CSHARP-5258: Fix as variable name in two code paths.

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/ExpressionToAggregationExpressionTranslator.cs

@@ -19,6 +19,7 @@
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Serializers;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
+using MongoDB.Driver.Linq.Linq3Implementation.Misc;
 using MongoDB.Driver.Linq.Linq3Implementation.Serializers;
 
 namespace MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators
@@ -123,6 +124,15 @@ public static AggregationExpression TranslateLambdaBody(
                 asRoot ?
                     context.CreateSymbolWithVarName(parameterExpression, varName: "ROOT", parameterSerializer, isCurrent: true) :
                     context.CreateSymbol(parameterExpression, parameterSerializer, isCurrent: false);
+
+            return TranslateLambdaBody(context, lambdaExpression, parameterSymbol);
+        }
+
+        public static AggregationExpression TranslateLambdaBody(
+            TranslationContext context,
+            LambdaExpression lambdaExpression,
+            Symbol parameterSymbol)
+        {
             var lambdaContext = context.WithSymbol(parameterSymbol);
             var translatedBody = Translate(lambdaContext, lambdaExpression.Body);
 

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MethodTranslators/FirstOrLastMethodToAggregationExpressionTranslator.cs

@@ -13,6 +13,7 @@
 * limitations under the License.
 */
 
+using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
@@ -97,12 +98,13 @@ public static AggregationExpression Translate(TranslationContext context, Method
                 if (method.IsOneOf(__withPredicateMethods))
                 {
                     var predicateLambda = ExpressionHelper.UnquoteLambdaIfQueryableMethod(method, arguments[1]);
-                    var predicateTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, predicateLambda, itemSerializer, asRoot: false);
-                    var predicateParameter = predicateLambda.Parameters[0];
+                    var parameterExpression = predicateLambda.Parameters.Single();
+                    var parameterSymbol = context.CreateSymbol(parameterExpression, itemSerializer, isCurrent: false);
+                    var predicateTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, predicateLambda, parameterSymbol);
                     sourceAst = AstExpression.Filter(
                         input: sourceAst,
                         cond: predicateTranslation.Ast,
-                        @as: predicateParameter.Name);
+                        @as: parameterSymbol.Var.Name);
                 }
 
                 AstExpression ast;

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MethodTranslators/WhereMethodToAggregationExpressionTranslator.cs

@@ -59,7 +59,7 @@ public static AggregationExpression Translate(TranslationContext context, Method
                 var ast = AstExpression.Filter(
                     sourceTranslation.Ast,
                     predicateTranslation.Ast,
-                    predicateParameter.Name,
+                    @as: predicateSymbol.Var.Name,
                     limitTranslation?.Ast);
 
                 var resultSerializer = NestedAsQueryableSerializer.CreateIEnumerableOrNestedAsQueryableSerializer(expression.Type, itemSerializer);

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp5258Tests.cs

@@ -0,0 +1,72 @@
+/* Copyright 2010-present MongoDB Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System.Linq;
+using FluentAssertions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    public class CSharp5258Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Select_First_with_predicate_should_work()
+        {
+            var collection = GetCollection();
+
+            var queryable = collection.AsQueryable()
+                .Select(_x => _x.List.First(_y => _y > 2));
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, "{ $project : { _v : { $arrayElemAt : [{ $filter : { input : '$List', as : 'v__0', cond : { $gt : ['$$v__0', 2] } } }, 0]  }, _id : 0 } }");
+
+            var results = queryable.ToList();
+            results.Should().Equal(3, 4);
+        }
+
+        [Fact]
+        public void Select_Where_should_work()
+        {
+            var collection = GetCollection();
+
+            var queryable = collection.AsQueryable()
+                .Select(_x => _x.List.Where(_y => _y % 2 == 0));
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, "{ $project : { _v : { $filter : { input : '$List', as : 'v__0', cond : { $eq : [{ $mod : ['$$v__0', 2] }, 0] } } }, _id : 0 } }");
+
+            var results = queryable.ToList();
+            results.Count.Should().Be(2);
+            results[0].Should().Equal();
+            results[1].Should().Equal(2, 4, 6);
+        }
+
+        private IMongoCollection<C> GetCollection()
+        {
+            var collection = GetCollection<C>("test");
+            CreateCollection(
+                collection,
+                new C { Id = 1, List = [1, 3, 5] },
+                new C { Id = 2, List = [2, 4, 6] });
+            return collection;
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            public int[] List { get; set; }
+        }
+    }
+}