[clang] [clang][bytecode] Ignore irrelevant InitializingPtrs in dynamic_cast (PR #206468)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

<details>
<summary>Changes</summary>

If the initializing pointer is unrealted to the one we're operating on, ignore it.

---
Full diff: https://github.com/llvm/llvm-project/pull/206468.diff


2 Files Affected:

- (modified) clang/lib/AST/ByteCode/Interp.cpp (+2-2) 
- (modified) clang/test/AST/ByteCode/dynamic-cast.cpp (+14) 


``````````diff
diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 0396482517046..249733fb1ea14 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -2058,10 +2058,10 @@ bool DynamicCast(InterpState &S, CodePtr OpPC, const Type *DestTypePtr,
   // Our given pointer, limited by the base that's currently being initialized,
   // if any.
   PtrView LimitedPtr;
-  if (S.InitializingPtrs.empty()) {
+  if (S.InitializingPtrs.empty() ||
+      S.InitializingPtrs.back().block() != Ptr.block()) {
     LimitedPtr = Ptr.stripBaseCasts().view();
   } else {
-    // FIXME: Is this always the correct block?
     LimitedPtr = S.InitializingPtrs.back();
     assert(LimitedPtr.block() == Ptr.block());
   }
diff --git a/clang/test/AST/ByteCode/dynamic-cast.cpp b/clang/test/AST/ByteCode/dynamic-cast.cpp
index a40b455cecabf..fa947e14cfa13 100644
--- a/clang/test/AST/ByteCode/dynamic-cast.cpp
+++ b/clang/test/AST/ByteCode/dynamic-cast.cpp
@@ -310,3 +310,17 @@ namespace Invalid {
   static_assert(&dynamic_cast<S&>((X&)x), ""); // both-error {{not an integral constant expression}} \
                                                // both-note {{initializer of 'x' is not a constant expression}}
 }
+
+namespace UnrelatedInitializingPtr {
+  struct A {
+    virtual void foo();
+  };
+  struct B : A {};
+  struct C : A {};
+  struct D : B {};
+
+  constexpr D d;
+  constexpr A &a = (B &)d;
+  constexpr auto p = dynamic_cast<C &>(a); // both-error {{must be initialized by a constant expression}} \
+                                           // both-note {{reference dynamic_cast failed: dynamic type 'UnrelatedInitializingPtr::D' of operand does not have a base class of type 'C'}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/206468