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

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

Author: necto

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.

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;
 }
 

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@-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@-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