Bad sequence point in switch with pattern matching in async method

[jcouv]

Set a breakpoint in GenerateDeconstructMethodCodeFixProvider.RegisterCodeFixesAsync and start debugging any of the tests for this fixer (such as TestDeconstructionDeclaration_Simple).
The stepping shows the return statement as executed, when it is not actually executed.

Unfortunately, I was not able to narrow down a small repro yet (the ones I tried behaved as expected).

This is using VisualStudio.15.Preview/15.8.0-pre.2.0+27729.1 on PR #28286

[jcouv]

This reminds me of a similar issue (not related to switch but possibly to pattern matching): #17076

Tagging @gafter for awareness

[gafter]

I was able to repro it using the following code:

using System.Threading.Tasks;

public class Program
{
    public static async Task Main()
    {
        switch (M())
        {
            case C c:
                break;
            default:
                return;
        }

        if (M() != null)
        {
        }
    }

    private static object M()
    {
        return new C();
    }
}

class C
{
}

[jcouv]

Awesome. Thanks a bunch!

[jcouv]

The sequence points seem correct.
Tagging @tmat for some advice.

        [Fact, WorkItem(28288, "https://github.com/dotnet/roslyn/issues/28288")]
        void Test()
        {
            var source = @"

public class Program
{
    public static async System.Threading.Tasks.Task Main()
    {
        switch (new Program())
        {
            case Program p:
                M(1);
                break;
            default:
                M(2);
                return;
        }

        await System.Threading.Tasks.Task.CompletedTask;
    }
    private static void M(int i) { }
}";
            var verifier = CompileAndVerify(source);
            verifier.VerifyIL("Program.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext()", @"{
  // Code size      169 (0xa9)
  .maxstack  3
  .locals init (int V_0,
                Program V_1,
                System.Runtime.CompilerServices.TaskAwaiter V_2,
                System.Exception V_3)
  // sequence point: }, sequence point: <hidden>
  IL_0000:  ldarg.0
  IL_0001:  ldfld      ""int Program.<Main>d__0.<>1__state""
  IL_0006:  stloc.0
  .try
  {
    // sequence point: <hidden>
    IL_0007:  ldloc.0
    IL_0008:  brfalse.s  IL_0059
    // sequence point: switch (new Program())
    IL_000a:  newobj     ""Program..ctor()""
    IL_000f:  stloc.1
    IL_0010:  ldloc.1
    IL_0011:  brfalse.s  IL_001d
    IL_0013:  ldloc.1
    IL_0014:  stloc.1
    // sequence point: M(1);
    IL_0015:  ldc.i4.1
    IL_0016:  call       ""void Program.M(int)""
    // sequence point: break;
    IL_001b:  br.s       IL_0025
    // sequence point: M(2);
    IL_001d:  ldc.i4.2
    IL_001e:  call       ""void Program.M(int)""
    // sequence point: return;
    IL_0023:  leave.s    IL_0095
    // sequence point: await System.Threading.Tasks.Task.CompletedTask;
    IL_0025:  call       ""System.Threading.Tasks.Task System.Threading.Tasks.Task.CompletedTask.get""
    IL_002a:  callvirt   ""System.Runtime.CompilerServices.TaskAwaiter System.Threading.Tasks.Task.GetAwaiter()""
    IL_002f:  stloc.2
    // sequence point: <hidden>
    IL_0030:  ldloca.s   V_2
    IL_0032:  call       ""bool System.Runtime.CompilerServices.TaskAwaiter.IsCompleted.get""
    IL_0037:  brtrue.s   IL_0075
    IL_0039:  ldarg.0
    IL_003a:  ldc.i4.0
    IL_003b:  dup
    IL_003c:  stloc.0
    IL_003d:  stfld      ""int Program.<Main>d__0.<>1__state""
    // async: yield
    IL_0042:  ldarg.0
    IL_0043:  ldloc.2
    IL_0044:  stfld      ""System.Runtime.CompilerServices.TaskAwaiter Program.<Main>d__0.<>u__1""
    IL_0049:  ldarg.0
    IL_004a:  ldflda     ""System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<Main>d__0.<>t__builder""
    IL_004f:  ldloca.s   V_2
    IL_0051:  ldarg.0
    IL_0052:  call       ""void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted<System.Runtime.CompilerServices.TaskAwaiter, Program.<Main>d__0>(ref System.Runtime.CompilerServices.TaskAwaiter, ref Program.<Main>d__0)""
    IL_0057:  leave.s    IL_00a8
    // async: resume
    IL_0059:  ldarg.0
    IL_005a:  ldfld      ""System.Runtime.CompilerServices.TaskAwaiter Program.<Main>d__0.<>u__1""
    IL_005f:  stloc.2
    IL_0060:  ldarg.0
    IL_0061:  ldflda     ""System.Runtime.CompilerServices.TaskAwaiter Program.<Main>d__0.<>u__1""
    IL_0066:  initobj    ""System.Runtime.CompilerServices.TaskAwaiter""
    IL_006c:  ldarg.0
    IL_006d:  ldc.i4.m1
    IL_006e:  dup
    IL_006f:  stloc.0
    IL_0070:  stfld      ""int Program.<Main>d__0.<>1__state""
    IL_0075:  ldloca.s   V_2
    IL_0077:  call       ""void System.Runtime.CompilerServices.TaskAwaiter.GetResult()""
    IL_007c:  leave.s    IL_0095
  }
  catch System.Exception
  {
    // sequence point: <hidden>
    IL_007e:  stloc.3
    IL_007f:  ldarg.0
    IL_0080:  ldc.i4.s   -2
    IL_0082:  stfld      ""int Program.<Main>d__0.<>1__state""
    IL_0087:  ldarg.0
    IL_0088:  ldflda     ""System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<Main>d__0.<>t__builder""
    IL_008d:  ldloc.3
    IL_008e:  call       ""void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)""
    IL_0093:  leave.s    IL_00a8
  }
  // sequence point: }
  IL_0095:  ldarg.0
  IL_0096:  ldc.i4.s   -2
  IL_0098:  stfld      ""int Program.<Main>d__0.<>1__state""
  // sequence point: <hidden>
  IL_009d:  ldarg.0
  IL_009e:  ldflda     ""System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program.<Main>d__0.<>t__builder""
  IL_00a3:  call       ""void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()""
  IL_00a8:  ret
}", sequencePoints: "", source: source);
        }

