const char *DiagStr, const char *DiagEnd,
     case DiagnosticsEngine::ak_nestednamespec:
     case DiagnosticsEngine::ak_declcontext:
     case DiagnosticsEngine::ak_attr:
+    case DiagnosticsEngine::ak_expr:
       getDiags()->ConvertArgToString(Kind, getRawArg(ArgNo),
                                      StringRef(Modifier, ModifierLen),
                                      StringRef(Argument, ArgumentLen),
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 200168087136b..6236bce743438 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -493,8 +493,8 @@ DeduceNullPtrTemplateArgument(Sema &S, TemplateParameterList *TemplateParams,
                                                         : CK_NullToPointer)
                     .get();
   return DeduceNonTypeTemplateArgument(
-      S, TemplateParams, NTTP, DeducedTemplateArgument(Value), Value->getType(),
-      Info, PartialOrdering, Deduced, HasDeducedAnyParam);
+      S, TemplateParams, NTTP, TemplateArgument(Value), Value->getType(), Info,
+      PartialOrdering, Deduced, HasDeducedAnyParam);
 }
 
 /// Deduce the value of the given non-type template parameter
@@ -508,8 +508,8 @@ DeduceNonTypeTemplateArgument(Sema &S, TemplateParameterList *TemplateParams,
                               SmallVectorImpl<DeducedTemplateArgument> &Deduced,
                               bool *HasDeducedAnyParam) {
   return DeduceNonTypeTemplateArgument(
-      S, TemplateParams, NTTP, DeducedTemplateArgument(Value), Value->getType(),
-      Info, PartialOrdering, Deduced, HasDeducedAnyParam);
+      S, TemplateParams, NTTP, TemplateArgument(Value), Value->getType(), Info,
+      PartialOrdering, Deduced, HasDeducedAnyParam);
 }
 
 /// Deduce the value of the given non-type template parameter
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 3d4a245eb8bd5..3040a30454b0c 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -1286,7 +1286,7 @@ TemplateArgumentLoc Sema::getTemplateArgumentPackExpansionPattern(
     Expr *Pattern = Expansion->getPattern();
     Ellipsis = Expansion->getEllipsisLoc();
     NumExpansions = Expansion->getNumExpansions();
-    return TemplateArgumentLoc(Pattern, Pattern);
+    return TemplateArgumentLoc(TemplateArgument(Pattern), Pattern);
   }
 
   case TemplateArgument::TemplateExpansion:
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 57fcc4b3b3682..2889a16dd0271 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -3981,7 +3981,7 @@ class TreeTransform {
       if (Result.isInvalid())
         return TemplateArgumentLoc();
 
-      return TemplateArgumentLoc(Result.get(), Result.get());
+      return TemplateArgumentLoc(TemplateArgument(Result.get()), Result.get());
     }
 
     case TemplateArgument::Template:
@@ -16125,8 +16125,8 @@ TreeTransform<Derived>::TransformSizeOfPackExpr(SizeOfPackExpr *E) {
             E->getPackLoc());
         if (DRE.isInvalid())
           return ExprError();
-        ArgStorage = new (getSema().Context)
-            PackExpansionExpr(DRE.get(), E->getPackLoc(), std::nullopt);
+        ArgStorage = TemplateArgument(new (getSema().Context) PackExpansionExpr(
+            DRE.get(), E->getPackLoc(), std::nullopt));
       }
       PackArgs = ArgStorage;
     }
diff --git a/clang/test/SemaHLSL/BuiltIns/RWBuffers.hlsl b/clang/test/SemaHLSL/BuiltIns/RWBuffers.hlsl
index 941e0a975d5f4..34930d8963688 100644
--- a/clang/test/SemaHLSL/BuiltIns/RWBuffers.hlsl
+++ b/clang/test/SemaHLSL/BuiltIns/RWBuffers.hlsl
@@ -60,13 +60,13 @@ RWBuffer<TemplatedVector<int> > r9;
 // arrays not allowed
 // expected-error@+3 {{constraints not satisfied for class template 'RWBuffer'}}
 // expected-note@*:* {{because 'half[4]' does not satisfy '__is_typed_resource_element_compatible'}}
-// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(__fp16[4])' evaluated to false}}
+// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(half[4])' evaluated to false}}
 RWBuffer<half[4]> r10;
 
