[regression] iOS app build fails when referencing package (e.g. Entity Framework Core 2.1) that depends on newer System.Memory in d15-9

[rolfbjarne]

Steps to Reproduce

1. Install Xamarin.iOS 11.16 (d15-9)
2. Download, extract, open & build https://github.com/xamarin/xamarin-macios/files/2350303/iOSEFCore212.zip

Expected Behavior

No build failure (just like in d15-8).

Actual Behavior

MTOUCH : error MT2101: Can't resolve the reference 'System.ReadOnlySpan`1<System.Char> System.MemoryExtensions::AsSpan(System.String,System.Int32,System.Int32)', referenced from the method 'System.ReadOnlySpan`1<System.Char> Microsoft.Extensions.Primitives.StringSegment::AsSpan()' in 'System.Memory, Version=4.0.1.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. [/Users/rolf/Downloads/iOSEFCore21/iOSEFCore21.csproj]

This is a repeat/regression of #4168, just that it only happens in d15-9.

Build Logs

Failing build log (with Xamarin.iOS 11.16.0.26): https://gist.github.com/rolfbjarne/9b1cecf7968f6958a667204dda49a09f
Working build log (with Xamarin.iOS 11.14.0.14): https://gist.github.com/rolfbjarne/a4e0a0859a627777f8302cda841ada19
Diff between build logs: https://gist.github.com/rolfbjarne/76c1dab569d0994497a8754910a5de63

The significant difference seems to be that we're shipping System.Memory.dll in d15-9, which we end up using, while in d15-8 we use the nuget-provided version:

https://gist.github.com/rolfbjarne/76c1dab569d0994497a8754910a5de63#file-build-diff-L47-L52
https://gist.github.com/rolfbjarne/76c1dab569d0994497a8754910a5de63#file-build-diff-L207

This is supported by the fact that if I remove the System.Memory.dll we ship in d15-9:

rm /Library/Frameworks/Xamarin.iOS.framework/Versions/11.16.0.26/lib/mono/Xamarin.iOS/Facades/System.Memory.dll

then the project builds fine.

The project also builds fine if I use a version of Xamarin.iOS that has bumped to mono 2018-06 (from PR #4277):

• XI version: mtouch 11.17.0.66 (PR Bump to mono:2018-06 #4277: c7654a0)
• Build log: https://gist.github.com/rolfbjarne/1778a1ed3f6fdc48aa03cc72d02b73d8

which seems to indicate that this is a problem in mono's 2018-04 branch (d15-8 used mono's 2018-02 branch, while d15-9 is using mono's 2018-04 branch).

[rolfbjarne]

@spouliot @marek-safar This is a regression in d15-9.

Could a potential solution be to not ship System.Memory.dll in d15-9?

[marek-safar]

That error is quite strange. We don't ship System.Memory (which is actually an issue on our side) but the current setup should still work. I'll check if I can reproduce it as well

[spouliot]

@marek-safar it was removed in 15.8 (mono 2018-02) but, IIRC, you mentioned there was a fix in progress for the next mono, i.e. my understanding is that System.Memory.dll was not removed in mono master (which became the 2018-04 we're presently using for 15.9).