[clang][bytecode] Don't outright reject dynamic casts

clang/lib/AST/ByteCode/Compiler.cpp

@@ -773,6 +773,11 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   case CK_ToVoid:
     return discard(SubExpr);
 
+  case CK_Dynamic:
+    // This initially goes through VisitCXXDynamicCastExpr, where we emit
+    // a diagnostic if appropriate.
+    return this->delegate(SubExpr);
+
   default:
     return this->emitInvalid(CE);
   }

clang/test/AST/ByteCode/cxx20.cpp

@@ -1201,3 +1201,13 @@ namespace NonPureVirtualCall {
 
   int main() { check(); }
 }
+
+namespace DyamicCast {
+  struct X {
+    virtual constexpr ~X() {}
+  };
+  struct Y : X {};
+  constexpr Y y;
+  constexpr const X *p = &y;
+  constexpr const Y *q = dynamic_cast<const Y*>(p);
+}

clang/test/SemaCXX/constant-expression-p2280r4.cpp

@@ -278,11 +278,13 @@ namespace dropped_note {
 namespace dynamic {
   struct A {virtual ~A();};
   struct B : A {};
-  void f(A& a) {
+  void f(A& a) { // interpreter-note 2{{declared here}}
     constexpr B* b = dynamic_cast<B*>(&a); // expected-error {{must be initialized by a constant expression}} \
-                                           // nointerpreter-note {{dynamic_cast applied to object 'a' whose dynamic type is not constant}}
+                                           // nointerpreter-note {{dynamic_cast applied to object 'a' whose dynamic type is not constant}} \
+                                           // interpreter-note {{pointer to 'a' is not a constant expression}}
     constexpr void* b2 = dynamic_cast<void*>(&a); // expected-error {{must be initialized by a constant expression}} \
-                                                  // nointerpreter-note {{dynamic_cast applied to object 'a' whose dynamic type is not constant}}
+                                                  // nointerpreter-note {{dynamic_cast applied to object 'a' whose dynamic type is not constant}} \
+                                                  // interpreter-note {{pointer to 'a' is not a constant expression}}
   }
 }
 
@@ -415,9 +417,7 @@ namespace InvalidConstexprFn {
   };
   constexpr int virtual_call(const PolyBase& b) { return b.get(); }
   constexpr auto* type(const PolyBase& b) { return &typeid(b); }
-  // FIXME: Intepreter doesn't support constexpr dynamic_cast yet.
-  constexpr const void* dyncast(const PolyBase& b) { return dynamic_cast<const void*>(&b); } // interpreter-error {{constexpr function never produces a constant expression}} \
-                                                                                             // interpreter-note 2 {{subexpression not valid in a constant expression}}
+  constexpr const void* dyncast(const PolyBase& b) { return dynamic_cast<const void*>(&b); }
   constexpr int sub(const int (&a)[], const int (&b)[]) { return a-b; }
   constexpr const int* add(const int &a) { return &a+3; }
 
@@ -427,7 +427,7 @@ namespace InvalidConstexprFn {
   static_assert(get_derived_member(Derived{}) == 0);
   static_assert(virtual_call(PolyDerived{}) == 1);
   static_assert(type(PolyDerived{}) != nullptr);
-  static_assert(dyncast(PolyDerived{}) != nullptr); // interpreter-error {{static assertion expression is not an integral constant expression}} interpreter-note {{in call}}
+  static_assert(dyncast(PolyDerived{}) != nullptr);
   static_assert(sub(arr, arr) == 0);
   static_assert(add(arr[0]) == &arr[3]);
 }