[clang][ExprConst] Reject field access with nullptr base (#113885)

Reject them if the base is null, not only if the entire pointer is null.

Fixes #113821

Author: tbaederr

clang/docs/ReleaseNotes.rst

@@ -148,6 +148,17 @@ C++ Specific Potentially Breaking Changes
     // Now diagnoses with an error.
     void f(int& i [[clang::lifetimebound]]);
 
+- Clang now rejects all field accesses on null pointers in constant expressions. The following code
+  used to work but will now be rejected:
+
+  .. code-block:: c++
+
+    struct S { int a; int b; };
+    constexpr const int *p = &((S*)nullptr)->b;
+
+  Previously, this code was erroneously accepted.
+
+
 ABI Changes in This Version
 ---------------------------
 

clang/lib/AST/ExprConstant.cpp

@@ -3261,8 +3261,8 @@ static bool HandleLValueDirectBase(EvalInfo &Info, const Expr *E, LValue &Obj,
     RL = &Info.Ctx.getASTRecordLayout(Derived);
   }
 
-  Obj.getLValueOffset() += RL->getBaseClassOffset(Base);
   Obj.addDecl(Info, E, Base, /*Virtual*/ false);
+  Obj.getLValueOffset() += RL->getBaseClassOffset(Base);
   return true;
 }
 
@@ -3286,8 +3286,8 @@ static bool HandleLValueBase(EvalInfo &Info, const Expr *E, LValue &Obj,
   // Find the virtual base class.
   if (DerivedDecl->isInvalidDecl()) return false;
   const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(DerivedDecl);
-  Obj.getLValueOffset() += Layout.getVBaseClassOffset(BaseDecl);
   Obj.addDecl(Info, E, BaseDecl, /*Virtual*/ true);
+  Obj.getLValueOffset() += Layout.getVBaseClassOffset(BaseDecl);
   return true;
 }
 
@@ -3330,8 +3330,8 @@ static bool HandleLValueMember(EvalInfo &Info, const Expr *E, LValue &LVal,
   }
 
   unsigned I = FD->getFieldIndex();
-  LVal.adjustOffset(Info.Ctx.toCharUnitsFromBits(RL->getFieldOffset(I)));
   LVal.addDecl(Info, E, FD);
+  LVal.adjustOffset(Info.Ctx.toCharUnitsFromBits(RL->getFieldOffset(I)));
   return true;
 }
 

clang/test/CXX/expr/expr.const/p2-0x.cpp

@@ -188,7 +188,7 @@ namespace UndefinedBehavior {
 
   namespace Ptr {
     struct A {};
-    struct B : A { int n; };
+    struct B : A { int n; int m; };
     B a[3][3];
     constexpr B *p = a[0] + 4; // expected-error {{constant expression}} expected-note {{element 4 of array of 3 elements}}
     B b = {};
@@ -204,6 +204,7 @@ namespace UndefinedBehavior {
     static_assert((A*)nb == 0, "");
     static_assert((B*)na == 0, "");
     constexpr const int &nf = nb->n; // expected-error {{constant expression}} expected-note {{cannot access field of null pointer}}
+    constexpr const int &mf = nb->m; // expected-error {{constant expression}} expected-note {{cannot access field of null pointer}}
     constexpr const int *np1 = (int*)nullptr + 0; // ok
     constexpr const int *np2 = &(*(int(*)[4])nullptr)[0]; // ok
     constexpr const int *np3 = &(*(int(*)[4])nullptr)[2]; // expected-error {{constant expression}} expected-note {{cannot perform pointer arithmetic on null pointer}}