Confirm expectations in TypeParameter_Return_01 unit-test

[AlekseyTs]

        [Fact]
        public void TypeParameter_Return_01()
        {
            var source =
@"
class C
{
    static U F1<T, U>(T t) => (U)(object)t; // 1 and 2
    static U F2<T, U>(T t) where U : class => (U)(object)t; // 3, 4 and 5
    static U F3<T, U>(T t) where U : struct => (U)(object)t; // 5
    static U F4<T, U>(T t) where T : class => (U)(object)t;
    static U F5<T, U>(T t) where T : struct => (U)(object)t;
}";

            var comp = CreateCompilation(new[] { source, NonNullTypesTrue, NonNullTypesAttributesDefinition });
            // PROTOTYPE(NullableReferenceTypes): Errors are different than expected.
            comp.VerifyDiagnostics(
                // (4,34): warning CS8600: Converting null literal or possible null value to non-nullable type.
                //     static U F1<T, U>(T t) => (U)(object)t;
                Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "(object)t").WithLocation(4, 34),
                // (5,50): warning CS8600: Converting null literal or possible null value to non-nullable type.
                //     static U F2<T, U>(T t) where U : class => (U)(object)t;
                Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "(object)t").WithLocation(5, 50),
                // (5,47): warning CS8600: Converting null literal or possible null value to non-nullable type.
                //     static U F2<T, U>(T t) where U : class => (U)(object)t;
                Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "(U)(object)t").WithLocation(5, 47),
                // (5,47): warning CS8603: Possible null reference return.
                //     static U F2<T, U>(T t) where U : class => (U)(object)t;
                Diagnostic(ErrorCode.WRN_NullReferenceReturn, "(U)(object)t").WithLocation(5, 47),
                // (6,51): warning CS8600: Converting null literal or possible null value to non-nullable type.
                //     static U F3<T, U>(T t) where U : struct => (U)(object)t;
                Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "(object)t").WithLocation(6, 51)
            );
        }