diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index add1582344a0e..83318c76db820 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -451,6 +451,7 @@ Bug Fixes to Attribute Support
   ``[[gnu::error("some error")]]`` now correctly triggers an error. (#GH146520)
 - Fix a crash when the function name is empty in the `swift_name` attribute. (#GH157075)
 - Fixes crashes or missing diagnostics with the `device_kernel` attribute. (#GH161905)
+- Fix handling of parameter indexes when an attribute is applied to a C++23 explicit object member function.
 
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/AST/Attr.h b/clang/include/clang/AST/Attr.h
index ce273c167aa22..14d7caa0e16d7 100644
--- a/clang/include/clang/AST/Attr.h
+++ b/clang/include/clang/AST/Attr.h
@@ -16,6 +16,7 @@
 #include "clang/AST/ASTFwd.h"
 #include "clang/AST/AttrIterator.h"
 #include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/AttrKinds.h"
 #include "clang/Basic/AttributeCommonInfo.h"
@@ -327,8 +328,8 @@ class ParamIdx {
   ParamIdx(unsigned Idx, const Decl *D)
       : Idx(Idx), HasThis(false), IsValid(true) {
     assert(Idx >= 1 && "Idx must be one-origin");
-    if (const auto *FD = dyn_cast<FunctionDecl>(D))
-      HasThis = FD->isCXXInstanceMember();
+    if (const auto *MethodDecl = dyn_cast<CXXMethodDecl>(D))
+      HasThis = MethodDecl->isImplicitObjectMemberFunction();
   }
 
   /// A type into which \c ParamIdx can be serialized.
diff --git a/clang/include/clang/Sema/Attr.h b/clang/include/clang/Sema/Attr.h
index 3f0b10212789a..5836231818eec 100644
--- a/clang/include/clang/Sema/Attr.h
+++ b/clang/include/clang/Sema/Attr.h
@@ -123,6 +123,12 @@ inline bool isInstanceMethod(const Decl *D) {
   return false;
 }
 
+inline bool hasImplicitObjectParameter(const Decl *D) {
+  if (const auto *MethodDecl = dyn_cast<CXXMethodDecl>(D))
+    return MethodDecl->isImplicitObjectMemberFunction();
+  return false;
+}
+
 /// Diagnose mutually exclusive attributes when present on a given
 /// declaration. Returns true if diagnosed.
 template <typename AttrTy>
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 52904c72d1cfc..c67ed99b1f49e 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -2608,13 +2608,13 @@ class Sema final : public SemaBase {
   };
 
   /// Given a function and its FormatAttr or FormatMatchesAttr info, attempts to
-  /// populate the FomatStringInfo parameter with the attribute's correct
+  /// populate the FormatStringInfo parameter with the attribute's correct
   /// format_idx and firstDataArg. Returns true when the format fits the
   /// function and the FormatStringInfo has been populated.
   static bool getFormatStringInfo(const Decl *Function, unsigned FormatIdx,
                                   unsigned FirstArg, FormatStringInfo *FSI);
   static bool getFormatStringInfo(unsigned FormatIdx, unsigned FirstArg,
-                                  bool IsCXXMember, bool IsVariadic,
+                                  bool HasImplicitThisParam, bool IsVariadic,
                                   FormatStringInfo *FSI);
 
   // Used by C++ template instantiation.
@@ -5119,7 +5119,7 @@ class Sema final : public SemaBase {
     // In C++ the implicit 'this' function parameter also counts.
     // Parameters are counted from one.
     bool HP = hasFunctionProto(D);
-    bool HasImplicitThisParam = isInstanceMethod(D);
+    bool HasImplicitThisParam = hasImplicitObjectParameter(D);
     bool IV = HP && isFunctionOrMethodVariadic(D);
     unsigned NumParams =
         (HP ? getFunctionOrMethodNumParams(D) : 0) + HasImplicitThisParam;
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index f4517877b04c8..ad2c2e4a97bb9 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -3542,9 +3542,7 @@ bool Sema::ValueIsRunOfOnes(CallExpr *TheCall, unsigned ArgNum) {
 
