Mono build is broken on OSX due to incorrect FrameworkPathOverride paths

[baronfel]

When building via msbuild or dotnet build or dotnet msbuild on an OSX system, any net4XX-targeting build fails because the FSharpBuild.Directory.Build.props hard-code mono paths that are standard on Linux, not on OSX. the meaningful lines are 45-50:

 <!-- mono -->
  <PropertyGroup Condition="'$(OS)' == 'Unix'">
    <MonoPackaging Condition="$(TargetFramework.StartsWith('net4'))">true</MonoPackaging>
    <FrameworkPathOverride Condition="'$(TargetFramework)' == 'net45'">/usr/lib/mono/4.5-api</FrameworkPathOverride>
    <FrameworkPathOverride Condition="'$(TargetFramework)' == 'net46'">/usr/lib/mono/4.6-api</FrameworkPathOverride>
  </PropertyGroup>

Repro steps

Provide the steps required to reproduce the problem

1. Clone down latest master
2. run dotnet build -f net462 src/fsharp/FSharp.Build/FSharp.Build.fsproj from the root directory

Expected behavior

The FSharp.Build dll is build for the net462 TFM.

Actual behavior

The FSharp.Build dll fails to build because it first builds FSharp.Core, and attempts to locate explicit references via the hard-coded /usr/lib/mono/4.5-api path. An example of the failing resolution is:

12:48:19.065  2:17>/Users/chethusk/oss/fsharp/artifacts/toolset/dotnet/sdk/2.1.500/Microsoft.Common.CurrentVersion.targets(2110,5): warning MSB3245: Could not resolve this reference. Could not locate the assembly "System". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors. [/Users/chethusk/oss/fsharp/src/fsharp/FSharp.Core/FSharp.Core.fsproj]
                             For SearchPath "{TargetFrameworkDirectory}". (TaskId:115)
                             Considered "/usr/lib/mono/4.5-api/System.winmd", but it didn't exist. (TaskId:115)
                             Considered "/usr/lib/mono/4.5-api/System.dll", but it didn't exist. (TaskId:115)
                             Considered "/usr/lib/mono/4.5-api/System.exe", but it didn't exist. (TaskId:115)
                             For SearchPath "{RawFileName}". (TaskId:115)
                             Considered treating "System" as a file name, but it didn't exist. (TaskId:115)
                     Primary reference "System.Data". (TaskId:115)

Known workarounds

None, because this property replacement overrides any FrameworkPathOverride that the user may set, eg via environmetn variable.

Related information

Provide any related information

• Operating system: OSX Mojave
• Branch: master
• .NET Runtime, CoreCLR or Mono Version: Mono 5.18 or 5.23, dotnet sdk 2.1.500 (as downloaded via the dotnet-install.sh script)
• Editing Tools (e.g. Visual Studio Version): VS Code w/ Ionide
• Indications of severity: high, this impacts the ability to merge and build on the fsharp/fsharp packaging repository.

[baronfel]

Note that this should be able to be set on a per-OS basis by using MSBuild builtins for platform detection: dotnet/msbuild#2468 (comment)

I've tried this locally myself and haven't been successful yet. In addition, we may not want to set this value explicitly if it's possible to detect that the user has set an override already. Finally, this matrix of reference directories should be expanded to the other -api folders:

I'd be glad to contribute all of this, just want to raise a flag.

[dsyme]

Fixed in #6181