[clang-tidy] Ignore cxxRewrittenBinaryOperator in defaulted function decls in modernize-use-nullptr

The check has produced false positives when checking the default implementation of the spaceship operator.
The default implementation should be skipped by the check.

Modified the existing test so that the check runs into the bug without this fix and add another test case.

Fixes llvm#53961.

Patch by Jens Massberg.

Reviewed By: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D138701

Author: jensmassberg

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

@@ -62,7 +62,9 @@ StatementMatcher makeCastSequenceMatcher() {
                         ImplicitCastToNull,
                         hasAncestor(cxxRewrittenBinaryOperator().bind(
                             "checkBinopOperands")))
-                        .bind(CastSequence))))));
+                        .bind(CastSequence))),
+                // Skip defaulted comparison operators.
+                unless(hasAncestor(functionDecl(isDefaulted()))))));
 }
 
 bool isReplaceableRange(SourceLocation StartLoc, SourceLocation EndLoc,

clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr-cxx20.cpp

@@ -1,9 +1,33 @@
 // RUN: %check_clang_tidy -std=c++20 %s modernize-use-nullptr %t
 
 namespace std {
+class strong_ordering;
+
+// Mock how STD defined unspecified parameters for the operators below.
+struct _CmpUnspecifiedParam {
+  consteval
+  _CmpUnspecifiedParam(int _CmpUnspecifiedParam::*) noexcept {}
+};
+
 struct strong_ordering {
-  int n;
-  constexpr operator int() const { return n; }
+  signed char value;
+
+  friend constexpr bool operator==(strong_ordering v,
+                                   _CmpUnspecifiedParam) noexcept {
+    return v.value == 0;
+  }
+  friend constexpr bool operator<(strong_ordering v,
+                                  _CmpUnspecifiedParam) noexcept {
+    return v.value < 0;
+  }
+  friend constexpr bool operator>(strong_ordering v,
+                                  _CmpUnspecifiedParam) noexcept {
+    return v.value > 0;
+  }
+  friend constexpr bool operator>=(strong_ordering v,
+                                   _CmpUnspecifiedParam) noexcept {
+    return v.value >= 0;
+  }
   static const strong_ordering equal, greater, less;
 };
 constexpr strong_ordering strong_ordering::equal = {0};
@@ -12,8 +36,10 @@ constexpr strong_ordering strong_ordering::less = {-1};
 } // namespace std
 
 class A {
+  int a;
 public:
   auto operator<=>(const A &other) const = default;
+  // CHECK-FIXES: auto operator<=>(const A &other) const = default;
 };
 
 void test_cxx_rewritten_binary_ops() {
@@ -32,3 +58,14 @@ void test_cxx_rewritten_binary_ops() {
   result = (a1 > ((a1 > (ptr == 0 ? a1 : a2)) ? a1 : a2));
   // CHECK-FIXES: result = (a1 > ((a1 > (ptr == nullptr ? a1 : a2)) ? a1 : a2));
 }
+
+template<class T1, class T2>
+struct P {
+  T1 x1;
+  T2 x2;
+  friend auto operator<=>(const P&, const P&) = default;
+  // CHECK-FIXES: friend auto operator<=>(const P&, const P&) = default;
+};
+
+bool foo(P<int,int> x, P<int, int> y) { return x < y; }
+// CHECK-FIXES: bool foo(P<int,int> x, P<int, int> y) { return x < y; }