Add a special case to differentiate begin/end calls

lib/Differentiator/ReverseModeVisitor.cpp

@@ -1565,6 +1565,23 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
       return StmtDiff(Call, zero);
     }
 
+    std::string FDName = FD->getNameAsString();
+    if (FDName == "begin" || FDName == "end") {
+      const Expr* arg = nullptr;
+      if (const auto* MCE = dyn_cast<CXXMemberCallExpr>(CE))
+        arg = MCE->getImplicitObjectArgument();
+      else
+        arg = CE->getArg(0);
+      if (const auto* CXXCE = dyn_cast<CXXConstructExpr>(arg))
+        arg = CXXCE->getArg(0);
+      StmtDiff argDiff = Visit(arg);
+      llvm::SmallVector<Expr*, 1> params{argDiff.getExpr()};
+      llvm::SmallVector<Expr*, 1> paramsDiff{argDiff.getExpr_dx()};
+      Expr* call = GetFunctionCall(FDName, "std", params);
+      Expr* callDiff = GetFunctionCall(FDName, "std", paramsDiff);
+      return {call, callDiff};
+    }
+
     auto NArgs = FD->getNumParams();
     // If the function has no args and is not a member function call then we
     // assume that it is not related to independent variables and does not