[Clang] Fix cast to BigIntType in hasUniqueObjectRepresentations

[royjacobson]

Summary

A bad QualType cast caused us not to detect _BigInt types if they were wrapped inside sugar types like SubstTemplateTypeParm.
Fix https://github.com/llvm/llvm-project/issues/62019

[cjdb]

LGTM, thanks for such a quick fix on a weekend :)

[shafik]

Thank you for this quick fix

clang/lib/AST/ASTContext.cpp

bool ASTContext::hasUniqueObjectRepresentations(QualType Ty) const {
 
   // (9.1) - T is trivially copyable...
   if (!Ty.isTriviallyCopyableType(*this))
     return false;
 
   // All integrals and enums are unique.
   if (Ty->isIntegralOrEnumerationType()) {
     // Except _BitInt types that have padding bits.
-    if (const auto *BIT = dyn_cast<BitIntType>(Ty))
+    if (const auto *BIT = Ty->getAs<BitIntType>())
       return getTypeSize(BIT) == BIT->getNumBits();
 
     return true;
   }
 
   // All other pointers are unique.
   if (Ty->isPointerType())
     return true;

clang/test/SemaCXX/type-traits.cpp

namespace ErrorType {
   struct S; //expected-note{{forward declaration of 'ErrorType::S'}}
 
   struct T {
         S t; //expected-error{{field has incomplete type 'S'}}
   };
   bool b = __has_unique_object_representations(T);
 };
 
+static_assert(!has_unique_object_representations<_BitInt(7)>::value, "BitInt:");
+static_assert(has_unique_object_representations<_BitInt(8)>::value, "BitInt:");
+static_assert(!has_unique_object_representations<_BitInt(127)>::value, "BitInt:");
+static_assert(has_unique_object_representations<_BitInt(128)>::value, "BitInt:");
+
+
 namespace PR46209 {
   // Foo has both a trivial assignment operator and a non-trivial one.
   struct Foo {
     Foo &operator=(const Foo &) & { return *this; }
     Foo &operator=(const Foo &) && = default;
   };
 
   // Bar's copy assignment calls Foo's non-trivial assignment.