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Document compat break between 16.0 and 16.1 from #34695 (#34904)

Fixes #34882
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gafter committed Apr 10, 2019
1 parent 703e87e commit 73f8d45101674cb229e934ecf9111a204e2569af
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  1. +10 −0 docs/compilers/CSharp/Compiler Breaking Changes - post VS2019.md
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## This document lists known breaking changes in Roslyn in *Visual Studio 2019 Update 1* and beyond compared to *Visual Studio 2019*.

*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. https://github.com/dotnet/roslyn/issues/34882 A new feature in C# `8.0` will permit using a constant pattern with an open type. For example, the following code will be permitted:
``` c#
bool M<T>(T t) => t is null;
```
However, in *Visual Studio 2019* we improperly permitted this to compile in language versions `7.0`, `7.1`, `7.2`, and `7.3`. In *Visual Studio 2019 Update 1* we will make it an error (as it was in *Visual Studio 2017*), and suggest updating to `preview` or `8.0`.

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