[clang] Enable -Wunique-object-duplication inside templated code (PR #125902)

[Hans Wennborg via cfe-commits]

================
@@ -154,4 +154,89 @@ namespace GlobalTest {
   };
 
   inline float Test::disallowedStaticMember2 = 2.3; // hidden-warning {{'disallowedStaticMember2' may be duplicated when built into a shared library: it is mutable, has hidden visibility, and external linkage}}
-} // namespace GlobalTest
\ No newline at end of file
+} // namespace GlobalTest
+
+/******************************************************************************
+ * Case three: Inside templates
+ ******************************************************************************/
+
+namespace TemplateTest {
+
+template <typename T>
+int disallowedTemplate1 = 0; // hidden-warning {{'disallowedTemplate1<int>' may be duplicated when built into a shared library: it is mutable, has hidden visibility, and external linkage}}
+
+template int disallowedTemplate1<int>; // hidden-note {{in instantiation of}}
+
+
+// Should work for implicit instantiation as well
+template <typename T>
+int disallowedTemplate2 = 0; // hidden-warning {{'disallowedTemplate2<int>' may be duplicated when built into a shared library: it is mutable, has hidden visibility, and external linkage}}
+
+int implicit_instantiate() {
+  return disallowedTemplate2<int>; // hidden-note {{in instantiation of}}
+}
+
+
+// Ensure we only get warnings for templates that are actually instantiated
+template <typename T>
+int maybeAllowedTemplate = 0; // Not instantiated, so no warning here
+
+template <typename T>
+int maybeAllowedTemplate<T*> = 1; // hidden-warning {{'maybeAllowedTemplate<int *>' may be duplicated when built into a shared library: it is mutable, has hidden visibility, and external linkage}}
+
+template <>
+int maybeAllowedTemplate<bool> = 2; // hidden-warning {{'maybeAllowedTemplate<bool>' may be duplicated when built into a shared library: it is mutable, has hidden visibility, and external linkage}}
+
+template int maybeAllowedTemplate<int*>; // hidden-note {{in instantiation of}}
+
+
+
+// Should work the same for static class members
+template <class T>
----------------
zmodem wrote:

ultra nit: for the template parameter you used `typename` above and `class` here. It doesn't really matter, but I'd pick one throughout for consistency.

https://github.com/llvm/llvm-project/pull/125902