[clang-tidy] Enable C23 support in modernize-use-nullptr (#89990)

C23 introduces the `nullptr` constant similar to C++11 which means that
the checker `modernize-use-nullptr` can be used on C23 code as well.

This PR enables the checker to be run on C23 and adds testcases.

See N3042:
https://open-std.org/JTC1/SC22/WG14/www/docs/n3042.htm

5 files changed, 146 insertions, 3 deletions

diff --git a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h
index 6c32a4edb4ff..f1591bae4465 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h
@@ -19,7 +19,7 @@ public:
   bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
     // FIXME this should be CPlusPlus11 but that causes test cases to
     // erroneously fail.
-    return LangOpts.CPlusPlus;
+    return LangOpts.CPlusPlus || LangOpts.C23;
   }
   void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 2867fc958030..f59d9c3b7a57 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -281,6 +281,10 @@ Changes in existing checks
   don't remove parentheses used in ``sizeof`` calls when they have array index
   accesses as arguments.
 
+- Improved :doc:`modernize-use-nullptr
+  <clang-tidy/checks/modernize/use-nullptr>` check to include support for
+  ``C23``, which also has introduced the ``nullptr`` keyword.
+
 - Improved :doc:`modernize-use-override
   <clang-tidy/checks/modernize/use-override>` check to also remove any trailing
   whitespace when deleting the ``virtual`` keyword.
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-nullptr.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-nullptr.rst
index 5e1ba858adf3..25e17fee0a3d 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-nullptr.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-nullptr.rst
@@ -4,7 +4,7 @@ modernize-use-nullptr
 =====================
 
 The check converts the usage of null pointer constants (e.g. ``NULL``, ``0``)
-to use the new C++11 ``nullptr`` keyword.
+to use the new C++11 and C23 ``nullptr`` keyword.
 
 Example
 -------
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr-c23.c b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr-c23.c
new file mode 100644
index 000000000000..6fb879b91e41
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr-c23.c
@@ -0,0 +1,139 @@
+// RUN: %check_clang_tidy %s modernize-use-nullptr %t -- -- -std=c23
+
+#define NULL 0
+
+void test_assignment() {
+  int *p1 = 0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use nullptr [modernize-use-nullptr]
+  // CHECK-FIXES: int *p1 = nullptr;
+  p1 = 0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: use nullptr
+  // CHECK-FIXES: p1 = nullptr;
+
+  int *p2 = NULL;
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use nullptr
+  // CHECK-FIXES: int *p2 = nullptr;
+
+  p2 = p1;
+  // CHECK-FIXES: p2 = p1;
+
+  const int null = 0;
+  int *p3 = &null;
+
+  p3 = NULL;
+  // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: use nullptr
+  // CHECK-FIXES: p3 = nullptr;
+
+  int *p4 = p3;
+
+  int i1 = 0;
+
+  int i2 = NULL;
+
+  int i3 = null;
+
+  int *p5, *p6, *p7;
+  p5 = p6 = p7 = NULL;
+  // CHECK-MESSAGES: :[[@LINE-1]]:18: warning: use nullptr
+  // CHECK-FIXES: p5 = p6 = p7 = nullptr;
+}
+
+void test_function(int *p) {}
+
+void test_function_no_ptr_param(int i) {}
+
+void test_function_call() {
+  test_function(0);
+  // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use nullptr
+  // CHECK-FIXES: test_function(nullptr);
+
+  test_function(NULL);
+  // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use nullptr
+  // CHECK-FIXES: test_function(nullptr);
+
+  test_function_no_ptr_param(0);
+}
+
+char *test_function_return1() {
+  return 0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use nullptr
+  // CHECK-FIXES: return nullptr;
+}
+
+void *test_function_return2() {
+  return NULL;
+  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use nullptr
+  // CHECK-FIXES: return nullptr;
+}
+
+int test_function_return4() {
+  return 0;
+}
+
+int test_function_return5() {
+  return NULL;
+}
+
+int *test_function_return_cast1() {
+  return(int)0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:9: warning: use nullptr
+  // CHECK-FIXES: return nullptr;
+}
+
+int *test_function_return_cast2() {
+#define RET return
+  RET(int)0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:6: warning: use nullptr
+  // CHECK-FIXES: RET nullptr;
+#undef RET
+}
+
+// Test parentheses expressions resulting in a nullptr.
+int *test_parentheses_expression1() {
+  return(0);
+  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use nullptr
+  // CHECK-FIXES: return(nullptr);
+}
+
+int *test_parentheses_expression2() {
+  return((int)(0.0f));
+  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use nullptr
+  // CHECK-FIXES: return(nullptr);
+}
+
+int *test_nested_parentheses_expression() {
+  return((((0))));
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use nullptr
+  // CHECK-FIXES: return((((nullptr))));
+}
+
+void test_const_pointers() {
+  const int *const_p1 = 0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:25: warning: use nullptr
+  // CHECK-FIXES: const int *const_p1 = nullptr;
+  const int *const_p2 = NULL;
+  // CHECK-MESSAGES: :[[@LINE-1]]:25: warning: use nullptr
+  // CHECK-FIXES: const int *const_p2 = nullptr;
+  const int *const_p3 = (int)0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:25: warning: use nullptr
+  // CHECK-FIXES: const int *const_p3 = nullptr;
+  const int *const_p4 = (int)0.0f;
+  // CHECK-MESSAGES: :[[@LINE-1]]:25: warning: use nullptr
+  // CHECK-FIXES: const int *const_p4 = nullptr;
+}
+
+void test_nested_implicit_cast_expr() {
+  int func0(void*, void*);
+  int func1(int, void*, void*);
+
+  (double)func1(0, 0, 0);
+  // CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use nullptr
+  // CHECK-MESSAGES: :[[@LINE-2]]:23: warning: use nullptr
+  // CHECK-FIXES: (double)func1(0, nullptr, nullptr);
+  (double)func1(func0(0, 0), 0, 0);
+  // CHECK-MESSAGES: :[[@LINE-1]]:23: warning: use nullptr
+  // CHECK-MESSAGES: :[[@LINE-2]]:26: warning: use nullptr
+  // CHECK-MESSAGES: :[[@LINE-3]]:30: warning: use nullptr
+  // CHECK-MESSAGES: :[[@LINE-4]]:33: warning: use nullptr
+  // CHECK-FIXES: (double)func1(func0(nullptr, nullptr), nullptr, nullptr);
+}
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr.c b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr.c
index c2ccbbd81171..1218b837199c 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr.c
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr.c
@@ -1,4 +1,4 @@
-// RUN: clang-tidy %s -checks=-*,modernize-use-nullptr -- | count 0
+// RUN: clang-tidy %s -checks=-*,modernize-use-nullptr -- -std=c17 | count 0
 
 // Note: this test expects no diagnostics, but FileCheck cannot handle that,
 // hence the use of | count 0.