Unexpected nullability warnings for conversions from base class to derived class

[AlekseyTs]

        [Fact]
        public void Test()
        {
            var source = @"
class Outer : Base
{
    void M1(Base? x1, Base y1)
    {
        if (x1 == null) return;
        Outer y = x1;
        x1.ToString();
        y = (Outer)x1;
        y = y1;
    }
}

class Base {}
";

            CreateCompilation(source, options: WithNonNullTypesTrue()).VerifyDiagnostics(
                // (7,19): error CS0266: Cannot implicitly convert type 'Base' to 'Outer'. An explicit conversion exists (are you missing a cast?)
                //         Outer y = x1;
                Diagnostic(ErrorCode.ERR_NoImplicitConvCast, "x1").WithArguments("Base", "Outer").WithLocation(7, 19),
                // (7,19): warning CS8619: Nullability of reference types in value of type 'Base' doesn't match target type 'Outer'.
                //         Outer y = x1;
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInAssignment, "x1").WithArguments("Base", "Outer").WithLocation(7, 19),
                // (10,13): error CS0266: Cannot implicitly convert type 'Base' to 'Outer'. An explicit conversion exists (are you missing a cast?)
                //         y = y1;
                Diagnostic(ErrorCode.ERR_NoImplicitConvCast, "y1").WithArguments("Base", "Outer").WithLocation(10, 13),
                // (10,13): warning CS8619: Nullability of reference types in value of type 'Base' doesn't match target type 'Outer'.
                //         y = y1;
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInAssignment, "y1").WithArguments("Base", "Outer").WithLocation(10, 13)
                );
        }

[cston]

Fixed in #31499.