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

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Arseniy Zaostrovnykh (necto)

<details>
<summary>Changes</summary>

After 1595988ee6f9732e7ea79928af8a470ad5ef7dbe
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
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 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>();
+}
+
+} // namespace templated

``````````

</details>


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