[Clang] Be less strict about diagnosing null pointer dereference. (#149648)

In #143667, we made constant evaluation fail on `*null_ptr`, as this is
UB. However, `&(*(foo*)0)` seems to be a common pattern, which made
#143667 too disruptive.

So instead of failing the evaluation, we note the UB, which let clang
recovers when checking for constant initialization.

Fixes #149500

1 files changed, 7 insertions, 3 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index cfc4729..e575405 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -9346,9 +9346,13 @@ bool LValueExprEvaluator::VisitUnaryDeref(const UnaryOperator *E) {
   // [C++26][expr.unary.op]
   // If the operand points to an object or function, the result
   // denotes that object or function; otherwise, the behavior is undefined.
-  return Success &&
-         (!E->getType().getNonReferenceType()->isObjectType() ||
-          findCompleteObject(Info, E, AK_Dereference, Result, E->getType()));
+  // Because &(*(type*)0) is a common pattern, we do not fail the evaluation
+  // immediately.
+  if (!Success || !E->getType().getNonReferenceType()->isObjectType())
+    return Success;
+  return bool(findCompleteObject(Info, E, AK_Dereference, Result,
+                                 E->getType())) ||
+         Info.noteUndefinedBehavior();
 }
 
 bool LValueExprEvaluator::VisitUnaryReal(const UnaryOperator *E) {