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Created attachment 21992 [details]
Sample solution attached detailing the compiler crash
Using Mono JIT compiler version 4.8.0 (Stable 126.96.36.1995/e4a3cf3 Mon Feb 27 08:30:18 UTC 2017)
Sample project attached with a very isolated reproducible example. Either build the solution using xbuild or use the provided commandline to compile it:
> mcs /noconfig /debug:full /debug+ /optimize- /out:obj/Debug/CrashTest.dll A.cs B.cs Properties/AssemblyInfo.cs ValueType.cs obj/Debug/.NETFramework,Version=v4.5.AssemblyAttribute.cs /target:library /define:"DEBUG;TRACE" /nostdlib /platform:AnyCPU /reference:/usr/lib/mono/4.5-api/System.dll /reference:/usr/lib/mono/4.5-api/System.Data.dll /reference:/usr/lib/mono/4.5-api/System.Xml.dll /reference:/usr/lib/mono/4.5-api/System.Core.dll /reference:/usr/lib/mono/4.5-api//mscorlib.dll /warn:4
The stacktrace is long and is ommitted in this report for brevity. A full copy of the stderr of the compiler is contained in crash.log in the project tarball. Compiling an async method which returns a Task<struct> crashes the mono compiler when the results of an await is used as a parameter to the constructor of another value type inline without the use of a temporary local for storage.
The offending line is here in A.cs:18:
> return new ValueType(local1, await b.ProduceStructure(input1, input2));
b.ProduceStructure produces a Task<long>. If I set the result of the awaited b.ProduceStructure is set to a local the compiler will emit successfully, so this is able to be fairly easily worked around, alas not a huge priority.
I feel that this is loosely related to #42224 - but was marked resolved in 4.6/stable and it is reproducible for me.
Thanks for your time in looking at this!
Fixed in master and 2017-04 branch