[clang][Interp] Fix ImplicitValueInitExprs for pointer types

This previously ran into an "unknown type" assertion when trying to emit
a 'Zero' op for a pointer type. Emit a NullPtr op instead.

Differential Revision: https://reviews.llvm.org/D137235

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -279,10 +279,15 @@ bool ByteCodeExprGen<Emitter>::VisitPointerArithBinOp(const BinaryOperator *E) {
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitImplicitValueInitExpr(const ImplicitValueInitExpr *E) {
-  if (std::optional<PrimType> T = classify(E))
-    return this->emitZero(*T, E);
+  std::optional<PrimType> T = classify(E);
 
-  return false;
+  if (!T)
+    return false;
+
+  if (E->getType()->isPointerType())
+    return this->emitNullPtr(E);
+
+  return this->emitZero(*T, E);
 }
 
 template <class Emitter>

clang/test/AST/Interp/literals.cpp

@@ -49,7 +49,10 @@ namespace IntegralCasts {
   static_assert(!nu, "");
 };
 
-
+constexpr int UninitI; // expected-error {{must be initialized by a constant expression}} \
+                       // ref-error {{must be initialized by a constant expression}}
+constexpr int *UninitPtr; // expected-error {{must be initialized by a constant expression}} \
+                          // ref-error {{must be initialized by a constant expression}}
 
 constexpr bool getTrue() { return true; }
 constexpr bool getFalse() { return false; }