[equmagg]

SizeOptIListSelectIterator<TSource, TResult>.ToList() checks the destination list’s Count immediately after constructing it with a capacity. Since Count is zero, the loop never executes and the override returns an empty list for a nonempty source.

This is a dead path only reachable via reflection,
but it is not documented as dead and might be used in the future, so better safe then sorry

repro

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;

IEnumerable<int> query = new[] { 1, 2, 3 }.Select(x => x * 10);

MethodInfo method = query.GetType().GetMethod(
    "ToList",
    BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly)
    ?? throw new InvalidOperationException("ToList override not found");

List<int> result = (List<int>)(method.Invoke(query, null)
    ?? throw new InvalidOperationException("ToList returned null"));

Console.WriteLine(query.GetType().Name);
Console.WriteLine($"[{string.Join(", ", result)}]"); // expect [10, 20, 30]

resolves #137126

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
12 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[jkotas]

This is a dead path only reachable via reflection

Is this path dead even in System.Linq.Enumerable.IsSizeOptimized=true configuration?

[equmagg]

Yes, it is not called from the public surface, even with IsSizeOptimized=true