Feature 'using declarations' is not available in C# 7.3. Please use language version 8.0 or greater - Error on one machine but works on another

When using Visual Studio Enterprise 16.3.7 on two separate machines one builds fine and the other machine throws the error.

Feature 'using declarations' is not available in C# 7.3. Please use language version 8.0 or greater.

This can easily be solved on the none working machine by setting LangVersion in .csproj as suggested here https://stackoverflow.com/a/48085575/3850405 or let Visual Studio automatically fix it like the print screen above.

<LangVersion>8.0</LangVersion>

What I can't understand is why one machine builds fine without this line in .csproj and the other machine needs it?

I received the same error, but I had simply forgotten to include the

<LangVersion>8.0</LangVersion>

attribute in ALL the .csproj files in the solution. The following is my current c# 8 setup:

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <Nullable>enable</Nullable>
    <NullableContextOptions>enable</NullableContextOptions>
  </PropertyGroup>

I found the following documents to be the most helpful when migrating from core 2.2 to 3.x.

MSDN 2.2 -> 3.0

MSDN 3.0 -> 3.1