Don't synthesize a parameter list out of nowhere.

src/Workspaces/CSharp/Portable/CodeGeneration/CSharpSyntaxGenerator.cs

@@ -2295,7 +2295,9 @@ public override IReadOnlyList<SyntaxNode> GetParameters(SyntaxNode declaration)
             var list = GetParameterList(declaration);
             return list != null
                 ? list.Parameters
-                : SpecializedCollections.EmptyReadOnlyList<SyntaxNode>();
+                : declaration is SimpleLambdaExpressionSyntax simpleLambda
+                    ? new[] { simpleLambda.Parameter }
+                    : SpecializedCollections.EmptyReadOnlyList<SyntaxNode>();
         }
 
         public override SyntaxNode InsertParameters(SyntaxNode declaration, int index, IEnumerable<SyntaxNode> parameters)
@@ -2385,7 +2387,6 @@ internal static BaseParameterListSyntax GetParameterList(SyntaxNode declaration)
                 SyntaxKind.DestructorDeclaration => ((DestructorDeclarationSyntax)declaration).ParameterList,
                 SyntaxKind.IndexerDeclaration => ((IndexerDeclarationSyntax)declaration).ParameterList,
                 SyntaxKind.ParenthesizedLambdaExpression => ((ParenthesizedLambdaExpressionSyntax)declaration).ParameterList,
-                SyntaxKind.SimpleLambdaExpression => SyntaxFactory.ParameterList(SyntaxFactory.SingletonSeparatedList(((SimpleLambdaExpressionSyntax)declaration).Parameter)),
                 SyntaxKind.LocalFunctionStatement => ((LocalFunctionStatementSyntax)declaration).ParameterList,
                 SyntaxKind.AnonymousMethodExpression => ((AnonymousMethodExpressionSyntax)declaration).ParameterList,
                 _ => (BaseParameterListSyntax)null,