Async await on dynamic expression is not detected.

[greenozon]

ILSpy version 4.0.0.4518-rc1

Error decompiling Microsoft.VisualStudio.Settings.PubSubReceiver.SubscribeAsync
 ---> System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
   at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource) in offset 46
   at System.Collections.Generic.List`1.get_Item(Int32 index) in offset 0
   at ICSharpCode.Decompiler.IL.InstructionCollection`1.get_Item(Int32 index) in offset 1
   at ICSharpCode.Decompiler.IL.ControlFlow.AsyncAwaitDecompiler.AnalyzeAwaitBlock(Block block, ILVariable& awaiter, IField& awaiterField, Int32& state) in offset 176
   at ICSharpCode.Decompiler.IL.ControlFlow.AsyncAwaitDecompiler.AnalyzeStateMachine(ILFunction function) in offset 228
   at ICSharpCode.Decompiler.IL.ControlFlow.AsyncAwaitDecompiler.Run(ILFunction function, ILTransformContext context) in offset 163
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in offset 581
-- continuing with outer exception (ICSharpCode.Decompiler.DecompilerException) --
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in offset 1068
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DoDecompile(IMethod method, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in offset 249
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DoDecompile(ITypeDefinition typeDef, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in offset 747
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DoDecompileTypes(IEnumerable`1 types, DecompileRun decompileRun, ITypeResolveContext decompilationContext, SyntaxTree syntaxTree) in offset 219
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DecompileTypes(IEnumerable`1 types) in offset 201
   at ICSharpCode.Decompiler.CSharp.WholeProjectDecompiler.<>c__DisplayClass30_0.<WriteCodeFilesInProject>b__2(IGrouping`2 file) in offset 68
   at System.Threading.Tasks.Parallel.<>c__DisplayClass17_0`1.<ForWorker>b__1() in offset 589
   at System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask) in offset 6
   at System.Threading.Tasks.Task.<>c__DisplayClass176_0.<ExecuteSelfReplicating>b__0(Object) in offset 134

target: https://www.sendspace.com/file/gjwrkb

[dgrunwald]

The cause seems to be await dynamicExpression;
The C# compiler uses a completely different code pattern here than for normal awaits, because the INotifyCompletion/ICriticalNotifyCompletion distinction now has to happen at run-time instead of compile-time.
We are fixing the crash, but adjusting the async/await transform is more complicated.

[siegfriedpammer]

A simple test case:

class Program
{
    dynamic GetDynamic()
    {
        return null;
    }

    public async Task<dynamic> TestDynamicWithAsync()
    {
        dynamic value = await GetDynamic();
        value.UseMe();
        return value;
    }
}