[clang][ExprConst] Consider integer pointers of value 0 nullptr (#150164)

When casting a 0 to a pointer type, the IsNullPtr flag was always set to
false, leading to weird results like a pointer with value 0 that isn't a
null pointer.

This caused

```c++
struct B { const int *p;};
template<B> void f() {}
template void f<B{nullptr}>();
template void f<B{fold(reinterpret_cast<int*>(0))}>();
```

to be valid code, since nullptr and (int*)0 aren't equal. This seems
weird and GCC doesn't behave like this.

1 files changed, 9 insertions, 5 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 34af9cc..3679327 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -9860,11 +9860,15 @@ bool PointerExprEvaluator::VisitCastExpr(const CastExpr *E) {
     if (Value.isInt()) {
       unsigned Size = Info.Ctx.getTypeSize(E->getType());
       uint64_t N = Value.getInt().extOrTrunc(Size).getZExtValue();
-      Result.Base = (Expr*)nullptr;
-      Result.InvalidBase = false;
-      Result.Offset = CharUnits::fromQuantity(N);
-      Result.Designator.setInvalid();
-      Result.IsNullPtr = false;
+      if (N == Info.Ctx.getTargetNullPointerValue(E->getType())) {
+        Result.setNull(Info.Ctx, E->getType());
+      } else {
+        Result.Base = (Expr *)nullptr;
+        Result.InvalidBase = false;
+        Result.Offset = CharUnits::fromQuantity(N);
+        Result.Designator.setInvalid();
+        Result.IsNullPtr = false;
+      }
       return true;
     } else {
       // In rare instances, the value isn't an lvalue.