Merge pull request #8223 from ian-twilightcoder/zero-as-null

[cherry-pick stable/20230725] [Sema] -Wzero-as-null-pointer-constant: don't warn for __null (llvm#69126)

Author: ian-twilightcoder

clang/docs/ReleaseNotes.rst

@@ -500,6 +500,10 @@ Improvements to Clang's diagnostics
   warnings. Instead, it will suggest casting the enum object based on its
   underlying type.
 
+- ``-Wzero-as-null-pointer-constant`` diagnostic is no longer emitted when using ``__null``
+  (or, more commonly, ``NULL`` when the platform defines it as ``__null``) to be more consistent
+  with GCC.
+
 Bug Fixes in This Version
 -------------------------
 - Fixed an issue where a class template specialization whose declaration is

clang/lib/Sema/Sema.cpp

@@ -596,7 +596,11 @@ void Sema::diagnoseZeroToNullptrConversion(CastKind Kind, const Expr *E) {
 
   if (Kind != CK_NullToPointer && Kind != CK_NullToMemberPointer)
     return;
-  if (E->IgnoreParenImpCasts()->getType()->isNullPtrType())
+
+  const Expr *EStripped = E->IgnoreParenImpCasts();
+  if (EStripped->getType()->isNullPtrType())
+    return;
+  if (isa<GNUNullExpr>(EStripped))
     return;
 
   if (Diags.isIgnored(diag::warn_zero_as_null_pointer_constant,
@@ -618,6 +622,8 @@ void Sema::diagnoseZeroToNullptrConversion(CastKind Kind, const Expr *E) {
 
   // If it is a macro from system header, and if the macro name is not "NULL",
   // do not warn.
+  // Note that uses of "NULL" will be ignored above on systems that define it
+  // as __null.
   SourceLocation MaybeMacroLoc = E->getBeginLoc();
   if (Diags.getSuppressSystemWarnings() &&
       SourceMgr.isInSystemMacro(MaybeMacroLoc) &&

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

@@ -16,10 +16,10 @@ int (S::*mp1) = 0; // expected-warning{{zero as null pointer constant}}
 void (*fp1)() = 0; // expected-warning{{zero as null pointer constant}}
 void* p1 = 0; // expected-warning{{zero as null pointer constant}}
 
-// NULL is an integer constant expression, so warn on it too:
-void* p2 = __null; // expected-warning{{zero as null pointer constant}}
-void (*fp2)() = __null; // expected-warning{{zero as null pointer constant}}
-int (S::*mp2) = __null; // expected-warning{{zero as null pointer constant}}
+// __null is not treated as an integer constant expression for GCC compatibility
+void* p2 = __null;
+void (*fp2)() = __null;
+int (S::*mp2) = __null;
 
 void f0(void* v = MACRO); // expected-warning{{zero as null pointer constant}}
 void f1(void* v = NULL); // expected-warning{{zero as null pointer constant}}