[clang] d049db8 - [clang] Fix false warning on reinterpret_casting unknown template type (#109430)

[via cfe-commits]

Author: Arseniy Zaostrovnykh
Date: 2025-01-20T10:02:57+01:00
New Revision: d049db83627d164e4353f59a5f0b4f87dd74b138

URL: https://github.com/llvm/llvm-project/commit/d049db83627d164e4353f59a5f0b4f87dd74b138
DIFF: https://github.com/llvm/llvm-project/commit/d049db83627d164e4353f59a5f0b4f87dd74b138.diff

LOG: [clang] Fix false warning on reinterpret_casting unknown template type (#109430)

After 1595988ee6f9732e7ea79928af8a470ad5ef7dbe
diag::warn_undefined_reinterpret_cast started raising on
non-instantiated template functions without sufficient knowledge whether
the reinterpret_cast is indeed UB.

Added: 
    

Modified: 
    clang/docs/ReleaseNotes.rst
    clang/lib/Sema/SemaCast.cpp
    clang/test/SemaCXX/reinterpret-cast.cpp

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2f7b9227386961..f6d5c346021d60 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -660,6 +660,8 @@ Improvements to Clang's diagnostics
 
 - Don't emit bogus dangling diagnostics when ``[[gsl::Owner]]`` and `[[clang::lifetimebound]]` are used together (#GH108272).
 
+- Don't emit bogus dignostic about an undefined behavior on ``reinterpret_cast<T>`` for non-instantiated template functions without sufficient knowledge whether it can actually lead to undefined behavior for ``T`` (#GH109430).
+
 - The ``-Wreturn-stack-address`` warning now also warns about addresses of
   local variables passed to function calls using the ``[[clang::musttail]]``
   attribute.

diff  --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index f98857f852b5af..54bc52fa2ac405 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -2094,6 +2094,10 @@ void Sema::CheckCompatibleReinterpretCast(QualType SrcType, QualType DestType,
     }
   }
 
+  if (SrcTy->isDependentType() || DestTy->isDependentType()) {
+    return;
+  }
+
   Diag(Range.getBegin(), DiagID) << SrcType << DestType << Range;
 }
 

diff  --git a/clang/test/SemaCXX/reinterpret-cast.cpp b/clang/test/SemaCXX/reinterpret-cast.cpp
index 45332fd15b5d4e..bfb808773b9004 100644
--- a/clang/test/SemaCXX/reinterpret-cast.cpp
+++ b/clang/test/SemaCXX/reinterpret-cast.cpp
@@ -302,3 +302,77 @@ void reinterpret_cast_allowlist () {
   (void)reinterpret_cast<unsigned char&>(b);
   (void)*reinterpret_cast<unsigned char*>(&b);
 }
+
+namespace templated {
+template <typename TARGETTYPE, typename UATYPE>
+void cast_uninstantiated() {
+  const UATYPE* data;
+  (void)*reinterpret_cast<const TARGETTYPE*>(data); // no warning
+}
+
+
+template <typename TARGETTYPE, typename UATYPE>
+void cast_instantiated_badly() {
+  const UATYPE* data;
+  (void)*reinterpret_cast<const TARGETTYPE*>(data); // expected-warning {{dereference of type 'const int *' that was reinterpret_cast from type 'const float *' has undefined behavior}}
+}
+
+template <typename TARGETTYPE, typename UATYPE>
+void cast_instantiated_well() {
+  const UATYPE* data;
+  (void)*reinterpret_cast<const TARGETTYPE*>(data); // no warning
+}
+
+template <typename TARGETTYPE>
+void cast_one_tmpl_arg_uninstantiated() {
+  const int* data;
+  (void)*reinterpret_cast<const TARGETTYPE*>(data); // no warning
+}
+
+template <typename TARGETTYPE>
+void cast_one_tmpl_arg_instantiated_badly() {
+  const float* data;
+  (void)*reinterpret_cast<const TARGETTYPE*>(data); // expected-warning {{dereference of type 'const int *' that was reinterpret_cast from type 'const float *' has undefined behavior}}
+}
+
+template <typename TARGETTYPE>
+void cast_one_tmpl_arg_instantiated_well() {
+  const float* data;
+  (void)*reinterpret_cast<const TARGETTYPE*>(data); // no warning
+}
+
+template <int size>
+void cast_nontype_template_true_positive_noninstantiated() {
+  const float *data;
+  const int arr[size];
+  (void)*reinterpret_cast<const int*>(data); // expected-warning {{dereference of type 'const int *' that was reinterpret_cast from type 'const float *' has undefined behavior}}
+}
+
+template <int size>
+void cast_nontype_template_true_negative_noninstantiated() {
+  const int data[size];
+  (void)*reinterpret_cast<const int*>(data); // no warning
+}
+
+void top() {
+  cast_instantiated_badly<int, float>();
+  // expected-note at -1 {{in instantiation of function template specialization 'templated::cast_instantiated_badly<int, float>' requested here}}
+  cast_instantiated_well<int, int>();
+  cast_one_tmpl_arg_instantiated_badly<int>();
+  // expected-note at -1 {{in instantiation of function template specialization 'templated::cast_one_tmpl_arg_instantiated_badly<int>' requested here}}
+  cast_one_tmpl_arg_instantiated_well<float>();
+}
+
+template<typename T, typename U>
+void cast_template_dependent_type_noninstantiated(T** x)
+{
+    (void)*reinterpret_cast<U**>(x);
+}
+
+template<typename T, typename U>
+void cast_template_dependent_member_type_noninstantiated(typename T::X x)
+{
+    (void)*reinterpret_cast<typename U::Y>(x);
+}
+
+} // namespace templated