PR44325 (and duplicates): don't issue -Wzero-as-null-pointer-constant

when rewriting 'a < b' as '(a <=> b) < 0'.

It's pretty common for comparison category types to use a pointer or
pointer-to-member type as their '0' parameter.

Author: zygoloid

clang/lib/Sema/Sema.cpp

@@ -537,6 +537,13 @@ void Sema::diagnoseZeroToNullptrConversion(CastKind Kind, const Expr* E) {
   if (E->IgnoreParenImpCasts()->getType()->isNullPtrType())
     return;
 
+  // Don't diagnose the conversion from a 0 literal to a null pointer argument
+  // in a synthesized call to operator<=>.
+  if (!CodeSynthesisContexts.empty() &&
+      CodeSynthesisContexts.back().Kind ==
+          CodeSynthesisContext::RewritingOperatorAsSpaceship)
+    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/cxx2a-three-way-comparison.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++2a -verify %s
+// RUN: %clang_cc1 -std=c++2a -verify %s -Wzero-as-null-pointer-constant
 
 // Keep this test before any declarations of operator<=>.
 namespace PR44786 {
@@ -40,3 +40,21 @@ namespace PR47893 {
   int &f(...);
   int &r = f(A(), A());
 }
+
+namespace PR44325 {
+  struct cmp_cat {};
+  bool operator<(cmp_cat, void*);
+  bool operator>(cmp_cat, int cmp_cat::*);
+
+  struct X {};
+  cmp_cat operator<=>(X, X);
+
+  bool b1 = X() < X(); // no warning
+  bool b2 = X() > X(); // no warning
+
+  // FIXME: It's not clear whether warning here is useful, but we can't really
+  // tell that this is a comparison category in general. This is probably OK,
+  // as comparisons against zero are only really intended for use in the
+  // implicit rewrite rules, not for explicit use by programs.
+  bool c = cmp_cat() < 0; // expected-warning {{zero as null pointer constant}}
+}