[Clang] Require base element type of __has_unique_object_representations to be complete (#95432)

Summary:
Fixes #95311

Previous behaviour was that `false` was silently returned, templated
classes were not instantiated and incomplete classes did not issue an
error.

---------

Co-authored-by: cor3ntin <[email protected]>

Test Plan: 

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Differential Revision: https://phabricator.intern.facebook.com/D60250880

Author: MitalAshok

clang/docs/ReleaseNotes.rst

@@ -835,6 +835,9 @@ Bug Fixes in This Version
 
 - Fixed Clang from generating dangling StringRefs when deserializing Exprs & Stmts (#GH98667)
 
+- ``__has_unique_object_representations`` correctly handles arrays of unknown bounds of
+  types by ensuring they are complete and instantiating them if needed. Fixes (#GH95311).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/AST/ASTContext.cpp

@@ -2831,6 +2831,10 @@ bool ASTContext::hasUniqueObjectRepresentations(
     return hasUniqueObjectRepresentations(getBaseElementType(Ty),
                                           CheckIfTriviallyCopyable);
 
+  assert((Ty->isVoidType() || !Ty->isIncompleteType()) &&
+         "hasUniqueObjectRepresentations should not be called with an "
+         "incomplete type");
+
   // (9.1) - T is trivially copyable...
   if (CheckIfTriviallyCopyable && !Ty.isTriviallyCopyableType(*this))
     return false;

clang/lib/Sema/SemaExprCXX.cpp

@@ -5069,6 +5069,10 @@ static bool CheckUnaryTypeTraitTypeCompleteness(Sema &S, TypeTrait UTT,
   case UTT_HasTrivialCopy:
   case UTT_HasTrivialDestructor:
   case UTT_HasVirtualDestructor:
+  // has_unique_object_representations<T> when T is an array is defined in terms
+  // of has_unique_object_representations<remove_all_extents_t<T>>, so the base
+  // type needs to be complete even if the type is an incomplete array type.
+  case UTT_HasUniqueObjectRepresentations:
     ArgTy = QualType(ArgTy->getBaseElementTypeUnsafe(), 0);
     [[fallthrough]];
 
@@ -5077,7 +5081,6 @@ static bool CheckUnaryTypeTraitTypeCompleteness(Sema &S, TypeTrait UTT,
   case UTT_IsDestructible:
   case UTT_IsNothrowDestructible:
   case UTT_IsTriviallyDestructible:
-  case UTT_HasUniqueObjectRepresentations:
     if (ArgTy->isIncompleteArrayType() || ArgTy->isVoidType())
       return true;
 

clang/test/SemaCXX/type-traits.cpp

@@ -3505,6 +3505,17 @@ static_assert(__has_unique_object_representations(_BitInt(8)), "BitInt:");
 static_assert(!__has_unique_object_representations(_BitInt(127)), "BitInt:");
 static_assert(__has_unique_object_representations(_BitInt(128)), "BitInt:");
 
+namespace GH95311 {
+
+template <int>
+class Foo {
+  int x;
+};
+static_assert(__has_unique_object_representations(Foo<0>[]));
+class Bar; // expected-note {{forward declaration of 'GH95311::Bar'}}
+static_assert(__has_unique_object_representations(Bar[])); // expected-error {{incomplete type}}
+
+}
 
 namespace PR46209 {
   // Foo has both a trivial assignment operator and a non-trivial one.