[clang][Interp] reinterpret casts aren't always fatal

clang/lib/AST/Interp/Compiler.cpp

@@ -426,7 +426,7 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
     if (CE->getType()->isAtomicType()) {
       if (!this->discard(SubExpr))
         return false;
-      return this->emitInvalidCast(CastKind::Reinterpret, CE);
+      return this->emitInvalidCast(CastKind::Reinterpret, /*Fatal=*/true, CE);
     }
 
     if (DiscardResult)
@@ -2465,10 +2465,13 @@ bool Compiler<Emitter>::VisitCXXThrowExpr(const CXXThrowExpr *E) {
 template <class Emitter>
 bool Compiler<Emitter>::VisitCXXReinterpretCastExpr(
     const CXXReinterpretCastExpr *E) {
-  if (!this->discard(E->getSubExpr()))
+  const Expr *SubExpr = E->getSubExpr();
+
+  bool TypesMatch = classify(E) == classify(SubExpr);
+  if (!this->emitInvalidCast(CastKind::Reinterpret, /*Fatal=*/!TypesMatch, E))
     return false;
 
-  return this->emitInvalidCast(CastKind::Reinterpret, E);
+  return this->delegate(SubExpr);
 }
 
 template <class Emitter>

clang/lib/AST/Interp/Interp.h

@@ -2787,13 +2787,16 @@ inline bool Unsupported(InterpState &S, CodePtr OpPC) {
 inline bool Error(InterpState &S, CodePtr OpPC) { return false; }
 
 /// Same here, but only for casts.
-inline bool InvalidCast(InterpState &S, CodePtr OpPC, CastKind Kind) {
+inline bool InvalidCast(InterpState &S, CodePtr OpPC, CastKind Kind,
+                        bool Fatal) {
   const SourceLocation &Loc = S.Current->getLocation(OpPC);
 
   // FIXME: Support diagnosing other invalid cast kinds.
-  if (Kind == CastKind::Reinterpret)
-    S.FFDiag(Loc, diag::note_constexpr_invalid_cast)
+  if (Kind == CastKind::Reinterpret) {
+    S.CCEDiag(Loc, diag::note_constexpr_invalid_cast)
         << static_cast<unsigned>(Kind) << S.Current->getRange(OpPC);
+    return !Fatal;
+  }
   return false;
 }
 

clang/lib/AST/Interp/Opcodes.td

@@ -739,7 +739,7 @@ def Invalid : Opcode {}
 def Unsupported : Opcode {}
 def Error : Opcode {}
 def InvalidCast : Opcode {
-  let Args = [ArgCastKind];
+  let Args = [ArgCastKind, ArgBool];
 }
 
 def InvalidDeclRef : Opcode {

clang/test/AST/Interp/codegen.cpp

@@ -31,3 +31,16 @@ namespace BaseClassOffsets {
   // CHECK: @_ZN16BaseClassOffsets1bE = global ptr getelementptr (i8, ptr @_ZN16BaseClassOffsets1cE, i64 4), align 8
   B* b = &c;
 }
+
+namespace reinterpretcast {
+  const unsigned int n = 1234;
+  extern const int &s = reinterpret_cast<const int&>(n);
+  // CHECK: @_ZN15reinterpretcastL1nE = internal constant i32 1234, align 4
+  // CHECK: @_ZN15reinterpretcast1sE = constant ptr @_ZN15reinterpretcastL1nE, align 8
+
+  void *f1(unsigned long l) {
+    return reinterpret_cast<void *>(l);
+  }
+  // CHECK: define {{.*}} ptr @_ZN15reinterpretcast2f1Em
+  // CHECK: inttoptr
+}