 bool Sema::getFormatStringInfo(const Decl *D, unsigned FormatIdx,
                                unsigned FirstArg, FormatStringInfo *FSI) {
-  bool IsCXXMember = false;
-  if (const auto *MD = dyn_cast<CXXMethodDecl>(D))
-    IsCXXMember = MD->isInstance();
+  bool HasImplicitThisParam = hasImplicitObjectParameter(D);
   bool IsVariadic = false;
   if (const FunctionType *FnTy = D->getFunctionType())
     IsVariadic = cast<FunctionProtoType>(FnTy)->isVariadic();
@@ -3553,11 +3551,12 @@ bool Sema::getFormatStringInfo(const Decl *D, unsigned FormatIdx,
   else if (const auto *OMD = dyn_cast<ObjCMethodDecl>(D))
     IsVariadic = OMD->isVariadic();
 
-  return getFormatStringInfo(FormatIdx, FirstArg, IsCXXMember, IsVariadic, FSI);
+  return getFormatStringInfo(FormatIdx, FirstArg, HasImplicitThisParam,
+                             IsVariadic, FSI);
 }
 
 bool Sema::getFormatStringInfo(unsigned FormatIdx, unsigned FirstArg,
-                               bool IsCXXMember, bool IsVariadic,
+                               bool HasImplicitThisParam, bool IsVariadic,
                                FormatStringInfo *FSI) {
   if (FirstArg == 0)
     FSI->ArgPassingKind = FAPK_VAList;
@@ -3571,7 +3570,7 @@ bool Sema::getFormatStringInfo(unsigned FormatIdx, unsigned FirstArg,
   // The way the format attribute works in GCC, the implicit this argument
   // of member functions is counted. However, it doesn't appear in our own
   // lists, so decrement format_idx in that case.
-  if (IsCXXMember) {
+  if (HasImplicitThisParam) {
     if(FSI->FormatIdx == 0)
       return false;
     --FSI->FormatIdx;
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 964a2a791e18f..a9e7b44ac9d73 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -3785,7 +3785,7 @@ static bool handleFormatAttrCommon(Sema &S, Decl *D, const ParsedAttr &AL,
 
   // In C++ the implicit 'this' function parameter also counts, and they are
   // counted from one.
-  bool HasImplicitThisParam = isInstanceMethod(D);
+  bool HasImplicitThisParam = hasImplicitObjectParameter(D);
   Info->NumArgs = getFunctionOrMethodNumParams(D) + HasImplicitThisParam;
 
   Info->Identifier = AL.getArgAsIdent(0)->getIdentifierInfo();
@@ -3926,7 +3926,7 @@ static void handleCallbackAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     return;
   }
 
-  bool HasImplicitThisParam = isInstanceMethod(D);
+  bool HasImplicitThisParam = hasImplicitObjectParameter(D);
   int32_t NumArgs = getFunctionOrMethodNumParams(D);
 
   FunctionDecl *FD = D->getAsFunction();
@@ -4110,7 +4110,7 @@ static void handleLifetimeCaptureByAttr(Sema &S, Decl *D,
 }
 
 void Sema::LazyProcessLifetimeCaptureByParams(FunctionDecl *FD) {
-  bool HasImplicitThisParam = isInstanceMethod(FD);
+  bool HasImplicitThisParam = hasImplicitObjectParameter(FD);
   SmallVector<LifetimeCaptureByAttr *, 1> Attrs;
   for (ParmVarDecl *PVD : FD->parameters())
     if (auto *A = PVD->getAttr<LifetimeCaptureByAttr>())
diff --git a/clang/test/CodeGenCXX/attr-callback.cpp b/clang/test/CodeGenCXX/attr-callback.cpp
index c3456d6c430ff..efa705b9d06dc 100644
--- a/clang/test/CodeGenCXX/attr-callback.cpp
+++ b/clang/test/CodeGenCXX/attr-callback.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple i386-unknown-unknown %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -triple i386-unknown-unknown -std=c++23 %s -emit-llvm -o - | FileCheck %s
 
 struct Base {
 
@@ -47,9 +47,30 @@ struct Derived_2 : public Base {
 // CHECK-NOT: !callback
 void Derived_2::virtual_1(void (*callback)(void)) {}
 
