Bug 53290 - Compilation difference between mcs & csc in Mono 4.9
Summary: Compilation difference between mcs & csc in Mono 4.9
Status: RESOLVED UPSTREAM
Alias: None
Product: Compilers
Classification: Mono
Component: C# ()
Version: unspecified
Hardware: PC Mac OS
: --- normal
Target Milestone: ---
Assignee: Marek Safar
URL:
Depends on:
Blocks:
 
Reported: 2017-03-13 20:14 UTC by Jonathan Pryor
Modified: 2017-03-14 14:01 UTC (History)
1 user (show)

Tags:
Is this bug a regression?: ---
Last known good build:

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Status:
RESOLVED UPSTREAM

Description Jonathan Pryor 2017-03-13 20:14:35 UTC
Consider the following code;

  using System.Threading.Tasks;
      
  class App {
    static void Foo ()
    {
    }
      
    static void Main ()
    {
      var r = Task.Run (Foo);
    }
  }

It compiles without error with `mcs`. It fails with `csc` from Mono 4.9:

> bo.cs(11,22): error CS0121: The call is ambiguous between the following methods or properties: 'Task.Run(Action)' and 'Task.Run(Func<Task>)'

The same happens with TryRoslyn:

https://tryroslyn.azurewebsites.net/#ATCuGcEsDsHNgMoE9wBcCmBbAdAFQBYBO6AhgCYyx4ngDW4A3ALABQI7wrIAxgDY3hgAQQAOI4AG8u7NCVSRuwAG4B7SGWAAxFSuAAKAJTSQUtuwC+x9ldnzFq9cACyJGPqNmTVkEpKFg/gC8wLg0tNgASqDQ+toqBsyewJZm5kA

Is this a Roslyn bug? An `mcs` bug?

Related: What happens when the non-Roslyn .NET 4.5 CSC.EXE is used?
Comment 1 Jonathan Pryor 2017-03-13 22:14:59 UTC
> What happens when the non-Roslyn .NET 4.5 CSC.EXE is used?

This is most amusing:

> bo.cs(11,21): error CS0407: 'void App.Foo()' has the wrong return type

bo.cs(11,21) is:

> var t = Task.Run (Foo);
>                   ^

I think I misplaced my jaw somewhere...

This doesn't appear to be specific to `Task<T>` either; replacing `Task.Run()` with static methods which overload based on `Action` and `Func<int>` results in the same error.

Which leads me to think that CSC's type inferencing *sucks*.
Comment 2 Marek Safar 2017-03-14 14:01:26 UTC
Relevant Roslyn issue https://github.com/dotnet/roslyn/issues/8942