[clang][bytecode] Diagnose comparisons of unrelated zero-sized pointers (#140695)

Author: tbaederr

clang/lib/AST/ByteCode/Interp.h

@@ -1135,6 +1135,14 @@ inline bool CmpHelperEQ<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
     }
   }
 
+  if (LHS.isUnknownSizeArray() && RHS.isUnknownSizeArray()) {
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.FFDiag(Loc, diag::note_constexpr_pointer_comparison_zero_sized)
+        << LHS.toDiagnosticString(S.getASTContext())
+        << RHS.toDiagnosticString(S.getASTContext());
+    return false;
+  }
+
   S.Stk.push<BoolT>(BoolT::from(Fn(ComparisonCategoryResult::Unordered)));
   return true;
 }

clang/test/AST/ByteCode/cxx11.cpp

@@ -243,3 +243,10 @@ namespace Volatile {
                   // both-note {{in call to 'f(0)'}}
   };
 }
+
+namespace ZeroSizeCmp {
+  extern void (*start[])();
+  extern void (*end[])();
+  static_assert(&start != &end, ""); // both-error {{constant expression}} \
+                                     // both-note {{comparison of pointers '&start' and '&end' to unrelated zero-sized objects}}
+}