[clang] [Sema] -Wzero-as-null-pointer-constant: don't warn for __null (PR #69126)

[Arseny Kapoulkine via cfe-commits]

https://github.com/zeux created https://github.com/llvm/llvm-project/pull/69126

The implementation of -Wzero-as-null-pointer-constant was done before the following fix has been committed to GCC:

https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=752e7593b0f19af233a0b7e72daab8413662b605;hp=298434c916c14e8adca2cab8a746aee29038c5b3

As a result, clang and gcc diverge on the use of `__null` and, consequently, on the use of `NULL` on systems like Linux/macOS where `NULL` is defined as `__null`.

This is a problem for compatibility between gcc and clang, particularly for code bases that support C++98 or for single-source libraries that are implemented in C, but compiled as C++ via inclusion into a C++ translation unit. Code like this can not be changed to use `nullptr`, as it needs to maintain compatibility with C before C23 or C++ before C++11, but warns on the use of `NULL` in clang.

The warning `Wzero-as-null-pointer-constant` is still useful with this change, as it allows to change `0` to `NULL`, which fixes
gcc warnings and helps the reader distinguish between pointers and non-pointers. Users who require a full C++11 modernization pass can still use clang-tidy for that purpose.

>From 357a21c38c1036a012affc85026fcba376ab7128 Mon Sep 17 00:00:00 2001
From: Arseny Kapoulkine <arseny.kapoulkine at gmail.com>
Date: Sun, 15 Oct 2023 13:20:31 -0700
Subject: [PATCH] [Sema] -Wzero-as-null-pointer-constant: don't warn for __null

The implementation of -Wzero-as-null-pointer-constant was done before
the following fix has been committed to GCC:

https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=752e7593b0f19af233a0b7e72daab8413662b605;hp=298434c916c14e8adca2cab8a746aee29038c5b3

As a result, clang and gcc diverge on the use of __null and,
consequently, on the use of NULL on systems like Linux/macOS where NULL
is defined as __null.

This is a problem for compatibility between gcc and clang, particularly
for code bases that support C++98 or for single-source libraries that
are implemented in C, but compiled as C++ via inclusion into a C++
translation unit. Code like this can not be changed to use nullptr, as
it needs to maintain compatibility with C before C23 or C++ before
C++11, but warns on the use of NULL.

Note: a limitation of this change is that we will still warn on use of
NULL when it is defined as 0, which could be a problem for clang-cl.
---
 clang/lib/Sema/Sema.cpp                  | 8 +++++++-
 clang/test/SemaCXX/warn-zero-nullptr.cpp | 8 ++++----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 67533ccbdf347c7..acb765559e6a8d4 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -590,7 +590,11 @@ 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,
@@ -612,6 +616,8 @@ 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) &&
diff --git a/clang/test/SemaCXX/warn-zero-nullptr.cpp b/clang/test/SemaCXX/warn-zero-nullptr.cpp
index 45f05fa5703b1c3..684572f8ef67d05 100644
--- a/clang/test/SemaCXX/warn-zero-nullptr.cpp
+++ b/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}}