 typedef vector<int, 8> int8;
 // expected-error@+3 {{constraints not satisfied for class template 'RWBuffer'}}
 // expected-note@*:* {{because 'vector<int, 8>' (vector of 8 'int' values) does not satisfy '__is_typed_resource_element_compatible'}}
-// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(int __attribute__((ext_vector_type(8))))' evaluated to false}}
+// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(vector<int, 8>)' evaluated to false}}
 RWBuffer<int8> r11;
 
 typedef int MyInt;
@@ -74,12 +74,12 @@ RWBuffer<MyInt> r12;
 
 // expected-error@+3 {{constraints not satisfied for class template 'RWBuffer'}}
 // expected-note@*:* {{because 'bool' does not satisfy '__is_typed_resource_element_compatible'}}
-// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(_Bool)' evaluated to false}}
+// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(bool)' evaluated to false}}
 RWBuffer<bool> r13;
 
 // expected-error@+3 {{constraints not satisfied for class template 'RWBuffer'}}
 // expected-note@*:* {{because 'vector<bool, 2>' (vector of 2 'bool' values) does not satisfy '__is_typed_resource_element_compatible'}}
-// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(_Bool __attribute__((ext_vector_type(2))))' evaluated to false}}
+// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(vector<bool, 2>)' evaluated to false}}
 RWBuffer<vector<bool, 2>> r14;
 
 enum numbers { one, two, three };
@@ -91,7 +91,7 @@ RWBuffer<numbers> r15;
 
 // expected-error@+3 {{constraints not satisfied for class template 'RWBuffer'}}
 // expected-note@*:* {{because 'vector<double, 3>' (vector of 3 'double' values) does not satisfy '__is_typed_resource_element_compatible'}}
-// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(double __attribute__((ext_vector_type(3))))' evaluated to false}}
+// expected-note@*:* {{because '__builtin_hlsl_is_typed_resource_element_compatible(vector<double, 3>)' evaluated to false}}
 RWBuffer<double3> r16;
 
 
diff --git a/clang/test/SemaTemplate/instantiate-expanded-type-constraint.cpp b/clang/test/SemaTemplate/instantiate-expanded-type-constraint.cpp
index 73fef87b97822..3edf243982958 100644
--- a/clang/test/SemaTemplate/instantiate-expanded-type-constraint.cpp
+++ b/clang/test/SemaTemplate/instantiate-expanded-type-constraint.cpp
@@ -8,7 +8,7 @@ constexpr bool is_same_v<T, T> = true;
 
 template<typename T, typename U>
 concept same_as = is_same_v<T, U>;
-// expected-note@-1{{because 'is_same_v<int, _Bool>' evaluated to false}}
+// expected-note@-1{{because 'is_same_v<int, bool>' evaluated to false}}
 
 template<typename T, typename... Us>
 concept either = (is_same_v<T, Us> || ...);
@@ -16,7 +16,7 @@ concept either = (is_same_v<T, Us> || ...);
 template<typename... Ts>
 struct T {
     template<same_as<Ts>... Us>
-    // expected-note@-1{{because 'same_as<int, _Bool>' evaluated to false}}
+    // expected-note@-1{{because 'same_as<int, bool>' evaluated to false}}
     static void foo(Us... u, int x) { };
     // expected-note@-1{{candidate template ignored: deduced too few arguments}}
     // expected-note@-2{{candidate template ignored: constraints not satisfied}}
diff --git a/clang/test/SemaTemplate/instantiate-requires-expr.cpp b/clang/test/SemaTemplate/instantiate-requires-expr.cpp
index ab5fac1f9e63e..47689b93db50f 100644
--- a/clang/test/SemaTemplate/instantiate-requires-expr.cpp
+++ b/clang/test/SemaTemplate/instantiate-requires-expr.cpp
@@ -72,8 +72,8 @@ namespace type_requirement {
 