Note to self: seq-points branch

[jcouv]

Another variant of this was spotted: #31665 (switch statement inside if/else)

[jcouv]

I'm able to repro this even without await suspensions.
Tagging @chuckries @tmat for advice. Thanks

        [Fact, WorkItem(28288, "https://github.com/dotnet/roslyn/issues/28288")]
        public void BadSequencePoint_TODO2()
        {
            var source = @"
using System.Threading.Tasks;

public class C
{
    public static async Task Main()
    {
        object o = new C();
        switch (o)
        {
            case C c:
                System.Console.Write(1);
                break;
            default:
                return;
        }

        if (M() != null)
        {
        }
    }

    private static object M()
    {
        return new C();
    }
}";
            // The debugger highlights the `break` then the `return` (instead of the `if`)
            var v = CompileAndVerify(source);
            v.VerifyIL("C.<Main>d__0.System.Runtime.CompilerServices.IAsyncStateMachine.MoveNext()", @"
{
  // Code size       73 (0x49)
  .maxstack  2
  .locals init (System.Exception V_0)
  .try
  {
    // sequence point: object o = new C();
    IL_0000:  newobj     ""C..ctor()""
    // sequence point: <hidden>
    IL_0005:  isinst     ""C""
    IL_000a:  brfalse.s  IL_0014
    // sequence point: System.Console.Write(1);
    IL_000c:  ldc.i4.1
    IL_000d:  call       ""void System.Console.Write(int)""
    // sequence point: break;
    IL_0012:  br.s       IL_0016
    // sequence point: return;
    IL_0014:  leave.s    IL_0035
    // sequence point: if (M() != null)
    IL_0016:  call       ""object C.M()""
    IL_001b:  pop
    // sequence point: <hidden>
    IL_001c:  leave.s    IL_0035
  }
  catch System.Exception
  {
    // sequence point: <hidden>
    IL_001e:  stloc.0
    IL_001f:  ldarg.0
    IL_0020:  ldc.i4.s   -2
    IL_0022:  stfld      ""int C.<Main>d__0.<>1__state""
    IL_0027:  ldarg.0
    IL_0028:  ldflda     ""System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder""
    IL_002d:  ldloc.0
    IL_002e:  call       ""void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetException(System.Exception)""
    IL_0033:  leave.s    IL_0048
  }
  // sequence point: }
  IL_0035:  ldarg.0
  IL_0036:  ldc.i4.s   -2
  IL_0038:  stfld      ""int C.<Main>d__0.<>1__state""
  // sequence point: <hidden>
  IL_003d:  ldarg.0
  IL_003e:  ldflda     ""System.Runtime.CompilerServices.AsyncTaskMethodBuilder C.<Main>d__0.<>t__builder""
  IL_0043:  call       ""void System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult()""
  IL_0048:  ret
}
", sequencePoints: "C+<Main>d__0.MoveNext", source: source);
        }

[gafter]

Fixed in #42824