Added early exit for defaulted FunctionDecls. This prevents matching of defaulted comparison operators. fixes #53355

[Febbe]

Updated in https://reviews.llvm.org/D118847

clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp

 void UseTrailingReturnTypeCheck::check(const MatchFinder::MatchResult &Result) {
   assert(PP && "Expected registerPPCallbacks() to have been called before so "
                "preprocessor is available");
 
   const auto *F = Result.Nodes.getNodeAs<FunctionDecl>("Func");
   const auto *Fr = Result.Nodes.getNodeAs<FriendDecl>("Friend");
   assert(F && "Matcher is expected to find only FunctionDecls");
 
-  if (F->getLocation().isInvalid())
+  // operator<=> generates also the AST of e.g. operator==, which is implicit
+  if (F->getLocation().isInvalid() || F->isImplicit())
     return;
 
-  // Skip functions which return just 'auto'.
-  const auto *AT = F->getDeclaredReturnType()->getAs<AutoType>();
-  if (AT != nullptr && !AT->isConstrained() &&
-      AT->getKeyword() == AutoTypeKeyword::Auto &&
-      !hasAnyNestedLocalQualifiers(F->getDeclaredReturnType()))
+  // Skip functions which return just 'auto' or are also defaulted operators.
+  if (const auto *AT = F->getDeclaredReturnType()->getAs<AutoType>();
+      AT != nullptr &&
+      ((!AT->isConstrained() && AT->getKeyword() == AutoTypeKeyword::Auto &&
+        !hasAnyNestedLocalQualifiers(F->getDeclaredReturnType())) ||
+       F->isDefaulted()))
     return;
 
   // TODO: implement those
   if (F->getDeclaredReturnType()->isFunctionPointerType() ||
       F->getDeclaredReturnType()->isMemberFunctionPointerType() ||
       F->getDeclaredReturnType()->isMemberPointerType()) {
     diag(F->getLocation(), Message);
     return;

clang-tools-extra/test/clang-tidy/checkers/modernize-use-trailing-return-type-cxx20.cpp

 int req1(T t) requires std::floating_point<T>;
 // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
 // CHECK-FIXES: {{^}}auto req1(T t) -> int requires std::floating_point<T>;{{$}}
 
 template <typename T>
 T req2(T t) requires requires { t + t; };
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
   // CHECK-FIXES: {{^}}auto req2(T t) -> T requires requires { t + t; };{{$}}
+
+//
+// Operator c++20 defaulted comparison operators
+//
+// Requires <compare>
+
+namespace std {
+  struct strong_ordering {
+    using value_type = signed char;
+    static strong_ordering const less;
+    static strong_ordering const equal;
+    static strong_ordering const equivalent;
+    static strong_ordering const greater;
+
+    constexpr strong_ordering(value_type v) : val(v) {}
+    template <typename T>
+    requires(T{0}) //
+        friend constexpr auto
+        operator==(strong_ordering v, T u) noexcept -> bool {
+      return v.val == u;
+    }
+
+    friend constexpr auto operator==(strong_ordering v, strong_ordering w) noexcept -> bool = default;
+
+    value_type val{};
+  };
+  inline constexpr strong_ordering strong_ordering::less{-1};
+  inline constexpr strong_ordering strong_ordering::equal{0};
+  inline constexpr strong_ordering strong_ordering::equivalent{0};
+  inline constexpr strong_ordering strong_ordering::greater{1};
+
+} // namespace std
+
+struct TestDefaultOperatorA {
+  int a{};
+  int b{};
+
+  friend auto operator<=>(const TestDefaultOperatorA &, const TestDefaultOperatorA &) noexcept = default;
+};
+
+struct TestDefaultOperatorB {
+  int a{};
+  int b{};
+  friend auto operator==(const TestDefaultOperatorB &, const TestDefaultOperatorB &) noexcept -> bool = default;
+  friend bool operator<(const TestDefaultOperatorB &, const TestDefaultOperatorB &) noexcept = default;
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  friend auto operator<(const TestDefaultOperatorB &, const TestDefaultOperatorB &) noexcept -> bool = default;{{$}}
+};