Query :: some queries with Queryable.Cast<T> don't work

[maumar]

Repro:

var query = from Post p in ctx.Posts
            select p.PostId;

var result = query.ToList();

the compiler introduces a Cast() (because type of "p" has been explicitly specified) relinq turns this into a subquery like so:

from p in  { from p' in ctx.Post select p' => Cast<Post>() } select p.PostId

then we lift the subquery:

from p in ctx.Posts select p.PostId => Cast<Post>()

this fails with the following:

Unhandled Exception: System.InvalidOperationException: No coercion operator is defined between types 'System.Int32' and 'Repro3499.Post'.
at System.Linq.Expressions.Expression.GetUserDefinedCoercionOrThrow(ExpressionType coercionType, Expression expression, Type convertToType)
at System.Linq.Expressions.Expression.Convert(Expression expression, Type type, MethodInfo method)
at Remotion.Linq.Clauses.ResultOperators.CastResultOperator.GetOutputDataInfo(IStreamedDataInfo inputInfo)
at Remotion.Linq.QueryModel.b__0(IStreamedDataInfo current, ResultOperatorBase resultOperator)
at System.Linq.Enumerable.Aggregate[TSource,TAccumulate](IEnumerable1 source, TAccumulate seed, Func3 func)
at Remotion.Linq.QueryModel.GetOutputDataInfo()
at Microsoft.Data.Entity.Query.EntityQueryModelVisitor.SingleResultToSequence(QueryModel queryModel) in D:\k\EntityFramework\src\EntityFramework.Core\Query\EntityQueryModelVisitor.cs:line 249
at Microsoft.Data.Entity.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel) in D:\k\EntityFramework\src\EntityFramework.Core\Query\EntityQueryModelVisitor.cs:line 163
at Microsoft.Data.Entity.Storage.Database.CompileQuery[TResult](QueryModel queryModel) in D:\k\EntityFramework\src\EntityFramework.Core\Storage\Database.cs:line 68
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at Microsoft.Data.Entity.Query.Internal.QueryCompiler.<>c__DisplayClass19_01.<CompileQuery>b__0() in D:\k\EntityFramework\src\EntityFramework.Core\Query\Internal\QueryCompiler.cs:line 183 at Microsoft.Data.Entity.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func1 compiler) in D:\k\EntityFramework\src\EntityFramework.Core\Query\Internal\CompiledQueryCache.cs:line 32
at Microsoft.Data.Entity.Query.Internal.QueryCompiler.CompileQuery[TResult](Expression query) in D:\k\EntityFramework\src\EntityFramework.Core\Query\Internal\QueryCompiler.cs:line 138
at Microsoft.Data.Entity.Query.Internal.QueryCompiler.Execute[TResult](Expression query) in D:\k\EntityFramework\src\EntityFramework.Core\Query\Internal\QueryCompiler.cs:line 84
at Microsoft.Data.Entity.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression) in D:\k\EntityFramework\src\EntityFramework.Core\Query\Internal\EntityQueryProvider.cs:line 37
at Remotion.Linq.QueryableBase1.GetEnumerator() at System.Collections.Generic.List1..ctor(IEnumerable`1 collection)

In this case the cast is redundant, but if we project a property from a derived type like so:

from VipCustomer c in ctx.Customers
select c.SomeVipCustomerProperty

the cast needs to be preserved

[mikary]

Verified that this is still an issue. May be related to / relevant for #4875

[smitpatel]

@maumar - This is somewhat related to our discussion from today.

[maumar]

it might also be due to incorrect assumption in the QM lifting, i.e. lifting casts is not always safe

[maumar]

Fixed in aacb31f