Possible issue with constructor parameters

[BillWagner]

See https://github.com/dotnet/csharpstandard/pull/219/files#r1171850693

The wording doesn't describe if there is any distinction between optional and required parameters in a constructor and if that distinction is considered part of the constructor signature.

[jskeet]

Marking as a bug because I believe it is, but I won't put time into this until after we've shipped C# 7, as I don't believe we're in a worse position than we were before.