Assertion hit in SyntheticBoundNodeFactory.Convert

[jcouv]

We can hit this assertion: Debug.Assert(useSiteInfo.Diagnostics.IsNullOrEmpty());
This is the second instance of this assertion getting hit (see issue #60060).

The following test crashes on the assertion:

        [Fact]
        public void TODO2()
        {
            string sourceRefLib = @"
public class Base { }
";

            var refLib = CreateEmptyCompilation(
                sourceRefLib,
                assemblyName: "RefLib",
                references: new[] { TestMetadata.Net20.mscorlib });

            refLib.VerifyEmitDiagnostics();

            string sourceMain = @"
public class Derived : Base { }
class C
{
    void M(Derived d)
    {
        _ = d is Base b;
    }
}
";

            var main = CreateEmptyCompilation(sourceMain, assemblyName: "Main",
                references: new MetadataReference[]
                {
                    new CSharpCompilationReference(refLib),
                    TestMetadata.Net451.mscorlib
                });

            var unifyReferenceWarning =
                // warning CS1701: Assuming assembly reference 'mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' used by 'RefLib' matches identity 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' of 'mscorlib', you may need to supply runtime policy
                Diagnostic(ErrorCode.WRN_UnifyReferenceMajMin).WithArguments("mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089", "RefLib", "mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089", "mscorlib");

            main.VerifyEmitDiagnostics(unifyReferenceWarning, unifyReferenceWarning, unifyReferenceWarning, unifyReferenceWarning, unifyReferenceWarning, unifyReferenceWarning);
        }

SyntheticBoundNodeFactory.Convert(TypeSymbol type, BoundExpression arg) line 1306
PatternLocalRewriter.TryLowerTypeTestAndCast(BoundDagTest test, BoundDagEvaluation evaluation, BoundExpression& sideEffect, BoundExpression& testExpression) line 491
IsPatternExpressionLinearLocalRewriter.ProduceLinearTestSequence(BoundDecisionDagNode node, LabelSymbol whenTrueLabel, LabelSymbol whenFalseLabel) line 254
IsPatternExpressionLinearLocalRewriter.LowerIsPatternAsLinearTestSequence(BoundIsPatternExpression isPatternExpression, BoundDecisionDag decisionDag, LabelSymbol whenTrueLabel, LabelSymbol whenFalseLabel) line 229
LocalRewriter.VisitIsPatternExpression(BoundIsPatternExpression node) line 24