VB: User-defined conversion rejected when its "in" conversion is narrowing and its "out" is widening

[jcouv]

Conversions fail (but are expected to succeed) when they involve:

• narrowing as "in" conversion (such as int to byte, or a narrowing tuple conversion (int, object) to (int, string))
• user-defined conversion
• widening conversion (such as widening nullable conversion

This issue exists in Roslyn and the native compiler.

Structure C
    Shared Sub Main()
        'Dim x As C = (1, Nothing)
        'Dim y As C? = (2, Nothing)

        Dim a As Integer = 1
        Dim b As Integer? = 1
        Dim x1 As C = a
        Dim y1 As C? = a
        Dim z1 As C? = b

    End Sub
    'Public Shared Widening Operator CType(ByVal d As (Integer, String)) As C
    '    Return New C()
    'End Operator


    Public Shared Widening Operator CType(ByVal d As Byte) As C
        Return New C()
    End Operator

End Structure

The issue was discovered in testing this PR: #18416 (comment)