Detect std::bit_cast only in C++20 mode

Author: Carlos Gálvez

clang-tools-extra/clang-tidy/bugprone/BitwisePointerCastCheck.cpp

@@ -14,13 +14,15 @@ using namespace clang::ast_matchers;
 namespace clang::tidy::bugprone {
 
 void BitwisePointerCastCheck::registerMatchers(MatchFinder *Finder) {
-  auto IsPointerType = refersToType(qualType(isAnyPointer()));
-  Finder->addMatcher(callExpr(hasDeclaration(functionDecl(allOf(
-                                  hasName("::std::bit_cast"),
-                                  hasTemplateArgument(0, IsPointerType),
-                                  hasTemplateArgument(1, IsPointerType)))))
-                         .bind("bit_cast"),
-                     this);
+  if (getLangOpts().CPlusPlus20) {
+    auto IsPointerType = refersToType(qualType(isAnyPointer()));
+    Finder->addMatcher(callExpr(hasDeclaration(functionDecl(allOf(
+                                    hasName("::std::bit_cast"),
+                                    hasTemplateArgument(0, IsPointerType),
+                                    hasTemplateArgument(1, IsPointerType)))))
+                           .bind("bit_cast"),
+                       this);
+  }
 
   auto IsDoublePointerType =
       hasType(qualType(pointsTo(qualType(isAnyPointer()))));

clang-tools-extra/docs/ReleaseNotes.rst

@@ -174,7 +174,7 @@ Changes in existing checks
 
 - Improved :doc:`modernize-avoid-c-arrays
   <clang-tidy/checks/modernize/avoid-c-arrays>` check to suggest using ``std::span``
-  as a replacement for parameters of incomplete C array type in C++20 and
+  as a replacement for parameters of incomplete C array type in C++20 and 
   ``std::array`` or ``std::vector`` before C++20.
 
 - Improved :doc:`modernize-min-max-use-initializer-list

clang-tools-extra/test/clang-tidy/checkers/bugprone/bitwise-pointer-cast-cxx20.cpp

@@ -0,0 +1,63 @@
+// RUN: %check_clang_tidy -std=c++20 %s bugprone-bitwise-pointer-cast %t
+
+void memcpy(void* to, void* dst, unsigned long long size)
+{
+  // Dummy implementation for the purpose of the test
+}
+
+namespace std
+{
+template <typename To, typename From>
+To bit_cast(From from)
+{
+  // Dummy implementation for the purpose of the test
+  To to{};
+  return to;
+}
+
+using ::memcpy;
+}
+
+void pointer2pointer()
+{
+  int x{};
+  float bad = *std::bit_cast<float*>(&x); // UB, but looks safe due to std::bit_cast
+  // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: do not use 'std::bit_cast' to cast between pointers [bugprone-bitwise-pointer-cast]
+  float good = std::bit_cast<float>(x);   // Well-defined
+
+  using IntPtr = int*;
+  using FloatPtr = float*;
+  IntPtr x2{};
+  float bad2 = *std::bit_cast<FloatPtr>(x2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: do not use 'std::bit_cast' to cast between pointers [bugprone-bitwise-pointer-cast]
+}
+
+void pointer2pointer_memcpy()
+{
+  int x{};
+  int* px{};
+  float y{};
+  float* py{};
+
+  memcpy(&py, &px, sizeof(px));
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: do not use 'memcpy' to cast between pointers [bugprone-bitwise-pointer-cast]
+  std::memcpy(&py, &px, sizeof(px));
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: do not use 'memcpy' to cast between pointers [bugprone-bitwise-pointer-cast]
+
+  std::memcpy(&y, &x, sizeof(x));
+}
+
+// Pointer-integer conversions are allowed by this check
+void int2pointer()
+{
+  unsigned long long addr{};
+  float* p = std::bit_cast<float*>(addr);
+  std::memcpy(&p, &addr, sizeof(addr));
+}
+
+void pointer2int()
+{
+  float* p{};
+  auto addr = std::bit_cast<unsigned long long>(p);
+  std::memcpy(&addr, &p, sizeof(p));
+}

clang-tools-extra/test/clang-tidy/checkers/bugprone/bitwise-pointer-cast.cpp

@@ -7,32 +7,10 @@ void memcpy(void* to, void* dst, unsigned long long size)
 
 namespace std
 {
-template <typename To, typename From>
-To bit_cast(From from)
-{
-  // Dummy implementation for the purpose of the test
-  To to{};
-  return to;
-}
-
 using ::memcpy;
 }
 
 void pointer2pointer()
-{
-  int x{};
-  float bad = *std::bit_cast<float*>(&x); // UB, but looks safe due to std::bit_cast
-  // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: do not use 'std::bit_cast' to cast between pointers [bugprone-bitwise-pointer-cast]
-  float good = std::bit_cast<float>(x);   // Well-defined
-
-  using IntPtr = int*;
-  using FloatPtr = float*;
-  IntPtr x2{};
-  float bad2 = *std::bit_cast<FloatPtr>(x2);
-  // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: do not use 'std::bit_cast' to cast between pointers [bugprone-bitwise-pointer-cast]
-}
-
-void pointer2pointer_memcpy()
 {
   int x{};
   int* px{};
@@ -51,11 +29,13 @@ void pointer2pointer_memcpy()
 void int2pointer()
 {
   unsigned long long addr{};
-  float* p = std::bit_cast<float*>(addr);
+  float* p{};
+  std::memcpy(&p, &addr, sizeof(addr));
 }
 
 void pointer2int()
 {
+  unsigned long long addr{};
   float* p{};
-  auto addr = std::bit_cast<unsigned long long>(p);
+  std::memcpy(&addr, &p, sizeof(p));
 }