[clang][ExprConst] Add diagnostics for invalid binary arithmetic

... between unrelated declarations or literals.

Author: tbaederr

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -91,11 +91,15 @@ def note_constexpr_pointer_subtraction_zero_size : Note<
   "subtraction of pointers to type %0 of zero size">;
 def note_constexpr_pointer_comparison_unspecified : Note<
   "comparison between '%0' and '%1' has unspecified value">;
+def note_constexpr_pointer_arith_unspecified : Note<
+  "arithmetic involving '%0' and '%1' has unspecified value">;
 def note_constexpr_pointer_constant_comparison : Note<
   "comparison of numeric address '%0' with pointer '%1' can only be performed "
   "at runtime">;
 def note_constexpr_literal_comparison : Note<
   "comparison of addresses of literals has unspecified value">;
+def note_constexpr_literal_arith : Note<
+  "arithmetic on addresses of literals has unspecified value">;
 def note_constexpr_opaque_call_comparison : Note<
   "comparison against opaque constant address '%0' can only be performed at "
   "runtime">;

clang/lib/AST/ExprConstant.cpp

@@ -14600,8 +14600,21 @@ bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
         return Error(E);
       const Expr *LHSExpr = LHSValue.Base.dyn_cast<const Expr *>();
       const Expr *RHSExpr = RHSValue.Base.dyn_cast<const Expr *>();
-      if (!LHSExpr || !RHSExpr)
-        return Error(E);
+      if (!LHSExpr || !RHSExpr) {
+        std::string LHS = LHSValue.toString(Info.Ctx, E->getLHS()->getType());
+        std::string RHS = RHSValue.toString(Info.Ctx, E->getRHS()->getType());
+        Info.FFDiag(E, diag::note_constexpr_pointer_arith_unspecified)
+            << LHS << RHS;
+        return false;
+      }
+
+      if (ArePotentiallyOverlappingStringLiterals(Info, LHSValue, RHSValue)) {
+        std::string LHS = LHSValue.toString(Info.Ctx, E->getLHS()->getType());
+        std::string RHS = RHSValue.toString(Info.Ctx, E->getRHS()->getType());
+        Info.FFDiag(E, diag::note_constexpr_literal_arith) << LHS << RHS;
+        return false;
+      }
+
       const AddrLabelExpr *LHSAddrExpr = dyn_cast<AddrLabelExpr>(LHSExpr);
       const AddrLabelExpr *RHSAddrExpr = dyn_cast<AddrLabelExpr>(RHSExpr);
       if (!LHSAddrExpr || !RHSAddrExpr)