Errors for scoped differences in override indexers are missing in some cases

[RikkiGibson]

public ref struct RS { }

public class Base
{
    public virtual RS this[scoped RS rs] { get => default; }
    public virtual RS this[scoped RS rs, int _] { get => default; set { }}
}

public class C : Base
{
    public override RS this[RS rs] { get => default; } // error CS8987: The 'scoped' modifier of parameter 'rs' doesn't match overridden or implemented member.
    public override RS this[RS rs, int _] { get => default; set { } } // no error
}

Expected: Both commented lines have errors
Actual: Only the first commented line has an error. It seems like adding a set accessor to a similar indexer declaration removes the error.

[RikkiGibson]

I think the issue is here. It is assumed that if setter is present, then only setter needs to be checked, but for the scoped checks to actually run, there needs to be a ref or ref struct output, as well as input, which is only found on the getter.

roslyn/src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol_ImplementationChecks.cs

Lines 1097 to 1122 in f8d563c

	if (overridingProperty.GetMethod is object)
	{
	MethodSymbol overriddenGetMethod = overriddenProperty.GetOwnOrInheritedGetMethod();
	checkValidMethodOverride(
	overridingProperty.GetMethod.GetFirstLocation(),
	overriddenGetMethod,
	overridingProperty.GetMethod,
	diagnostics,
	checkReturnType: true,
	// Don't check parameters on the getter if there is a setter
	// because they will be a subset of the setter
	checkParameters: overridingProperty.SetMethod is null ||
	overriddenGetMethod?.AssociatedSymbol != overriddenProperty ||
	overriddenProperty.GetOwnOrInheritedSetMethod()?.AssociatedSymbol != overriddenProperty);
	}
	if (overridingProperty.SetMethod is object)
	{
	var ownOrInheritedOverriddenSetMethod = overriddenProperty.GetOwnOrInheritedSetMethod();
	checkValidMethodOverride(
	overridingProperty.SetMethod.GetFirstLocation(),
	ownOrInheritedOverriddenSetMethod,
	overridingProperty.SetMethod,
	diagnostics,
	checkReturnType: false,
	checkParameters: true);