Fixing issue #907

First, I added a failing unit test, in two parts:

* A new AsyncAwaitValueTaskStateMachine class in Samples.cs
  in coverlet.core.tests/Samples, in line with the existing
  AsyncAwaitStateMachine class, but returning a completed
  ValueTask instead of a completed Task.

* A new test in CecilSymbolHelperTests.cs in
  coverlet.core.tests/Symbols that expects the "await" in
  that sample class not to have a branch in it.

After it failed, I updated CecilSymbolHelper to include
get_IsCompleted from System.Runtime.CompilerServices.ValueTaskAwaiter,
as suggested by @a-jackson in issue #907.  The test passed.

One open question is whether it might be worth adding a
ValueTask-based analogue for the Instrumentation.AsyncAwait.cs
samples.

Author: alexthornton1

src/coverlet.core/Symbols/CecilSymbolHelper.cs

@@ -172,6 +172,7 @@ instruction.Previous.Operand is MethodReference operand &&
                     operand.Name == "get_IsCompleted" &&
                     (
                         operand.DeclaringType.FullName.StartsWith("System.Runtime.CompilerServices.TaskAwaiter") ||
+                        operand.DeclaringType.FullName.StartsWith("System.Runtime.CompilerServices.ValueTaskAwaiter") ||
                         operand.DeclaringType.FullName.StartsWith("System.Runtime.CompilerServices.ConfiguredTaskAwaitable") ||
                         operand.DeclaringType.FullName.StartsWith("System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable")
                     )

test/coverlet.core.tests/Samples/Samples.cs

@@ -189,6 +189,14 @@ async public Task AsyncAwait()
         }
     }
 
+    public class AsyncAwaitValueTaskStateMachine
+    {
+        async public ValueTask AsyncAwait()
+        {
+            await default(ValueTask);
+        }
+    }
+
     [ExcludeFromCoverage]
     public class ClassExcludedByCoverletCodeCoverageAttr
     {

test/coverlet.core.tests/Symbols/CecilSymbolHelperTests.cs

@@ -294,6 +294,22 @@ public void GetBranchPoints_IgnoresBranchesIn_AsyncAwaitStateMachine()
             Assert.Empty(points);
         }
 
+        [Fact]
+        public void GetBranchPoints_IgnoresBranchesIn_AsyncAwaitValueTaskStateMachine()
+        {
+            // arrange
+            var nestedName = typeof(AsyncAwaitValueTaskStateMachine).GetNestedTypes(BindingFlags.NonPublic).First().Name;
+            var type = _module.Types.FirstOrDefault(x => x.FullName == typeof(AsyncAwaitValueTaskStateMachine).FullName);
+            var nestedType = type.NestedTypes.FirstOrDefault(x => x.FullName.EndsWith(nestedName));
+            var method = nestedType.Methods.First(x => x.FullName.EndsWith("::MoveNext()"));
+
+            // act
+            var points = _cecilSymbolHelper.GetBranchPoints(method);
+
+            // assert
+            Assert.Empty(points);
+        }
+        
         [Fact]
         public void GetBranchPoints_ExceptionFilter()
         {