   template<typename T> requires
   false_v<requires { typename T::template temp<T>; }>
-  // expected-note@-1 {{because 'false_v<requires { typename contains_template<int>::template temp<type_requirement::contains_template<int> >; }>' evaluated to false}}
-  // expected-note@-2 {{because 'false_v<requires { typename contains_template<short>::template temp<type_requirement::contains_template<short> >; }>' evaluated to false}}
+  // expected-note@-1 {{because 'false_v<requires { typename contains_template<int>::template temp<type_requirement::contains_template<int>>; }>' evaluated to false}}
+  // expected-note@-2 {{because 'false_v<requires { typename contains_template<short>::template temp<type_requirement::contains_template<short>>; }>' evaluated to false}}
   struct r2 {};
 
   using r2i1 = r2<contains_template<int>>; // expected-error{{constraints not satisfied for class template 'r2' [with T = type_requirement::contains_template<int>]}}
diff --git a/clang/test/SemaTemplate/trailing-return-short-circuit.cpp b/clang/test/SemaTemplate/trailing-return-short-circuit.cpp
index 0d1c9b52b0e85..4ef7888d23dc5 100644
--- a/clang/test/SemaTemplate/trailing-return-short-circuit.cpp
+++ b/clang/test/SemaTemplate/trailing-return-short-circuit.cpp
@@ -39,13 +39,13 @@ void usage() {
   Foo(true);
   // expected-error@-1{{no matching function for call to 'Foo'}}
   // expected-note@#FOO{{candidate template ignored: constraints not satisfied [with T = bool]}}
-  // expected-note@#FOO_REQ{{because 'sizeof(_Bool) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#FOO_REQ{{because 'sizeof(bool) > 2' (1 > 2) evaluated to false}}
   // expected-note@#FOO_REQ{{because substituted constraint expression is ill-formed: type 'bool' cannot be used prior to '::' because it has no members}}
 
   TrailingReturn(true);
   // expected-error@-1{{no matching function for call to 'TrailingReturn'}}
   // expected-note@#TRAILING{{candidate template ignored: constraints not satisfied [with T = bool]}}
-  // expected-note@#TRAILING_REQ{{because 'sizeof(_Bool) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#TRAILING_REQ{{because 'sizeof(bool) > 2' (1 > 2) evaluated to false}}
   // expected-note@#TRAILING_REQ_VAL{{because substituted constraint expression is ill-formed: type 'bool' cannot be used prior to '::' because it has no members}}
 
   // Fails the 1st check, fails 2nd because ::value is false.
diff --git a/libcxx/test/libcxx/algorithms/cpp17_iterator_concepts.verify.cpp b/libcxx/test/libcxx/algorithms/cpp17_iterator_concepts.verify.cpp
index 99f67c8a795a9..c4462b26f5c92 100644
--- a/libcxx/test/libcxx/algorithms/cpp17_iterator_concepts.verify.cpp
+++ b/libcxx/test/libcxx/algorithms/cpp17_iterator_concepts.verify.cpp
@@ -173,7 +173,7 @@ void check_bidirectional_iterator_requirements() {
   _LIBCPP_REQUIRE_CPP17_BIDIRECTIONAL_ITERATOR(missing_postdecrement, ""); // expected-error {{static assertion failed}}
   // expected-note@*:* {{cannot decrement value of type 'missing_postdecrement'}}
   _LIBCPP_REQUIRE_CPP17_BIDIRECTIONAL_ITERATOR(not_returning_iter_reference, ""); // expected-error {{static assertion failed}}
-  // expected-note@*:* {{because type constraint 'same_as<int, __iter_reference<not_returning_iter_reference> >' was not satisfied}}
+  // expected-note-re@*:* {{because type constraint 'same_as<int, __iter_reference<not_returning_iter_reference>{{ ?}}>' was not satisfied}}
   // clang-format on
 }