List patterns: no error is emitted when Length is missing or inaccessible

[sa-exe]

Version Used: main (27 Jan 2022, 3b32fc3)

Steps to Reproduce:

using System;

_ = new S() is [];

struct S
{
  public int this[Index i] => i.Value;
  // Introducing a private Length property won't change the result
  // private int Length => 42;
}

Expected Behavior: a compiler error is emitted saying that the Length property is required but missing/inaccessible.

Actual Behavior:
error CS7038: Failed to emit module '_': Unable to determine specific cause of the failure.

Relates to test plan #51289

[RikkiGibson]

fyi @alrz @jcouv

[jcouv]

Thanks @SomeAnonDev for reporting this. The fix was merged for 17.2p2.