Compiler Breaking Changes VS2017

This document lists known breaking changes in Roslyn 2.0 (VS2017) from Roslyn 1.* (VS2015) and native C# compiler (VS2013 and previous).

Breaks are formatted with a monotonically increasing numbered list to allow them to referenced via shorthand (i.e., "known break #1"). Each entry should include a short description of the break, followed by either a link to the issue describing the full details of the break or the full details of the break inline.

1. Previously, we would not find a best type among multiple types (for ternary expressions and method type inference) when the types differed only in dynamic-ness and would involve some nesting. For example, we could not find a best type between KeyValuePair<dynamic, object> and KeyValuePair<object, dynamic>. Starting with VS 2017, we can find a best type (we merge dynamic-ness). In this example, KeyValuePair<dynamic, dynamic> is the best type. This can affect overload resolution. For instance, allowing a better overload to be picked, which was previously discarded. See issues #12585, #14247, #14213, and their corresponding PRs for more details.