[clang] [clang][dataflow] Model assignment to derived class from base. (PR #85064)

[via cfe-commits]

================
@@ -35,45 +69,27 @@ void clang::dataflow::copyRecord(RecordStorageLocation &Src,
   });
   assert(compatibleTypes);
 
-  for (auto [Field, DstFieldLoc] : Dst.children()) {
-    StorageLocation *SrcFieldLoc = Src.getChild(*Field);
-
-    assert(Field->getType()->isReferenceType() ||
-           (SrcFieldLoc != nullptr && DstFieldLoc != nullptr));
-
-    if (Field->getType()->isRecordType()) {
-      copyRecord(cast<RecordStorageLocation>(*SrcFieldLoc),
-                 cast<RecordStorageLocation>(*DstFieldLoc), Env);
-    } else if (Field->getType()->isReferenceType()) {
-      Dst.setChild(*Field, SrcFieldLoc);
-    } else {
-      if (Value *Val = Env.getValue(*SrcFieldLoc))
-        Env.setValue(*DstFieldLoc, *Val);
-      else
-        Env.clearValue(*DstFieldLoc);
-    }
-  }
-
-  for (const auto &[Name, SynthFieldLoc] : Src.synthetic_fields()) {
-    if (SynthFieldLoc->getType()->isRecordType()) {
-      copyRecord(*cast<RecordStorageLocation>(SynthFieldLoc),
-                 cast<RecordStorageLocation>(Dst.getSyntheticField(Name)), Env);
-    } else {
-      if (Value *Val = Env.getValue(*SynthFieldLoc))
-        Env.setValue(Dst.getSyntheticField(Name), *Val);
-      else
-        Env.clearValue(Dst.getSyntheticField(Name));
-    }
+  if (SrcType == DstType || (SrcDecl != nullptr && DstDecl != nullptr &&
+                             SrcDecl->isDerivedFrom(DstDecl))) {
+    for (auto [Field, DstFieldLoc] : Dst.children())
----------------
martinboehme wrote:

The two `lambda()` calls would be in the if and the else branch, respectively? I.e.

```cxx
  if (SrcType == DstType || (SrcDecl != nullptr && DstDecl != nullptr &&
                             SrcDecl->isDerivedFrom(DstDecl))) {
    lambda(Dst);
    // And synthetic fields
  } else {
    lambda(Src);
    // And synthetic fields
  }
```

The problem is that this doesn't work because it changes behavior. This is what we have in the if branch right now:

```cxx
    for (auto [Field, DstFieldLoc] : Dst.children())
      copyField(Field, Src.getChild(*Field), DstFieldLoc, Dst, Env);
```

Changing this to `lambda(Dst)` would mean that the behavior becomes:

```
    for (auto [Field, SrcFieldLoc] : Dst.children())
      copyField(Field, SrcFieldLoc, Dst.getChild(*Field), Dst, Env);
```

Note that the name `SrcFieldLoc` is now essentially wrong, and the `copyField()` becomes a no-op because it's copying a field to itself.

https://github.com/llvm/llvm-project/pull/85064