+class ExplicitParameterObject {
+  __attribute__((callback(1, 0))) void implicit_this_idx(void (*callback)(ExplicitParameterObject*));
+  __attribute__((callback(1, this))) void implicit_this_identifier(void (*callback)(ExplicitParameterObject*));
+  __attribute__((callback(2, 1))) void explicit_this_idx(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*));
+  __attribute__((callback(2, self))) void explicit_this_identifier(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*));
+};
+
+// CHECK-DAG: define{{.*}} void @_ZN23ExplicitParameterObject17implicit_this_idxEPFvPS_E({{[^!]*!callback}} ![[cid3:[0-9]+]]
+void ExplicitParameterObject::implicit_this_idx(void (*callback)(ExplicitParameterObject*)) {}
+
+// CHECK-DAG: define{{.*}} void @_ZN23ExplicitParameterObject24implicit_this_identifierEPFvPS_E({{[^!]*!callback}} ![[cid3]]
+void ExplicitParameterObject::implicit_this_identifier(void (*callback)(ExplicitParameterObject*)) {}
+
+// CHECK-DAG: define{{.*}} void @_ZNH23ExplicitParameterObject17explicit_this_idxEPS_PFvS0_E({{[^!]*!callback}} ![[cid3]]
+void ExplicitParameterObject::explicit_this_idx(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*)) {}
+
+// CHECK-DAG: define{{.*}} void @_ZNH23ExplicitParameterObject24explicit_this_identifierEPS_PFvS0_E({{[^!]*!callback}} ![[cid3]]
+void ExplicitParameterObject::explicit_this_identifier(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*)) {}
+
 // CHECK-DAG: ![[cid0]] = !{![[cid0b:[0-9]+]]}
 // CHECK-DAG: ![[cid0b]] = !{i64 1, i1 false}
 // CHECK-DAG: ![[cid1]] = !{![[cid1b:[0-9]+]]}
 // CHECK-DAG: ![[cid1b]] = !{i64 2, i1 false}
 // CHECK-DAG: ![[cid2]] = !{![[cid2b:[0-9]+]]}
 // CHECK-DAG: ![[cid2b]] = !{i64 1, i64 0, i64 -1, i64 0, i1 false}
+// CHECK-DAG: ![[cid3]] = !{![[cid3b:[0-9]+]]}
+// CHECK-DAG: ![[cid3b]] = !{i64 1, i64 0, i1 false}
diff --git a/clang/test/SemaCXX/attr-callback-broken.cpp b/clang/test/SemaCXX/attr-callback-broken.cpp
index a5469b22ba350..53b331a49251b 100644
--- a/clang/test/SemaCXX/attr-callback-broken.cpp
+++ b/clang/test/SemaCXX/attr-callback-broken.cpp
@@ -1,7 +1,12 @@
-// RUN: %clang_cc1 %s -verify -fsyntax-only
+// RUN: %clang_cc1 %s -std=c++23 -verify -fsyntax-only
 
 class C_in_class {
 #define HAS_THIS
 #include "../Sema/attr-callback-broken.c"
 #undef HAS_THIS
 };
+
+class ExplicitParameterObject {
+  __attribute__((callback(2, 0))) void explicit_this_idx(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*));           // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
+  __attribute__((callback(2, this))) void explicit_this_identifier(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*)); // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
+};
diff --git a/clang/test/SemaCXX/attr-callback.cpp b/clang/test/SemaCXX/attr-callback.cpp
index ee02f7d3d24f7..ff5a241e92f74 100644
--- a/clang/test/SemaCXX/attr-callback.cpp
+++ b/clang/test/SemaCXX/attr-callback.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -verify -fsyntax-only
+// RUN: %clang_cc1 %s -std=c++23 -verify -fsyntax-only
 
 // expected-no-diagnostics
 
@@ -6,6 +6,11 @@ class C_in_class {
 #include "../Sema/attr-callback.c"
 };
 
