[clang] Skip defaulted functions in zero-as-null-pointer-constant.

The zero-as-null-pointer-constant check should not fire if it is inside
a defaulted function, e.g. defaulted spaceship operators.
Add C++20 tests with spaceship operators.

Fixes #50221

Differential Revision: https://reviews.llvm.org/D138727

clang/lib/Sema/Sema.cpp

@@ -597,6 +597,12 @@ void Sema::diagnoseZeroToNullptrConversion(CastKind Kind, const Expr *E) {
           CodeSynthesisContext::RewritingOperatorAsSpaceship)
     return;
 
+  // Ignore null pointers in defaulted comparison operators.
+  FunctionDecl *FD = getCurFunctionDecl();
+  if (FD && FD->isDefaulted()) {
+    return;
+  }
+
   // If it is a macro from system header, and if the macro name is not "NULL",
   // do not warn.
   SourceLocation MaybeMacroLoc = E->getBeginLoc();

clang/test/SemaCXX/warn-zero-nullptr-cxx20.cpp

@@ -0,0 +1,51 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wzero-as-null-pointer-constant -std=c++20
+
+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 {
+  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};
+constexpr strong_ordering strong_ordering::greater = {1};
+constexpr strong_ordering strong_ordering::less = {-1};
+} // namespace std
+
+struct A {
+  int a;
+  constexpr auto operator<=>(const A &other) const = default;
+};
+
+void test_cxx_rewritten_binary_ops() {
+  A a1, a2;
+  bool result;
+  result = (a1 < a2);
+  result = (a1 >= a2);
+  int *ptr = 0; // expected-warning{{zero as null pointer constant}}
+  result = (a1 > (ptr == 0 ? a1 : a2)); // expected-warning{{zero as null pointer constant}}
+  result = (a1 > ((a1 > (ptr == 0 ? a1 : a2)) ? a1 : a2)); // expected-warning{{zero as null pointer constant}}
+}