Drop async modifier from abstract (bodyless) converted methods

CodeConverter/CSharp/DeclarationNodeVisitor.cs

@@ -854,7 +854,17 @@ public override async Task<CSharpSyntaxNode> VisitMethodStatement(VBSyntax.Metho
         );
 
         bool canHaveMethodBody = declaredSymbol?.CanHaveMethodBody() != false;
-        return hasBody && canHaveMethodBody ? decl : decl.WithSemicolonToken(SemicolonToken);
+        return hasBody && canHaveMethodBody ? decl : WithoutAsyncModifier(decl).WithSemicolonToken(SemicolonToken);
+    }
+
+    /// <summary>
+    /// VB allows <c>MustOverride Async Function</c>, but C# reports CS1994 for <c>async</c> on a method without a body.
+    /// <c>async</c> is an implementation detail rather than part of the signature, so overriding methods can still be async.
+    /// </summary>
+    private static MethodDeclarationSyntax WithoutAsyncModifier(MethodDeclarationSyntax decl)
+    {
+        var asyncModifier = decl.Modifiers.FirstOrDefault(m => m.IsKind(CS.SyntaxKind.AsyncKeyword));
+        return asyncModifier == default ? decl : decl.WithModifiers(decl.Modifiers.Remove(asyncModifier));
     }
 
     public override async Task<CSharpSyntaxNode> VisitEventBlock(VBSyntax.EventBlockSyntax node)

Tests/CSharp/MemberTests/MemberTests.cs

@@ -1662,6 +1662,88 @@ internal string Format(string prefix = null, string suffix = null)
         suffix = suffix ?? ']'.ToString();
         return prefix + suffix;
     }
+}");
+    }
+
+    [Fact]
+    public async Task MustOverrideAsyncFunctionDropsAsyncModifierAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(
+            @"Imports System.Threading.Tasks
+
+MustInherit Class TestClass
+    Public MustOverride Async Function TestMethod() As Task
+End Class
+
+Class ChildClass
+    Inherits TestClass
+
+    Public Overrides Async Function TestMethod() As Task
+        Await Task.Delay(1)
+    End Function
+End Class", @"using System.Threading.Tasks;
+
+internal abstract partial class TestClass
+{
+    public abstract Task TestMethod();
+}
+
+internal partial class ChildClass : TestClass
+{
+
+    public override async Task TestMethod()
+    {
+        await Task.Delay(1);
+    }
+}");
+    }
+
+    [Fact]
+    public async Task MustOverrideAsyncSubAndGenericFunctionDropsAsyncModifierAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(
+            @"Imports System.Threading.Tasks
+
+MustInherit Class TestClass
+    Protected MustOverride Async Sub Fire()
+    Public MustOverride Async Function GetAsync(x As Integer) As Task(Of Integer)
+End Class", @"using System.Threading.Tasks;
+
+internal abstract partial class TestClass
+{
+    protected abstract void Fire();
+    public abstract Task<int> GetAsync(int x);
+}");
+    }
+
+    [Fact]
+    public async Task PartialAsyncMethodAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(
+            @"Imports System.Threading.Tasks
+
+Partial Class TestClass
+    Private Partial Sub Defined()
+    End Sub
+End Class
+
+Partial Class TestClass
+    Private Async Sub Defined()
+        Await Task.Delay(1)
+    End Sub
+End Class", @"using System.Threading.Tasks;
+
+internal partial class TestClass
+{
+    partial void Defined();
+}
+
+internal partial class TestClass
+{
+    async partial void Defined()
+    {
+        await Task.Delay(1);
+    }
 }");
     }
 }