fix #2786

Author: jonsequitur

src/System.CommandLine.Tests/ParsingValidationTests.cs

@@ -355,6 +355,44 @@ public void A_custom_validator_can_be_added_to_a_command()
                 .Contain("Options '--one' and '--two' cannot be used together.");
         }
 
+        [Fact]
+        public void GetValue_in_command_validator_does_not_suppress_argument_validation_errors()
+        {
+            var fileArg = new Argument<FileInfo>("file");
+            fileArg.AcceptExistingOnly();
+
+            var root = new RootCommand { fileArg };
+
+            root.Validators.Add(result =>
+            {
+                _ = result.GetValue(fileArg);
+            });
+
+            var parseResult = root.Parse("nonexistent.xyz");
+
+            parseResult.Errors.Should().ContainSingle()
+                .Which.Message.Should().Be(LocalizationResources.FileDoesNotExist("nonexistent.xyz"));
+        }
+
+        [Fact]
+        public void GetValue_in_command_validator_does_not_suppress_option_argument_validation_errors()
+        {
+            var fileOption = new Option<FileInfo>("--file");
+            fileOption.AcceptExistingOnly();
+
+            var root = new RootCommand { fileOption };
+
+            root.Validators.Add(result =>
+            {
+                _ = result.GetValue(fileOption);
+            });
+
+            var parseResult = root.Parse("--file nonexistent.xyz");
+
+            parseResult.Errors.Should().ContainSingle()
+                .Which.Message.Should().Be(LocalizationResources.FileDoesNotExist("nonexistent.xyz"));
+        }
+
         [Fact]
         public void A_custom_validator_can_be_added_to_an_option()
         {

src/System.CommandLine/Parsing/ArgumentResult.cs

@@ -12,6 +12,7 @@ namespace System.CommandLine.Parsing
     public sealed class ArgumentResult : SymbolResult
     {
         private ArgumentConversionResult? _conversionResult;
+        private bool _validatorsHaveBeenRun;
         private bool _onlyTakeHasBeenCalled;
 
         internal ArgumentResult(
@@ -31,8 +32,27 @@ internal ArgumentResult(
 
         public bool Implicit { get; private set; }
 
-        internal ArgumentConversionResult GetArgumentConversionResult() =>
-            _conversionResult ??= ValidateAndConvert(useValidators: true);
+        internal ArgumentConversionResult GetArgumentConversionResult()
+        {
+            if (_conversionResult is not null)
+            {
+                if (!_validatorsHaveBeenRun && Argument.HasValidators)
+                {
+                    // GetValueOrDefault was called before GetArgumentConversionResult,
+                    // which cached a conversion result without running validators.
+                    // Run them now so that validators like AcceptExistingOnly() are not skipped.
+                    _validatorsHaveBeenRun = true;
+                    for (var i = 0; i < Argument.Validators.Count; i++)
+                    {
+                        Argument.Validators[i](this);
+                    }
+                }
+
+                return _conversionResult;
+            }
+
+            return _conversionResult = ValidateAndConvert(useValidators: true);
+        }
 
         /// <summary>
         /// Gets the parsed value or the default value for <see cref="Argument"/>.
@@ -141,6 +161,8 @@ private ArgumentConversionResult ValidateAndConvert(bool useValidators)
             //        => GetValueOrDefault => ValidateAndConvert (again)
             if (useValidators && Argument.HasValidators)
             {
+                _validatorsHaveBeenRun = true;
+
                 for (var i = 0; i < Argument.Validators.Count; i++)
                 {
                     Argument.Validators[i](this);

src/System.CommandLine/Parsing/OptionResult.cs

@@ -11,8 +11,6 @@ namespace System.CommandLine.Parsing
     /// </summary>
     public sealed class OptionResult : SymbolResult
     {
-        private ArgumentConversionResult? _argumentConversionResult;
-
         internal OptionResult(
             Option option,
             SymbolResultTree symbolResultTree,
@@ -54,15 +52,13 @@ internal OptionResult(
         /// </summary>
         /// <returns>The parsed value or the default value for <see cref="Option"/></returns>
         public T GetValueOrDefault<T>() =>
-            ArgumentConversionResult
-                .ConvertIfNeeded(typeof(T))
-                .GetValueOrDefault<T>();
+            GetResult(Option.Argument)!.GetValueOrDefault<T>();
 
         internal bool IsArgumentLimitReached
             => Option.Argument.Arity.MaximumNumberOfValues == (Implicit ? Tokens.Count - 1 : Tokens.Count);
 
         internal ArgumentConversionResult ArgumentConversionResult
-            => _argumentConversionResult ??= GetResult(Option.Argument)!.GetArgumentConversionResult();
+            => GetResult(Option.Argument)!.GetArgumentConversionResult();
 
         internal override bool UseDefaultValueFor(ArgumentResult argumentResult)
         {