List patterns: x is [..var y] with a custom collection type causes a crash

[sa-exe]

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

Steps to Reproduce:

using System;

var x = new S();
_ = x is [..var y];

struct S
{
  public int Length { get; }
  public int this[int i] => 42;
  public S this[Range r] => default;
}

Expected Behavior: the code compiles

Actual Behavior:

System.InvalidOperationException: This program location is thought to be unreachable.
   at Microsoft.CodeAnalysis.CSharp.Binder.MakeIsPatternExpression(SyntaxNode node, BoundExpression expression, BoundPattern pattern, TypeSymbol boolType, Boolean hasErrors, BindingDiagnosticBag diagnostics) in /_/src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs:line 60
   at Microsoft.CodeAnalysis.CSharp.Binder.BindIsPatternExpression(IsPatternExpressionSyntax node, BindingDiagnosticBag diagnostics) in /_/src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs:line 20
...

https://sharplab.io/#v2:EYLgtghglgdgNAFxAJwK7wCYgNQB8ACATAIwCwAUBQG4TIAEAHnQLx0wCmA7nQMoAUASgDcFAPotGdKAGc6AbQB0CmvQCeAXRGVy0hGgDGCXhQDeFOnXwBmKTCMAZdjADmCABZ0TdZ+wRC6AL7mljawRu4ycmFS6iwAfHQALIRaFta8dBHScgBKEC7sdMixzAkY7ABmEKgANn4UAUA==