Assert fails in `NullableWalker.AsMemberOfType`

[jjonescz]

Version Used: 20250912.15 (d92fd99, main), Debug.

Steps to Reproduce:

.NET Lab

using System;

class C
{
    bool M(ReadOnlySpan<int> a, Span<int> b)
    {
        return b == a;
    }
}

(Target framework: net9.0)

This assert fails:

roslyn/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

Line 8840 in d92fd99

Debug.Assert(false);

System.InvalidOperationException: Assertion failed
   at System.Diagnostics.Debug.Fail(String message, String detailMessage)
   at System.Diagnostics.Debug.Assert(Boolean condition)
   at Microsoft.CodeAnalysis.CSharp.NullableWalker.AsMemberOfType(TypeSymbol type, Symbol symbol)
   at Microsoft.CodeAnalysis.CSharp.NullableWalker.ReInferBinaryOperator(SyntaxNode syntax, MethodSymbol method, BoundExpression leftOperand, BoundExpression rightOperand, TypeWithState leftUnderlyingType, TypeWithState rightUnderlyingType)

[RikkiGibson]

Added small subset of the stack trace in description.

Interesting that a == b does not fail the assertion here.

[jjonescz]

We seem to be trying to make operator ==(ROS<int>, ROS<int>) a member of type Span<int> which is of course not possible. Probably a == b works because we will have ReadOnlySpan<int> instead or some such. Might be related to first-class span conversions.