[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

Author: cor3ntin

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) {

clang/test/AST/ByteCode/const-eval.c

@@ -51,8 +51,6 @@ struct s {
 };
 
 EVAL_EXPR(19, ((int)&*(char*)10 == 10 ? 1 : -1));
-// ref-error@-1 {{expression is not an integer constant expression}} \
-// ref-note@-1 {{dereferencing a null pointer}}
 
 #ifndef NEW_INTERP
 EVAL_EXPR(20, __builtin_constant_p(*((int*) 10)));

clang/test/Sema/const-eval.c

@@ -41,9 +41,6 @@ struct s {
 };
 
 EVAL_EXPR(19, ((int)&*(char*)10 == 10 ? 1 : -1));
-// expected-error@-1 {{not an integer constant expression}} \
-// expected-note@-1 {{dereferencing a null pointer is not allowed in a constant expression}}
-
 
 EVAL_EXPR(20, __builtin_constant_p(*((int*) 10)));
 
@@ -153,3 +150,7 @@ struct PR35214_X {
 int PR35214_x;
 int PR35214_y = ((struct PR35214_X *)&PR35214_x)->arr[1]; // expected-error {{not a compile-time constant}}
 int *PR35214_z = &((struct PR35214_X *)&PR35214_x)->arr[1]; // ok, &PR35214_x + 2
+
+
+int * GH149500_p = &(*(int *)0x400);
+static const void *GH149500_q = &(*(const struct sysrq_key_op *)0);

clang/test/SemaCXX/constant-expression-cxx14.cpp

@@ -1445,3 +1445,8 @@ static_assert(test_member_null(), "");
 
 }
 }
+
+namespace GH149500 {
+  unsigned int * p = &(*(unsigned int *)0x400);
+  static const void *q = &(*(const struct sysrq_key_op *)0);
+}