[Clang] Require base element type of __has_unique_object_representations to be complete

[MitalAshok]

Fixes #95311

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

[llvmbot]

@llvm/pr-subscribers-clang

Author: Mital Ashok (MitalAshok)

Changes

Fixes #95311

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

---

Full diff: https://github.com/llvm/llvm-project/pull/95432.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/lib/AST/ASTContext.cpp (+6)
• (modified) clang/lib/Sema/SemaExprCXX.cpp (+4-1)
• (modified) clang/test/SemaCXX/type-traits.cpp (+11)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8c2f737836a9d..4359213286fa0 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -660,6 +660,9 @@ Bug Fixes in This Version
 - Correctly reject declarations where a statement is required in C.
   Fixes #GH92775
 
+- ``__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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index aa22825602a40..f3b698b3f0c59 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -2791,6 +2791,12 @@ bool ASTContext::hasUniqueObjectRepresentations(
     return hasUniqueObjectRepresentations(getBaseElementType(Ty),
                                           CheckIfTriviallyCopyable);
 
+  if (Ty->isVoidType())
+    return false;
+
+  assert(!Ty->isIncompleteType() && "hasUniqueObjectRepresentations should not "
+                                    "be called with an incomplete type");
+
   // (9.1) - T is trivially copyable...
   if (CheckIfTriviallyCopyable && !Ty.isTriviallyCopyableType(*this))
     return false;
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index f3af8dee6b090..127621a31470d 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -5149,6 +5149,10 @@ 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]];
 
@@ -5157,7 +5161,6 @@ 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;
 
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index d40605f56f1ed..b5cefb18fa13e 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/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.

[shafik]

It is not clear why we need this change as well.

[MitalAshok]

For the new !Ty->isIncompleteType() assertion below. Before void would go to the !isTriviallyCopyable path, this is just a more explicit way of handling __has_unique_object_representations(void). The assertion could also be assert(Ty->isVoidType() || !Ty->isIncompleteType()) to follow the spec better?

[cor3ntin]

I'd prefer that, I think

[cor3ntin]

LGTM