Give non-empty names to copy parameters of anonymous methods.

It is unexpected to have parameters originated from source having empty names in correct programs.
Fixes #4527.

src/Compilers/CSharp/Portable/Symbols/Source/LambdaSymbol.cs

@@ -346,7 +346,7 @@ private static ParameterSymbol CopyParameter(ParameterSymbol parameter, MethodSy
                 parameter.Type,
                 parameter.Ordinal,
                 parameter.RefKind,
-                    string.Empty); // Make sure nothing binds to this.
+                GeneratedNames.LambdaCopyParameterName(parameter)); // Make sure nothing binds to this.
         }
 
         public sealed override bool Equals(object symbol)

src/Compilers/CSharp/Portable/Symbols/Synthesized/GeneratedNames.cs

@@ -460,5 +460,10 @@ internal static string ReusableHoistedLocalFieldName(int number)
             Debug.Assert((char)GeneratedNameKind.ReusableHoistedLocalField == '7');
             return "<>7__wrap" + StringExtensions.GetNumeral(number);
         }
+
+        internal static string LambdaCopyParameterName(ParameterSymbol sourceParameter)
+        {
+            return "<" + sourceParameter.Name + ">";
+        }
     }
 }

src/Compilers/CSharp/Test/Semantic/FlowAnalysis/RegionAnalysisTests.cs

@@ -1593,7 +1593,7 @@ public static void Main()
             Assert.Null(GetSymbolNamesJoined(analysis.AlwaysAssigned));
             Assert.Equal("p", GetSymbolNamesJoined(analysis.Captured));
             Assert.Equal("i", GetSymbolNamesJoined(analysis.UnsafeAddressTaken));
-            Assert.Equal("", GetSymbolNamesJoined(analysis.VariablesDeclared));
+            Assert.Equal("<p>", GetSymbolNamesJoined(analysis.VariablesDeclared));
 
             Assert.Equal("p", GetSymbolNamesJoined(analysis.DataFlowsIn));
             Assert.Null(GetSymbolNamesJoined(analysis.DataFlowsOut));
@@ -1601,7 +1601,7 @@ public static void Main()
             Assert.Equal("p", GetSymbolNamesJoined(analysis.ReadInside));
             Assert.Equal("i", GetSymbolNamesJoined(analysis.ReadOutside));
 
-            Assert.Equal("", GetSymbolNamesJoined(analysis.WrittenInside));
+            Assert.Equal("<p>", GetSymbolNamesJoined(analysis.WrittenInside));
             Assert.Equal("i, p, d", GetSymbolNamesJoined(analysis.WrittenOutside));
         }
 

src/Compilers/CSharp/Test/Semantic/Semantics/LambdaTests.cs

@@ -1396,5 +1396,46 @@ public interface IColumn { }
             var compilation = CreateCompilationWithMscorlib(source, new[] { SystemCoreRef, CSharpRef }, options: TestOptions.ReleaseExe);
             CompileAndVerify(compilation, expectedOutput: "Select<T, S>");
         }
+
+        [Fact, WorkItem(4527, "https://github.com/dotnet/roslyn/issues/4527")]
+        public void AnonymousMethodExpressionWithoutParameterList()
+        {
+            var source =
+@"
+using System;
+using System.Threading.Tasks;
+
+namespace RoslynAsyncDelegate
+{
+    class Program
+    {
+        static EventHandler MyEvent;
+
+        static void Main(string[] args)
+        {
+           MyEvent += async delegate { await Task.Delay(0); };
+        }
+    }
+}
+
+";
+            var compilation = CreateCompilationWithMscorlib45(source, options: TestOptions.DebugExe);
+
+            var tree = compilation.SyntaxTrees[0];
+            var model = compilation.GetSemanticModel(tree);
+
+            var node1 = tree.GetRoot().DescendantNodes().Where(n => n.IsKind(SyntaxKind.AnonymousMethodExpression)).Single();
+
+            Assert.Equal("async delegate { await Task.Delay(0); }", node1.ToString());
+
+            Assert.Equal("void System.EventHandler.Invoke(System.Object sender, System.EventArgs e)", model.GetTypeInfo(node1).ConvertedType.GetMembers("Invoke").Single().ToTestDisplayString());
+
+            var lambdaParameters = ((MethodSymbol)(model.GetSymbolInfo(node1)).Symbol).Parameters;
+
+            Assert.Equal("System.Object <sender>", lambdaParameters[0].ToTestDisplayString());
+            Assert.Equal("System.EventArgs <e>", lambdaParameters[1].ToTestDisplayString());
+
+            CompileAndVerify(compilation);
+        }
     }
 }