replace ParseArgument<T> with Func<ArgumentResult, T>, fixes #1939

src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt

@@ -17,8 +17,8 @@ System.CommandLine
     .ctor(System.String name, System.String description = null)
     .ctor(System.String name, Func<T> getDefaultValue, System.String description = null)
     .ctor(Func<T> getDefaultValue)
-    .ctor(System.String name, ParseArgument<T> parse, System.Boolean isDefault = False, System.String description = null)
-    .ctor(ParseArgument<T> parse, System.Boolean isDefault = False)
+    .ctor(System.String name, Func<System.CommandLine.Parsing.ArgumentResult,T> parse, System.Boolean isDefault = False, System.String description = null)
+    .ctor(Func<System.CommandLine.Parsing.ArgumentResult,T> parse, System.Boolean isDefault = False)
     public System.Type ValueType { get; }
   public struct ArgumentArity : System.ValueType, System.IEquatable<ArgumentArity>
     public static ArgumentArity ExactlyOne { get; }
@@ -202,8 +202,8 @@ System.CommandLine
   public class Option<T> : Option, IValueDescriptor<T>, System.CommandLine.Binding.IValueDescriptor, System.CommandLine.Completions.ICompletionSource
     .ctor(System.String name, System.String description = null)
     .ctor(System.String[] aliases, System.String description = null)
-    .ctor(System.String name, ParseArgument<T> parseArgument, System.Boolean isDefault = False, System.String description = null)
-    .ctor(System.String[] aliases, ParseArgument<T> parseArgument, System.Boolean isDefault = False, System.String description = null)
+    .ctor(System.String name, Func<System.CommandLine.Parsing.ArgumentResult,T> parseArgument, System.Boolean isDefault = False, System.String description = null)
+    .ctor(System.String[] aliases, Func<System.CommandLine.Parsing.ArgumentResult,T> parseArgument, System.Boolean isDefault = False, System.String description = null)
     .ctor(System.String name, Func<T> getDefaultValue, System.String description = null)
     .ctor(System.String[] aliases, Func<T> getDefaultValue, System.String description = null)
     public ArgumentArity Arity { get; set; }
@@ -417,11 +417,6 @@ System.CommandLine.Parsing
     public Token Token { get; }
     public System.Object GetValueOrDefault()
     public T GetValueOrDefault<T>()
-  public delegate ParseArgument<out T> : System.MulticastDelegate, System.ICloneable, System.Runtime.Serialization.ISerializable
-    .ctor(System.Object object, System.IntPtr method)
-    public System.IAsyncResult BeginInvoke(ArgumentResult result, System.AsyncCallback callback, System.Object object)
-    public T EndInvoke(System.IAsyncResult result)
-    public T Invoke(ArgumentResult result)
   public class ParseError
     public System.String Message { get; }
     public SymbolResult SymbolResult { get; }

src/System.CommandLine/Argument{T}.cs

@@ -70,7 +70,7 @@ public Argument(Func<T> getDefaultValue) : this()
         /// <exception cref="ArgumentNullException">Thrown when <paramref name="parse"/> is null.</exception>
         public Argument(
             string? name,
-            ParseArgument<T> parse, 
+            Func<ArgumentResult, T> parse, 
             bool isDefault = false,
             string? description = null) : this(name, description)
         {
@@ -108,7 +108,7 @@ public Argument(
         /// </summary>
         /// <param name="parse">A custom argument parser.</param>
         /// <param name="isDefault"><see langword="true"/> to use the <paramref name="parse"/> result as default value.</param>
-        public Argument(ParseArgument<T> parse, bool isDefault = false) : this(null!, parse, isDefault)
+        public Argument(Func<ArgumentResult, T> parse, bool isDefault = false) : this(null!, parse, isDefault)
         {
         }
 

src/System.CommandLine/Option{T}.cs

@@ -27,7 +27,7 @@ public Option(
         /// <inheritdoc/>
         public Option(
             string name,
-            ParseArgument<T> parseArgument,
+            Func<ArgumentResult, T> parseArgument,
             bool isDefault = false,
             string? description = null) 
             : base(name, description, 
@@ -37,7 +37,7 @@ public Option(
         /// <inheritdoc/>
         public Option(
             string[] aliases,
-            ParseArgument<T> parseArgument,
+            Func<ArgumentResult, T> parseArgument,
             bool isDefault = false,
             string? description = null) 
             : base(aliases, description, new Argument<T>(parseArgument ?? throw new ArgumentNullException(nameof(parseArgument)), isDefault))