The outcome of the test seems to depend on which framework target we use! For example, net451 makes lang="C#" fail (if this commit is applied), whereas net452 passes!
The difference between C# and F# is confusing. The Name of second (Case2AB) is _Case2AB for F# and null for C#...
See https://github.com/messagetemplates/messagetemplates-fsharp/commit/ec3a3cdb96a413406119c3dba83a9ee726c112a4 and https://github.com/messagetemplates/messagetemplates-fsharp/blob/ec3a3cdb96a413406119c3dba83a9ee726c112a4/test/FsMessageTemplates.Tests/FsTests.Capture.fs#L183