[clang] [Clang][RFC] Introduce a trait to determine the structure binding size (PR #131515)

[Aaron Ballman via cfe-commits]

================
@@ -1642,6 +1661,56 @@ void Sema::CheckCompleteDecompositionDeclaration(DecompositionDecl *DD) {
     DD->setInvalidDecl();
 }
 
+std::optional<unsigned> Sema::GetDecompositionElementCount(QualType T,
+                                                           SourceLocation Loc) {
+  const ASTContext &Ctx = getASTContext();
+  assert(!T->isDependentType());
+
+  Qualifiers Quals;
+  QualType Unqual = Context.getUnqualifiedArrayType(T, Quals);
+  Quals.removeCVRQualifiers();
+  T = Context.getQualifiedType(Unqual, Quals);
+
+  if (auto *CAT = Ctx.getAsConstantArrayType(T))
+    return CAT->getSize().getZExtValue();
+  if (auto *VT = T->getAs<VectorType>())
+    return VT->getNumElements();
+  if (T->getAs<ComplexType>())
+    return 2;
+
+  llvm::APSInt TupleSize(Ctx.getTypeSize(Ctx.getSizeType()));
+  switch (isTupleLike(*this, Loc, T, TupleSize)) {
+  case IsTupleLike::Error:
+    return {};
+  case IsTupleLike::TupleLike:
+    return TupleSize.getExtValue();
+  case IsTupleLike::NotTupleLike:
+    break;
+  }
+
+  CXXRecordDecl *OrigRD = T->getAsCXXRecordDecl();
+  if (!OrigRD || OrigRD->isUnion())
+    return std::nullopt;
+
+  if (RequireCompleteType(Loc, T, diag::err_incomplete_type))
+    return std::nullopt;
+
+  CXXCastPath BasePath;
+  DeclAccessPair BasePair =
+      findDecomposableBaseClass(*this, Loc, OrigRD, BasePath);
+  const CXXRecordDecl *RD = cast_or_null<CXXRecordDecl>(BasePair.getDecl());
----------------
AaronBallman wrote:

```suggestion
  const auto *RD = cast_or_null<CXXRecordDecl>(BasePair.getDecl());
```

https://github.com/llvm/llvm-project/pull/131515