unify CustomParser and DefaultValueFactory via new SetValueFactory API

Author: jonsequitur

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

@@ -15,6 +15,7 @@
     public Func<System.CommandLine.Parsing.ArgumentResult,T> DefaultValueFactory { get; set; }
     public System.Boolean HasDefaultValue { get; }
     public System.Type ValueType { get; }
+    public System.Void SetValueFactory(Func<System.CommandLine.Parsing.ArgumentResult,T> valueFactory, ValueFactoryInvocation invocation = WhenTokensMatched)
   public struct ArgumentArity : System.ValueType, System.IEquatable<ArgumentArity>
     public static ArgumentArity ExactlyOne { get; }
     public static ArgumentArity OneOrMore { get; }
@@ -99,6 +100,7 @@
     public Option<T> AcceptLegalFileNamesOnly()
     public Option<T> AcceptLegalFilePathsOnly()
     public Option<T> AcceptOnlyFromAmong(System.String[] values)
+    public System.Void SetValueFactory(Func<System.CommandLine.Parsing.ArgumentResult,T> valueFactory, ValueFactoryInvocation invocation = WhenTokensMatched)
   public static class OptionValidation
     public static Option<System.IO.FileInfo> AcceptExistingOnly(this Option<System.IO.FileInfo> option)
     public static Option<System.IO.DirectoryInfo> AcceptExistingOnly(this Option<System.IO.DirectoryInfo> option)
@@ -148,6 +150,10 @@
     public System.Collections.Generic.IEnumerable<Symbol> Parents { get; }
     public System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem> GetCompletions(System.CommandLine.Completions.CompletionContext context)
     public System.String ToString()
+  public enum ValueFactoryInvocation : System.Enum, System.IComparable, System.IConvertible, System.IFormattable, System.ISpanFormattable
+    WhenTokensMatched=1
+    WhenTokensNotMatched=2
+    Always=3
   public class VersionOption : Option
     .ctor()
     .ctor(System.String name, System.String[] aliases)

src/System.CommandLine.Tests/CustomParsingTests.cs

@@ -13,6 +13,8 @@
 
 namespace System.CommandLine.Tests;
 
+#pragma warning disable CS0618 // Tests in this class intentionally exercise the obsolete legacy parsing API.
+
 public class CustomParsingTests
 {
     [Fact]
@@ -986,4 +988,6 @@ public void GetResult_by_name_can_be_used_recursively_within_custom_option_parse
         parseResult.GetValue<string>("--second").Should().Be("two");
         parseResult.GetValue<string>("--third").Should().Be("three");
     }
-}
+}
+
+#pragma warning restore CS0618

src/System.CommandLine.Tests/System.CommandLine.Tests.csproj

@@ -4,7 +4,7 @@
     <TargetFrameworks>$(NetMinimum);$(NetFrameworkMinimum)</TargetFrameworks>
     <GenerateProgramFile>false</GenerateProgramFile>
     <DefaultExcludesInProjectFolder>$(DefaultExcludesInProjectFolder);TestApps\**</DefaultExcludesInProjectFolder>
-    <NoWarn>$(NoWarn);CS8632</NoWarn> <!-- Suppress nullable warning for files included as links -->
+    <NoWarn>$(NoWarn);CS8632;CS0618</NoWarn> <!-- Suppress nullable warning for linked files and obsolete warnings for legacy API compatibility tests -->
   </PropertyGroup>
 
   <ItemGroup>