[Fact]
        public void LambdaReturnValue_NestedNullability_Ref()
        {
            var source =
@"delegate ref V D<T, U, V>(ref T t, ref U u);
interface I<T> { }
interface IIn<in T> { }
interface IOut<out T> { }
class C
{
    static V F<T, U, V>(D<T, U, V> d) => throw null;
    static void G(bool b)
    {
        // I<object>
        F((ref I<object?> a1, ref I<object?> b1) => { if (b) return ref a1; return ref b1; });
        F((ref I<object?> a2, ref I<object> b2) => { if (b) return ref a2; return ref b2; });
        F((ref I<object> a3, ref I<object?> b3) => { if (b) return ref a3; return ref b3; });
        F((ref I<object> a4, ref I<object> b4) => { if (b) return ref a4; return ref b4; });
        // IIn<object>
        F((ref IIn<object?> c1, ref IIn<object?> d1) => { if (b) return ref c1; return ref d1; });
        F((ref IIn<object?> c2, ref IIn<object> d2) => { if (b) return ref c2; return ref d2; });
        F((ref IIn<object> c3, ref IIn<object?> d3) => { if (b) return ref c3; return ref d3; });
        F((ref IIn<object> c4, ref IIn<object> d4) => { if (b) return ref c4; return ref d4; });
        // IOut<object>
        F((ref IOut<object?> e1, ref IOut<object?> f1) => { if (b) return ref e1; return ref f1; });
        F((ref IOut<object?> e2, ref IOut<object> f2) => { if (b) return ref e2; return ref f2; });
        F((ref IOut<object> e3, ref IOut<object?> f3) => { if (b) return ref e3; return ref f3; });
        F((ref IOut<object> e4, ref IOut<object> f4) => { if (b) return ref e4; return ref f4; });
    }
}";
            var comp = CreateCompilation(new[] { source }, options: WithNonNullTypesTrue());
            // https://github.com/dotnet/roslyn/issues/30432: Report warnings for combinations of `IIn<object?>` and `IIn<object!>`
            // and combinations of  `IOut<object?>` and `IOut<object!>`.
            comp.VerifyDiagnostics(
                // (12,11): warning CS8622: Nullability of reference types in type of parameter 'a2' of 'lambda expression' doesn't match the target delegate 'D<I<object>, I<object>, I<object>>'.
                //         F((ref I<object?> a2, ref I<object> b2) => { if (b) return ref a2; return ref b2; });
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, "(ref I<object?> a2, ref I<object> b2) => { if (b) return ref a2; return ref b2; }").WithArguments("a2", "lambda expression", "D<I<object>, I<object>, I<object>>").WithLocation(12, 11),
                // (12,11): warning CS8622: Nullability of reference types in type of parameter 'b2' of 'lambda expression' doesn't match the target delegate 'D<I<object>, I<object>, I<object>>'.
                //         F((ref I<object?> a2, ref I<object> b2) => { if (b) return ref a2; return ref b2; });
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, "(ref I<object?> a2, ref I<object> b2) => { if (b) return ref a2; return ref b2; }").WithArguments("b2", "lambda expression", "D<I<object>, I<object>, I<object>>").WithLocation(12, 11),
                // (13,11): warning CS8622: Nullability of reference types in type of parameter 'a3' of 'lambda expression' doesn't match the target delegate 'D<I<object>, I<object>, I<object>>'.
                //         F((ref I<object> a3, ref I<object?> b3) => { if (b) return ref a3; return ref b3; });
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, "(ref I<object> a3, ref I<object?> b3) => { if (b) return ref a3; return ref b3; }").WithArguments("a3", "lambda expression", "D<I<object>, I<object>, I<object>>").WithLocation(13, 11),
                // (13,11): warning CS8622: Nullability of reference types in type of parameter 'b3' of 'lambda expression' doesn't match the target delegate 'D<I<object>, I<object>, I<object>>'.
                //         F((ref I<object> a3, ref I<object?> b3) => { if (b) return ref a3; return ref b3; });
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, "(ref I<object> a3, ref I<object?> b3) => { if (b) return ref a3; return ref b3; }").WithArguments("b3", "lambda expression", "D<I<object>, I<object>, I<object>>").WithLocation(13, 11));
            comp.VerifyTypes();
        }