[clang] [clang][bytecode] Fix crash when dynamic_cast'ing a root pointer (PR #217540)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/217540

We can't call getBase(), so move the isRoot() check before.

>From 8b4a48f4a6dca5c2291c9bb62cc006708671d58b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Thu, 20 Aug 2026 08:55:15 +0200
Subject: [PATCH] [clang][bytecode] Fix crash when dynamic_cast'ing a root
 pointer

We can't call getBase(), so move the isRoot() check before.
---
 clang/lib/AST/ByteCode/Interp.cpp        |  7 ++++---
 clang/test/AST/ByteCode/dynamic-cast.cpp | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 7704550a12d82..8f0a36877aa2b 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -2246,15 +2246,16 @@ bool DynamicCast(InterpState &S, CodePtr OpPC, const Type *DestTypePtr,
     if (R.valid()) {
       Result = Iter.atField(*R.Offset);
       break;
-    } else if (R.Ambiguous) {
+    }
+    if (R.Ambiguous) {
       Ambiguous = true;
       break;
     }
 
-    // This moves us DOWN the type hierarchy.
-    Iter = Iter.getBase();
     if (Iter.isRoot() || !Iter.isBaseClass())
       break;
+    // This moves us DOWN the type hierarchy.
+    Iter = Iter.getBase();
   }
 
   if (Ambiguous)
diff --git a/clang/test/AST/ByteCode/dynamic-cast.cpp b/clang/test/AST/ByteCode/dynamic-cast.cpp
index 83a745e271a28..8a4678b092c2e 100644
--- a/clang/test/AST/ByteCode/dynamic-cast.cpp
+++ b/clang/test/AST/ByteCode/dynamic-cast.cpp
@@ -352,3 +352,18 @@ namespace VirtualBase {
   }
   static_assert(test());
 }
+
+namespace RootPtr {
+  struct S {
+    constexpr virtual int foo() { return 0; }
+  };
+
+  struct T {};
+
+  struct U : virtual T {
+    constexpr S *bar() const { return (S *)this; } // both-note {{cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression}}
+  };
+  constexpr U u;
+  static_assert(dynamic_cast<T *>(u.bar())); // both-error {{not an integral constant expression}} \
+                                             // both-note {{in call to}}
+}