[clang] cf7864b - [clang][bytecode] Allow downcasts of constexpr-unknown values (#204084)

[via cfe-commits]

Author: Timm Baeder
Date: 2026-06-16T16:17:04+02:00
New Revision: cf7864b62ae3bac28b44024cbbcfd3800c8f35f9

URL: https://github.com/llvm/llvm-project/commit/cf7864b62ae3bac28b44024cbbcfd3800c8f35f9
DIFF: https://github.com/llvm/llvm-project/commit/cf7864b62ae3bac28b44024cbbcfd3800c8f35f9.diff

LOG: [clang][bytecode] Allow downcasts of constexpr-unknown values (#204084)

They might be diagnosed later.

Added: 
    

Modified: 
    clang/lib/AST/ByteCode/Interp.cpp
    clang/test/SemaCXX/constant-expression-p2280r4.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 954e4e2cddb8d..e5bf9c0c590ac 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -558,9 +558,16 @@ bool CheckDowncast(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
     return true;
 
   const auto *E = cast<CastExpr>(S.Current->getExpr(OpPC));
-  QualType TargetQT = E->getType()->getPointeeType();
+  QualType ExprTy = E->getType();
+  if (ExprTy->isPointerOrReferenceType())
+    ExprTy = ExprTy->getPointeeType();
+
+  QualType TargetQT = ExprTy;
   QualType MostDerivedQT = Ptr.getDeclPtr().getType();
 
+  if (MostDerivedQT->isPointerOrReferenceType())
+    MostDerivedQT = MostDerivedQT->getPointeeType();
+
   S.CCEDiag(E, diag::note_constexpr_invalid_downcast)
       << MostDerivedQT << TargetQT;
 
@@ -1578,9 +1585,6 @@ bool GetPtrDerivedPop(InterpState &S, CodePtr OpPC, uint32_t Off, bool NullOK,
     return true;
   }
 
-  if (isConstexprUnknown(Ptr))
-    return false;
-
   if (!Ptr.getFieldDesc()->isRecord()) {
     S.Stk.push<Pointer>(Ptr);
     return true;

diff  --git a/clang/test/SemaCXX/constant-expression-p2280r4.cpp b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
index e282688ed2248..bb5499e57ac78 100644
--- a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
+++ b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
@@ -296,14 +296,14 @@ namespace casting {
   struct B : A {};
   struct C : A {};
   extern A &a;
-  extern B &b; // nointerpreter-note {{declared here}}
+  extern B &b; // expected-note {{declared here}}
   constexpr B &t1 = (B&)a; // expected-error {{must be initialized by a constant expression}} \
-                           // nointerpreter-note {{cannot cast object of dynamic type 'A' to type 'B'}}
+                           // expected-note {{cannot cast object of dynamic type 'A' to type 'B'}}
   constexpr B &t2 = (B&)(A&)b; // expected-error {{must be initialized by a constant expression}} \
-                               // nointerpreter-note {{initializer of 'b' is not a constant expression}}
-  constexpr bool t3 = &b + 1 == &(B&)(A&)b; // interpreter-error {{constexpr variable 't3' must be initialized by a constant expression}}
+                               // expected-note {{initializer of 'b' is not a constant expression}}
+  constexpr bool t3 = &b + 1 == &(B&)(A&)b;
   constexpr C &t4 = (C&)(A&)b; // expected-error {{must be initialized by a constant expression}} \
-                               // nointerpreter-note {{cannot cast object of dynamic type 'B' to type 'C'}}
+                               // expected-note {{cannot cast object of dynamic type 'B' to type 'C'}}
 }
 
 namespace pointer_comparisons {