Add a validation message for an invalid decoder

Sources/ArgumentParser/Parsable Types/ParsableArgumentsValidation.swift

@@ -146,14 +146,29 @@ struct ParsableArgumentsCodingKeyValidator: ParsableArgumentsValidator {
 
   /// This error indicates that an option, a flag, or an argument of
   /// a `ParsableArguments` is defined without a corresponding `CodingKey`.
-  struct Error: ParsableArgumentsValidatorError, CustomStringConvertible {
+  struct MissingKeysError: ParsableArgumentsValidatorError, CustomStringConvertible {
     let missingCodingKeys: [String]
 
     var description: String {
+      let resolution = """
+        To resolve this error, make sure that all properties have corresponding
+        cases in your custom `CodingKey` enumeration.
+        """
+
       if missingCodingKeys.count > 1 {
-        return "Arguments \(missingCodingKeys.map({ "`\($0)`" }).joined(separator: ",")) are defined without corresponding `CodingKey`s."
+        return """
+          Arguments \(missingCodingKeys.map({ "`\($0)`" }).joined(separator: ",")) \
+          are defined without corresponding `CodingKey`s.
+
+          \(resolution)
+          """
       } else {
-        return "Argument `\(missingCodingKeys[0])` is defined without a corresponding `CodingKey`."
+        return """
+          Argument `\(missingCodingKeys[0])` is defined without a corresponding \
+          `CodingKey`.
+
+          \(resolution)
+          """
       }
     }
 
@@ -162,6 +177,23 @@ struct ParsableArgumentsCodingKeyValidator: ParsableArgumentsValidator {
     }
   }
 
+  struct InvalidDecoderError: ParsableArgumentsValidatorError, CustomStringConvertible {
+    let type: ParsableArguments.Type
+
+    var description: String {
+      """
+      The implementation of `init(from:)` for `\(type)`
+      is not compatible with ArgumentParser. To resolve this issue, make sure
+      that `init(from:)` calls the `container(keyedBy:)` method on the given
+      decoder and decodes each of its properties using the returned decoder.
+      """
+    }
+
+    var kind: ValidatorErrorKind {
+      .failure
+    }
+  }
+
   static func validate(_ type: ParsableArguments.Type) -> ParsableArgumentsValidatorError? {
     let argumentKeys: [String] = Mirror(reflecting: type.init())
       .children
@@ -179,11 +211,11 @@ struct ParsableArgumentsCodingKeyValidator: ParsableArgumentsValidator {
     }
     do {
       let _ = try type.init(from: Validator(argumentKeys: argumentKeys))
-      fatalError("The validator should always throw.")
+      return InvalidDecoderError(type: type)
     } catch let result as Validator.ValidationResult {
       switch result {
       case .missingCodingKeys(let keys):
-        return Error(missingCodingKeys: keys)
+        return MissingKeysError(missingCodingKeys: keys)
       case .success:
         return nil
       }
@@ -244,9 +276,11 @@ struct NonsenseFlagsValidator: ParsableArgumentsValidator {
       """
       One or more Boolean flags is declared with an initial value of `true`.
       This results in the flag always being `true`, no matter whether the user
-      specifies the flag or not. To resolve this error, change the default to
-      `false`, provide a value for the `inversion:` parameter, or remove the
-      `@Flag` property wrapper altogether.
+      specifies the flag or not.
+
+      To resolve this error, change the default to `false`, provide a value
+      for the `inversion:` parameter, or remove the `@Flag` property wrapper
+      altogether.
 
       Affected flag(s):
       \(names.joined(separator: "\n"))

Tests/ArgumentParserUnitTests/ParsableArgumentsValidationTests.swift

@@ -85,29 +85,53 @@ final class ParsableArgumentsValidationTests: XCTestCase {
     XCTAssertNil(ParsableArgumentsCodingKeyValidator.validate(B.self))
 
     if let error = ParsableArgumentsCodingKeyValidator.validate(C.self)
-      as? ParsableArgumentsCodingKeyValidator.Error
+      as? ParsableArgumentsCodingKeyValidator.MissingKeysError
     {
       XCTAssert(error.missingCodingKeys == ["count"])
     } else {
       XCTFail()
     }
 
     if let error = ParsableArgumentsCodingKeyValidator.validate(D.self)
-      as? ParsableArgumentsCodingKeyValidator.Error
+      as? ParsableArgumentsCodingKeyValidator.MissingKeysError
     {
       XCTAssert(error.missingCodingKeys == ["phrase"])
     } else {
       XCTFail()
     }
 
     if let error = ParsableArgumentsCodingKeyValidator.validate(E.self)
-      as? ParsableArgumentsCodingKeyValidator.Error
+      as? ParsableArgumentsCodingKeyValidator.MissingKeysError
     {
       XCTAssert(error.missingCodingKeys == ["phrase", "includeCounter"])
     } else {
       XCTFail()
     }
   }
+
+  private struct TypeWithInvalidDecoder: ParsableArguments {
+    @Argument(help: "The phrase to repeat.")
+    var phrase: String = ""
+
+    @Option(help: "The number of times to repeat 'phrase'.")
+    var count: Int = 0
+
+    init() {}
+
+    init(from decoder: Decoder) throws {
+      self.init()
+    }
+  }
+
+  func testCustomDecoderValidation() throws {
+    if let error = ParsableArgumentsCodingKeyValidator.validate(TypeWithInvalidDecoder.self)
+      as? ParsableArgumentsCodingKeyValidator.InvalidDecoderError
+    {
+      XCTAssert(error.type == TypeWithInvalidDecoder.self)
+    } else {
+      XCTFail()
+    }
+  }
 
   private struct F: ParsableArguments {
     @Argument()
@@ -413,9 +437,11 @@ final class ParsableArgumentsValidationTests: XCTestCase {
         """
         One or more Boolean flags is declared with an initial value of `true`.
         This results in the flag always being `true`, no matter whether the user
-        specifies the flag or not. To resolve this error, change the default to
-        `false`, provide a value for the `inversion:` parameter, or remove the
-        `@Flag` property wrapper altogether.
+        specifies the flag or not.
+
+        To resolve this error, change the default to `false`, provide a value
+        for the `inversion:` parameter, or remove the `@Flag` property wrapper
+        altogether.
 
         Affected flag(s):
         --nonsense
@@ -448,9 +474,11 @@ final class ParsableArgumentsValidationTests: XCTestCase {
         """
         One or more Boolean flags is declared with an initial value of `true`.
         This results in the flag always being `true`, no matter whether the user
-        specifies the flag or not. To resolve this error, change the default to
-        `false`, provide a value for the `inversion:` parameter, or remove the
-        `@Flag` property wrapper altogether.
+        specifies the flag or not.
+
+        To resolve this error, change the default to `false`, provide a value
+        for the `inversion:` parameter, or remove the `@Flag` property wrapper
+        altogether.
 
         Affected flag(s):
         --stuff