diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -104,6 +104,8 @@
   instantiated in one module and whose definition is instantiated in another
   module may end up with members associated with the wrong declaration of the
   class, which can result in miscompiles in some cases.
+- Fix crash on use of a variadic overloaded operator.
+  (`#42535 <https://github.com/llvm/llvm-project/issues/42535>_`)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -9850,7 +9850,7 @@
 }
 
 static bool haveSameParameterTypes(ASTContext &Context, const FunctionDecl *F1,
-                                   const FunctionDecl *F2, unsigned NumParams) {
+                                   const FunctionDecl *F2) {
   if (declaresSameEntity(F1, F2))
     return true;
 
@@ -9863,8 +9863,14 @@
     return F->getParamDecl(I++)->getType();
   };
 
+  unsigned F1NumParams = F1->getNumParams() + isa<CXXMethodDecl>(F1);
+  unsigned F2NumParams = F2->getNumParams() + isa<CXXMethodDecl>(F2);
+
+  if (F1NumParams != F2NumParams)
+    return false;
+
   unsigned I1 = 0, I2 = 0;
-  for (unsigned I = 0; I != NumParams; ++I) {
+  for (unsigned I = 0; I != F1NumParams; ++I) {
     QualType T1 = NextParam(F1, I1, I == 0);
     QualType T2 = NextParam(F2, I2, I == 0);
     assert(!T1.isNull() && !T2.isNull() && "Unexpected null param types");
@@ -10032,8 +10038,7 @@
     case ImplicitConversionSequence::Worse:
       if (Cand1.Function && Cand2.Function &&
           Cand1.isReversed() != Cand2.isReversed() &&
-          haveSameParameterTypes(S.Context, Cand1.Function, Cand2.Function,
-                                 NumArgs)) {
+          haveSameParameterTypes(S.Context, Cand1.Function, Cand2.Function)) {
         // Work around large-scale breakage caused by considering reversed
         // forms of operator== in C++20:
         //
@@ -13997,6 +14002,10 @@
         std::swap(Args[0], Args[1]);
 
       if (FnDecl) {
+
+        if (FnDecl->isInvalidDecl())
+          return ExprError();
+
         Expr *Base = nullptr;
         // We matched an overloaded operator. Build a call to that
         // operator.
@@ -14029,7 +14038,7 @@
           llvm::SmallVector<FunctionDecl*, 4> AmbiguousWith;
           for (OverloadCandidate &Cand : CandidateSet) {
             if (Cand.Viable && Cand.Function && Cand.isReversed() &&
-                haveSameParameterTypes(Context, Cand.Function, FnDecl, 2)) {
+                haveSameParameterTypes(Context, Cand.Function, FnDecl)) {
               for (unsigned ArgIdx = 0; ArgIdx < 2; ++ArgIdx) {
                 if (CompareImplicitConversionSequences(
                         *this, OpLoc, Cand.Conversions[ArgIdx],
diff --git a/clang/test/SemaCXX/overloaded-operator-decl.cpp b/clang/test/SemaCXX/overloaded-operator-decl.cpp
--- a/clang/test/SemaCXX/overloaded-operator-decl.cpp
+++ b/clang/test/SemaCXX/overloaded-operator-decl.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s 
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
 struct X { 
   X();
   X(int); 
@@ -58,3 +59,67 @@
     A()(i);
   }
 }
+
+namespace GH42535 {
+class E {
+  E& operator=(const E& rhs, ...); // expected-error{{overloaded 'operator=' cannot be variadic}}
+  E& operator+=(const E& rhs, ...); // expected-error{{overloaded 'operator+=' cannot be variadic}}
+
+};
+void operator+(E, ...) {} // expected-error{{overloaded 'operator+' cannot be variadic}}
+void operator-(E, ...) {} // expected-error{{overloaded 'operator-' cannot be variadic}}
+void operator*(E, ...) {} // expected-error{{overloaded 'operator*' cannot be variadic}}
+void operator/(E, ...) {} // expected-error{{overloaded 'operator/' must be a binary operator}}
+void operator/(E, E, ...) {} // expected-error{{overloaded 'operator/' cannot be variadic}}
+void operator%(E, ...) {} // expected-error{{overloaded 'operator%' must be a binary operator}}
+void operator%(E, E, ...) {} // expected-error{{overloaded 'operator%' cannot be variadic}}
+E& operator++(E&, ...); // expected-error{{overloaded 'operator++' cannot be variadic}}
+E& operator--(E&, ...); // expected-error{{overloaded 'operator--' cannot be variadic}}
+bool operator<(const E& lhs, ...); // expected-error{{overloaded 'operator<' must be a binary operator}}
+bool operator<(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator>(const E& lhs, ...); // expected-error{{overloaded 'operator>' must be a binary operator}}
+bool operator>(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator>=(const E& lhs, ...); // expected-error{{overloaded 'operator>=' must be a binary operator}}
+bool operator>=(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator<=(const E& lhs, ...); // expected-error{{overloaded 'operator<=' must be a binary operator}}
+bool operator<=(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator==(const E& lhs, ...); // expected-error{{overloaded 'operator==' must be a binary operator}}
+bool operator==(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator!=(const E& lhs, ...); // expected-error{{overloaded 'operator!=' must be a binary operator}}
+bool operator!=(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator&&(const E& lhs, ...); // expected-error{{overloaded 'operator&&' must be a binary operator}}
+bool operator&&(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator||(const E& lhs, ...); // expected-error{{overloaded 'operator||' must be a binary operator}}
+bool operator||(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator>>(const E& lhs, ...); // expected-error{{overloaded 'operator>>' must be a binary operator}}
+bool operator>>(const E& lhs, const E& rhs, ...); // expected-error{{cannot be variadic}}
+bool operator&(const E& lhs, ...); // expected-error{{cannot be variadic}}
+#if __cplusplus >= 202002L
+auto operator<=>(const E& lhs, ...);  // expected-error{{overloaded 'operator<=>' must be a binary operator}}
+#endif
+void d() {
+  E() + E();
+  E() - E();
+  E() * E();
+  E() / E();
+  E() % E();
+  ++E(); // expected-error{{cannot increment value of type 'E'}}
+  --E(); // expected-error{{cannot decrement value of type 'E'}}
+  E() < E();
+  E() > E();
+  E() <= E();
+  E() >= E();
+  E() == E();
+  E() != E();
+#if __cplusplus >= 202002L
+  E() <=> E();
+#endif
+  E e;
+  E e1 = e;
+  e += e1;
+  E() && E();
+  E() || E();
+  E() & E();
+  E() >> E();
+}
+}