+class ExplicitParameterObject {
+  __attribute__((callback(2, 1))) void explicit_this_idx(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*));
+  __attribute__((callback(2, self))) void explicit_this_identifier(this ExplicitParameterObject* self, void (*callback)(ExplicitParameterObject*));
+};
+
 struct Base {
 
   void no_args_1(void (*callback)(void));
diff --git a/clang/test/SemaCXX/attr-format.cpp b/clang/test/SemaCXX/attr-format.cpp
index adc05fc46776c..c0aeb5d07dfe9 100644
--- a/clang/test/SemaCXX/attr-format.cpp
+++ b/clang/test/SemaCXX/attr-format.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -Wformat-nonliteral -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++23 -Wformat-nonliteral -verify %s
 #include <stdarg.h>
 
 int printf(const char *fmt, ...) __attribute__((format(printf, 1, 2)));
@@ -11,6 +11,10 @@ struct S {
   // the format argument is argument 2 here.
   void g(const char*, ...) __attribute__((format(printf, 2, 3)));
   const char* g2(const char*) __attribute__((format_arg(2)));
+  // From C++23 'this' can also be specified explicitly.
+  void g3(this S&, const char *, ...) __attribute__((format(printf, 2, 3)));
+  void g4(this const char* s, ...) __attribute__((format(printf, 1, 2)));
+  consteval operator const char*() const { return "%f"; } // #g4_fmt_string
 
   void h(const char*, ...) __attribute__((format(printf, 1, 4))); // \
       expected-error{{implicit this argument as the format string}}
@@ -18,10 +22,17 @@ struct S {
       expected-error{{out of bounds}}
   const char* h3(const char*) __attribute__((format_arg(1))); // \
       expected-error{{invalid for the implicit this argument}}
+  void h4(this S&, const char *, ...) __attribute__((format(printf, 1, 3))); // \
+      expected-error {{format argument not a string type}}
 
   void operator() (const char*, ...) __attribute__((format(printf, 2, 3)));
 };
 
+void s() {
+  S().g4(4); // expected-warning {{format specifies type 'double' but the argument has type 'int'}}
+             // expected-note@#g4_fmt_string {{format string is defined here}}
+}
+
 // PR5521
 struct A { void a(const char*,...) __attribute((format(printf,2,3))); };
 void b(A x) {
diff --git a/clang/test/SemaCXX/attr-lifetime-capture-by.cpp b/clang/test/SemaCXX/attr-lifetime-capture-by.cpp
index 70a5fe5a45376..8606592c6b771 100644
--- a/clang/test/SemaCXX/attr-lifetime-capture-by.cpp
+++ b/clang/test/SemaCXX/attr-lifetime-capture-by.cpp
@@ -44,4 +44,7 @@ struct T {
   {
     s.captureInt(x);
   }
+
+  void explicit_this1(this T& self, const int &x [[clang::lifetime_capture_by(self)]]);
+  void explicit_this2(this T& self, const int &x [[clang::lifetime_capture_by(this)]]); // expected-error {{argument references unavailable implicit 'this'}}
 };
diff --git a/clang/test/SemaCXX/attr-nonnull.cpp b/clang/test/SemaCXX/attr-nonnull.cpp
index 6f9119b519d09..0fba6b50cb354 100644
--- a/clang/test/SemaCXX/attr-nonnull.cpp
+++ b/clang/test/SemaCXX/attr-nonnull.cpp
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
-// RUN: %clang_cc1 -fsyntax-only -verify %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s -fexperimental-new-constant-interpreter
 struct S {
   S(const char *) __attribute__((nonnull(2)));
 
@@ -11,6 +11,13 @@ struct S {
 
   void h(const char*) __attribute__((nonnull(1))); // \
       expected-error{{invalid for the implicit this argument}}
+
+  void i(this S* self, const char*) __attribute__((nonnull(1)));
+
+  void j(this S* self, const char*) __attribute__((nonnull(2)));
+
+  void k(this S* self, const char*) __attribute__((nonnull(3))); // \
+      expected-error{{'nonnull' attribute parameter 1 is out of bounds}}
 };
 
 void test() {