infer DirectoryInfo when trailing slash is Path.AltDirectorySeparatorChar

Author: jonsequitur

src/System.CommandLine.Tests/Binding/ModelBindingCommandHandlerTests.cs

@@ -297,6 +297,7 @@ public void When_argument_type_is_more_specific_than_parameter_type_then_paramet
         [InlineData(typeof(FileSystemInfo), true, nameof(ExistingFile))]
         [InlineData(typeof(FileSystemInfo), true, nameof(ExistingDirectory))]
         [InlineData(typeof(FileSystemInfo), true, nameof(NonexistentPathWithTrailingSlash))]
+        [InlineData(typeof(FileSystemInfo), true, nameof(NonexistentPathWithTrailingAltSlash))]
         [InlineData(typeof(FileSystemInfo), true, nameof(NonexistentPathWithoutTrailingSlash))]
 
         [InlineData(typeof(string[]), false)]
@@ -545,7 +546,18 @@ public void Apply(InvocationContext context)
                           .Should()
                           .Be(NonexistentPathWithTrailingSlash()),
                 variationName: nameof(NonexistentPathWithTrailingSlash)),
-                
+
+            BindingTestCase.Create<FileSystemInfo>(
+                NonexistentPathWithTrailingAltSlash(),
+                fsi => fsi.Should()
+                          .BeOfType<DirectoryInfo>()
+                          .Which
+                          .FullName
+                          .Should()
+                          .Be(NonexistentPathWithTrailingSlash(), 
+                              "DirectoryInfo replaces Path.AltDirectorySeparatorChar with Path.DirectorySeparatorChar on Windows"),
+                variationName: nameof(NonexistentPathWithTrailingAltSlash)),
+
             BindingTestCase.Create<FileSystemInfo>(
                 NonexistentPathWithoutTrailingSlash(),
                 fsi => fsi.Should()
@@ -582,6 +594,8 @@ private static string NonexistentPathWithoutTrailingSlash()
 
         private static string NonexistentPathWithTrailingSlash() => 
             NonexistentPathWithoutTrailingSlash() + Path.DirectorySeparatorChar;
+        private static string NonexistentPathWithTrailingAltSlash() => 
+            NonexistentPathWithoutTrailingSlash() + Path.AltDirectorySeparatorChar;
 
         private static string ExistingFile() =>
             Directory.GetFiles(ExistingDirectory()).FirstOrDefault() ?? 

src/System.CommandLine/Binding/ArgumentConverter.cs

@@ -23,7 +23,8 @@ internal static class ArgumentConverter
                     return new DirectoryInfo(value);
                 }
 
-                if (value.EndsWith(Path.DirectorySeparatorChar.ToString()))
+                if (value.EndsWith(Path.DirectorySeparatorChar.ToString()) ||
+                    value.EndsWith(Path.AltDirectorySeparatorChar.ToString()))
                 {
                     return new DirectoryInfo(value);
                 }