[clang] Fix false warning on reinterpret_casting unknown template type

[necto]

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

This change fixes it.

CPP-5704

[llvmbot]

@llvm/pr-subscribers-clang

Author: Arseniy Zaostrovnykh (necto)

Changes

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

This change fixes it.

CPP-5704

---

Full diff: https://github.com/llvm/llvm-project/pull/109430.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaCast.cpp (+4)
• (modified) clang/test/SemaCXX/reinterpret-cast.cpp (+62)

diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index 6ac6201843476b..dd65b301343e6c 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -2092,6 +2092,10 @@ void Sema::CheckCompatibleReinterpretCast(QualType SrcType, QualType DestType,
     }
   }
 
+  if (SrcTy->isTemplateTypeParmType() || DestTy->isTemplateTypeParmType()) {
+    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..773aa8ee0e7894 100644
--- a/clang/test/SemaCXX/reinterpret-cast.cpp
+++ b/clang/test/SemaCXX/reinterpret-cast.cpp
@@ -302,3 +302,65 @@ 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>();
+}
+
+} // namespace templated

[sdkrystian]

This should probably beif (SrcTy->isDependentType() || DestTy->isDependentType()). Otherwise we will continue to warn for the following:

template<typename T, typename U>
void f(T** x) 
{
    *reinterpret_cast<U**>(x);
}

[necto]

Good point
Fixed in 541e0af

[cor3ntin]

LGTM.
Do you need me to merge that for you?

[necto]

LGTM. Do you need me to merge that for you?

Yes, please

[cor3ntin]

Hum, this actually need a release note, can you edit clang/docs/ReleaseNotes.rst? Thanks

[necto]

Hum, this actually need a release note, can you edit clang/docs/ReleaseNotes.rst? Thanks

Added: e0db8fb

[necto]

@cor3ntin is it good to go now?